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2026-11-06 21:43:33 +05:30
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+# binary-tree-traversals
+
+Runnable companion for the ankurm.com article
+[Binary Tree Traversals in Java: Inorder, Preorder, Postorder and Level Order (Recursive and Iterative)](https://ankurm.com/binary-tree-traversal-java-inorder-preorder-postorder-level-order/).
+
+Recursive and iterative inorder, preorder and postorder (two-stack and one-stack), Morris traversal,
+level order (plain, by level, zigzag), height, diameter (naive O(n^2) vs one pass, with counted node
+visits), tree construction from traversals, and a `StackOverflowError` demo on a deep skewed tree.
+
+| Component | Version | Verified against |
+|---|---|---|
+| JDK | 25.0.5+8 (Temurin, LTS) | `java -version` |
+| JUnit | 6.1.3 | `maven-metadata.xml` on Maven Central (same as the sibling modules) |
+| Maven | 3.9.11 | `mvn -version` |
+
+```bash
+export JAVA_HOME=/path/to/jdk-25 && export PATH="$JAVA_HOME/bin:$PATH"
+cd binary-tree-traversals
+mvn test                 # 15 tests; rewrites output/01-11
+./scripts/run-all.sh     # also regenerates output/12-14 (StackOverflowError depth at three -Xss sizes)
+```
+
+| Source (`src/main/java/com/ankurm/interviewlab/bintree/`) | What it is |
+|---|---|
+| `TreeNode`, `Trees` | node class; sample tree, random/skewed/perfect/BST builders, build from inorder+preorder or inorder+postorder |
+| `RecursiveTraversals` | the three textbook recursive orders |
+| `IterativeTraversals` | explicit-stack inorder/preorder/postorder, Morris in/pre-order, `StepListener` hook for the step tables |
+| `LevelOrder` | queue BFS, by-level grouping, zigzag |
+| `TreeMetrics` | counts, height, diameter (naive and one pass, with visit counters) |
+| `DeepRecursionDemo` | 1,000,000-node skewed tree: recursion overflows, iteration does not |
+
+`output/12` to `output/14` are measurements: the overflow depth depends on the machine, the JVM and
+`-Xss`, so they are not asserted. Everything else is asserted by the tests that write it.
@@ -0,0 +1,21 @@
+Sample tree: every traversal, recursive and iterative
+=====================================================
+
+tree (value(left,right), - = no child): 1(2(4,5(7,-)),3(-,6(8,9)))
+
+recursive inorder        : [4, 2, 7, 5, 1, 3, 8, 6, 9]
+recursive preorder       : [1, 2, 4, 5, 7, 3, 6, 8, 9]
+recursive postorder      : [4, 7, 5, 2, 8, 9, 6, 3, 1]
+
+iterative inorder        : [4, 2, 7, 5, 1, 3, 8, 6, 9]
+iterative preorder       : [1, 2, 4, 5, 7, 3, 6, 8, 9]
+iterative postorder (2 stacks): [4, 7, 5, 2, 8, 9, 6, 3, 1]
+iterative postorder (1 stack) : [4, 7, 5, 2, 8, 9, 6, 3, 1]
+
+Morris inorder           : [4, 2, 7, 5, 1, 3, 8, 6, 9]
+Morris preorder          : [1, 2, 4, 5, 7, 3, 6, 8, 9]
+tree after Morris        : 1(2(4,5(7,-)),3(-,6(8,9)))  (unchanged)
+
+level order (queue)       : [1, 2, 3, 4, 5, 6, 7, 8, 9]
+level order by level      : [[1], [2, 3], [4, 5, 6], [7, 8, 9]]
+zigzag                    : [[1], [3, 2], [4, 5, 6], [9, 8, 7]]
@@ -0,0 +1,22 @@
+Iterative inorder on the sample tree, step by step
+==================================================
+
+step | action              | stack (bottom..top) | output so far
+1    | push 1              | [1]                 | []
+2    | push 2              | [1 2]               | []
+3    | push 4              | [1 2 4]             | []
+4    | pop 4, visit        | [1 2]               | [4]
+5    | pop 2, visit        | [1]                 | [4 2]
+6    | push 5              | [1 5]               | [4 2]
+7    | push 7              | [1 5 7]             | [4 2]
+8    | pop 7, visit        | [1 5]               | [4 2 7]
+9    | pop 5, visit        | [1]                 | [4 2 7 5]
+10   | pop 1, visit        | []                  | [4 2 7 5 1]
+11   | push 3              | [3]                 | [4 2 7 5 1]
+12   | pop 3, visit        | []                  | [4 2 7 5 1 3]
+13   | push 6              | [6]                 | [4 2 7 5 1 3]
+14   | push 8              | [6 8]               | [4 2 7 5 1 3]
+15   | pop 8, visit        | [6]                 | [4 2 7 5 1 3 8]
+16   | pop 6, visit        | []                  | [4 2 7 5 1 3 8 6]
+17   | push 9              | [9]                 | [4 2 7 5 1 3 8 6]
+18   | pop 9, visit        | []                  | [4 2 7 5 1 3 8 6 9]
@@ -0,0 +1,13 @@
+Iterative preorder on the sample tree, step by step
+===================================================
+
+step | action                          | stack (bottom..top) | output so far
+1    | pop 1, visit, push its children | [3 2]               | [1]
+2    | pop 2, visit, push its children | [3 5 4]             | [1 2]
+3    | pop 4, visit, push its children | [3 5]               | [1 2 4]
+4    | pop 5, visit, push its children | [3 7]               | [1 2 4 5]
+5    | pop 7, visit, push its children | [3]                 | [1 2 4 5 7]
+6    | pop 3, visit, push its children | [6]                 | [1 2 4 5 7 3]
+7    | pop 6, visit, push its children | [9 8]               | [1 2 4 5 7 3 6]
+8    | pop 8, visit, push its children | [9]                 | [1 2 4 5 7 3 6 8]
+9    | pop 9, visit, push its children | []                  | [1 2 4 5 7 3 6 8 9]
@@ -0,0 +1,22 @@
+Iterative postorder with two stacks, step by step
+=================================================
+
+step | action                                | s1 (bottom..top) | s2 (bottom..top)    | output so far
+1    | move 1 to s2, push its children on s1 | [2 3]            | [1]                 | []
+2    | move 3 to s2, push its children on s1 | [2 6]            | [1 3]               | []
+3    | move 6 to s2, push its children on s1 | [2 8 9]          | [1 3 6]             | []
+4    | move 9 to s2, push its children on s1 | [2 8]            | [1 3 6 9]           | []
+5    | move 8 to s2, push its children on s1 | [2]              | [1 3 6 9 8]         | []
+6    | move 2 to s2, push its children on s1 | [4 5]            | [1 3 6 9 8 2]       | []
+7    | move 5 to s2, push its children on s1 | [4 7]            | [1 3 6 9 8 2 5]     | []
+8    | move 7 to s2, push its children on s1 | [4]              | [1 3 6 9 8 2 5 7]   | []
+9    | move 4 to s2, push its children on s1 | []               | [1 3 6 9 8 2 5 7 4] | []
+10   | pop 4 from s2 to output               | []               | [1 3 6 9 8 2 5 7]   | [4]
+11   | pop 7 from s2 to output               | []               | [1 3 6 9 8 2 5]     | [4 7]
+12   | pop 5 from s2 to output               | []               | [1 3 6 9 8 2]       | [4 7 5]
+13   | pop 2 from s2 to output               | []               | [1 3 6 9 8]         | [4 7 5 2]
+14   | pop 8 from s2 to output               | []               | [1 3 6 9]           | [4 7 5 2 8]
+15   | pop 9 from s2 to output               | []               | [1 3 6]             | [4 7 5 2 8 9]
+16   | pop 6 from s2 to output               | []               | [1 3]               | [4 7 5 2 8 9 6]
+17   | pop 3 from s2 to output               | []               | [1]                 | [4 7 5 2 8 9 6 3]
+18   | pop 1 from s2 to output               | []               | []                  | [4 7 5 2 8 9 6 3 1]
@@ -0,0 +1,26 @@
+Iterative postorder with one stack, step by step
+================================================
+
+step | action                                   | stack (bottom..top) | output so far
+1    | push 1                                   | [1]                 | []
+2    | push 2                                   | [1 2]               | []
+3    | push 4                                   | [1 2 4]             | []
+4    | pop 4, visit                             | [1 2]               | [4]
+5    | peek 2: right child 5 not done, go right | [1 2]               | [4]
+6    | push 5                                   | [1 2 5]             | [4]
+7    | push 7                                   | [1 2 5 7]           | [4]
+8    | pop 7, visit                             | [1 2 5]             | [4 7]
+9    | pop 5, visit                             | [1 2]               | [4 7 5]
+10   | pop 2, visit                             | [1]                 | [4 7 5 2]
+11   | peek 1: right child 3 not done, go right | [1]                 | [4 7 5 2]
+12   | push 3                                   | [1 3]               | [4 7 5 2]
+13   | peek 3: right child 6 not done, go right | [1 3]               | [4 7 5 2]
+14   | push 6                                   | [1 3 6]             | [4 7 5 2]
+15   | push 8                                   | [1 3 6 8]           | [4 7 5 2]
+16   | pop 8, visit                             | [1 3 6]             | [4 7 5 2 8]
+17   | peek 6: right child 9 not done, go right | [1 3 6]             | [4 7 5 2 8]
+18   | push 9                                   | [1 3 6 9]           | [4 7 5 2 8]
+19   | pop 9, visit                             | [1 3 6]             | [4 7 5 2 8 9]
+20   | pop 6, visit                             | [1 3]               | [4 7 5 2 8 9 6]
+21   | pop 3, visit                             | [1]                 | [4 7 5 2 8 9 6 3]
+22   | pop 1, visit                             | []                  | [4 7 5 2 8 9 6 3 1]
@@ -0,0 +1,13 @@
+Level order with a queue, step by step
+======================================
+
+step | action                            | queue (front..back) | output so far
+1    | poll 1, visit, offer its children | [2 3]               | [1]
+2    | poll 2, visit, offer its children | [3 4 5]             | [1 2]
+3    | poll 3, visit, offer its children | [4 5 6]             | [1 2 3]
+4    | poll 4, visit, offer its children | [5 6]               | [1 2 3 4]
+5    | poll 5, visit, offer its children | [6 7]               | [1 2 3 4 5]
+6    | poll 6, visit, offer its children | [7 8 9]             | [1 2 3 4 5 6]
+7    | poll 7, visit, offer its children | [8 9]               | [1 2 3 4 5 6 7]
+8    | poll 8, visit, offer its children | [9]                 | [1 2 3 4 5 6 7 8]
+9    | poll 9, visit, offer its children | []                  | [1 2 3 4 5 6 7 8 9]
@@ -0,0 +1,20 @@
+Morris inorder on the sample tree, step by step
+===============================================
+
+step | action                                                | threads (from node) | output so far
+1    | at 1: thread 5 -> 1, go left                          | [5]                 | []
+2    | at 2: thread 4 -> 2, go left                          | [5 4]               | []
+3    | at 4: no left child, visit, go right                  | [5 4]               | [4]
+4    | at 2: thread from 4 found, remove it, visit, go right | [5]                 | [4 2]
+5    | at 5: thread 7 -> 5, go left                          | [5 7]               | [4 2]
+6    | at 7: no left child, visit, go right                  | [5 7]               | [4 2 7]
+7    | at 5: thread from 7 found, remove it, visit, go right | [5]                 | [4 2 7 5]
+8    | at 1: thread from 5 found, remove it, visit, go right | []                  | [4 2 7 5 1]
+9    | at 3: no left child, visit, go right                  | []                  | [4 2 7 5 1 3]
+10   | at 6: thread 8 -> 6, go left                          | [8]                 | [4 2 7 5 1 3]
+11   | at 8: no left child, visit, go right                  | [8]                 | [4 2 7 5 1 3 8]
+12   | at 6: thread from 8 found, remove it, visit, go right | []                  | [4 2 7 5 1 3 8 6]
+13   | at 9: no left child, visit, go right                  | []                  | [4 2 7 5 1 3 8 6 9]
+
+tree before: 1(2(4,5(7,-)),3(-,6(8,9)))
+tree after : 1(2(4,5(7,-)),3(-,6(8,9)))
@@ -0,0 +1,14 @@
+Sample tree: size, height and diameter
+======================================
+
+nodes                         : 9
+leaves                        : 4
+height, counted in nodes      : 4
+height, counted in edges      : 3
+height via level order (BFS)  : 4
+diameter, counted in nodes    : 7
+diameter, counted in edges    : 6
+one longest path              : [7, 5, 2, 1, 3, 6, 8]
+height of the subtree at node : {1=4, 2=3, 3=3, 4=1, 5=2, 6=2, 7=1, 8=1, 9=1}
+node visits, naive O(n^2) way : 26
+node visits, one-pass way     : 9
@@ -0,0 +1,11 @@
+Diameter: node visits, naive vs one pass (counted, not timed)
+=============================================================
+
+tree                      nodes   naive visits     one-pass   diameter
+sample (9)                    9             26            9          7
+perfect, 4 levels            15             49           15          7
+perfect, 7 levels           127            769          127         13
+perfect, 10 levels         1023           9217         1023         19
+left-skewed 100             100           5050          100        100
+left-skewed 1000           1000         500500         1000       1000
+left-skewed 2000           2000        2001000         2000       2000
@@ -0,0 +1,21 @@
+Constructing a tree from two traversals
+=======================================
+
+inorder   : [4, 2, 5, 1, 6, 3]
+preorder  : [1, 2, 4, 5, 3, 6]
+
+from inorder + preorder, one line per recursive call:
+root 1  inorder slice [0..5]  left size 3, right size 2
+  root 2  inorder slice [0..2]  left size 1, right size 1
+    root 4  inorder slice [0..0]  left size 0, right size 0
+    root 5  inorder slice [2..2]  left size 0, right size 0
+  root 3  inorder slice [4..5]  left size 1, right size 0
+    root 6  inorder slice [4..4]  left size 0, right size 0
+result    : 1(2(4,5),3(6,-))
+postorder of the result: [4, 5, 2, 6, 3, 1]
+
+from inorder + postorder the same tree comes back: 1(2(4,5),3(6,-))
+
+two different trees with the same preorder AND postorder:
+  1(2,-)  preorder [1, 2]  postorder [2, 1]
+  1(-,2)  preorder [1, 2]  postorder [2, 1]
@@ -0,0 +1,8 @@
+Inorder of a binary search tree is sorted
+=========================================
+
+inserted in this order : [50, 30, 70, 20, 40, 60, 80]
+tree                   : 50(30(20,40),70(60,80))
+preorder               : [50, 30, 20, 40, 70, 60, 80]
+inorder                : [20, 30, 40, 50, 60, 70, 80]
+postorder              : [20, 40, 30, 60, 80, 70, 50]
@@ -0,0 +1,19 @@
+JVM: Eclipse Adoptium 25.0.5, ThreadStackSize=1024 KB, cores=2
+Built a left-skewed tree with 1000000 nodes using a loop (no recursion).
+
+Depth reached by a recursive probe before StackOverflowError (3 attempts in one JVM):
+  attempt 1: StackOverflowError after reaching depth 21258
+  attempt 2: StackOverflowError after reaching depth 29514
+  attempt 3: StackOverflowError after reaching depth 29514
+
+RecursiveTraversals.inorder on the same tree:
+  caught java.lang.StackOverflowError
+  stack trace length: 1024 frames (the JVM truncates traces; the real depth is larger)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      ...
+
+IterativeTraversals.inorder on the same tree:
+  visited 1000000 nodes, first=1000000, last=1, no error
@@ -0,0 +1,19 @@
+JVM: Eclipse Adoptium 25.0.5, ThreadStackSize=512 KB, cores=2
+Built a left-skewed tree with 1000000 nodes using a loop (no recursion).
+
+Depth reached by a recursive probe before StackOverflowError (3 attempts in one JVM):
+  attempt 1: StackOverflowError after reaching depth 5175
+  attempt 2: StackOverflowError after reaching depth 12829
+  attempt 3: StackOverflowError after reaching depth 13130
+
+RecursiveTraversals.inorder on the same tree:
+  caught java.lang.StackOverflowError
+  stack trace length: 1024 frames (the JVM truncates traces; the real depth is larger)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      ...
+
+IterativeTraversals.inorder on the same tree:
+  visited 1000000 nodes, first=1000000, last=1, no error
@@ -0,0 +1,19 @@
+JVM: Eclipse Adoptium 25.0.5, ThreadStackSize=4096 KB, cores=2
+Built a left-skewed tree with 1000000 nodes using a loop (no recursion).
+
+Depth reached by a recursive probe before StackOverflowError (3 attempts in one JVM):
+  attempt 1: StackOverflowError after reaching depth 51051
+  attempt 2: StackOverflowError after reaching depth 127818
+  attempt 3: StackOverflowError after reaching depth 127818
+
+RecursiveTraversals.inorder on the same tree:
+  caught java.lang.StackOverflowError
+  stack trace length: 1024 frames (the JVM truncates traces; the real depth is larger)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      at com.ankurm.interviewlab.bintree.RecursiveTraversals.inorder(RecursiveTraversals.java:20)
+      ...
+
+IterativeTraversals.inorder on the same tree:
+  visited 1000000 nodes, first=1000000, last=1, no error
@@ -0,0 +1,65 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  <groupId>com.ankurm.interviewlab</groupId>
+  <artifactId>binary-tree-traversals</artifactId>
+  <version>1.0.0</version>
+  <packaging>jar</packaging>
+
+  <name>java-interview-lab :: binary-tree-traversals</name>
+  <description>
+    Runnable companion code for ankurm.com's "Binary Tree Traversals in Java" article: recursive and
+    iterative inorder, preorder and postorder, Morris traversal, level order, height, diameter, tree
+    construction from traversals, and a StackOverflowError demonstration on a deep skewed tree.
+  </description>
+
+  <properties>
+    <maven.compiler.release>25</maven.compiler.release>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <junit.version>6.1.3</junit.version>
+  </properties>
+
+  <dependencyManagement>
+    <dependencies>
+      <dependency>
+        <groupId>org.junit</groupId>
+        <artifactId>junit-bom</artifactId>
+        <version>${junit.version}</version>
+        <type>pom</type>
+        <scope>import</scope>
+      </dependency>
+    </dependencies>
+  </dependencyManagement>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.junit.jupiter</groupId>
+      <artifactId>junit-jupiter</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>org.junit.platform</groupId>
+      <artifactId>junit-platform-launcher</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.14.0</version>
+      </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <version>3.6.0</version>
+        <configuration>
+          <argLine>-Xmx512m -Duser.timezone=UTC -Dfile.encoding=UTF-8</argLine>
+        </configuration>
+      </plugin>
+    </plugins>
+  </build>
+</project>
@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+# Regenerates every file in output/. Needs JDK 25 and Maven. The first Maven run must be online.
+# 01-11 are written by JUnit tests that assert the same facts (deterministic).
+# 12-14 are MEASUREMENTS of StackOverflowError depth: they depend on the machine, the JVM build
+# and the -Xss setting, and the depth even varies between attempts inside one JVM.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+mkdir -p output
+mvn -B -q test
+mvn -B -q compile
+run() { java "$@" -cp target/classes com.ankurm.interviewlab.bintree.DeepRecursionDemo 2>&1 | grep -v '^Picked up JAVA_TOOL_OPTIONS' ; }
+run            > output/12-deep-recursion-default-stack.txt
+run -Xss512k   > output/13-deep-recursion-xss512k.txt
+run -Xss4m     > output/14-deep-recursion-xss4m.txt
+echo "done:"; ls -1 output
@@ -0,0 +1,73 @@
+package com.ankurm.interviewlab.bintree;
+
+import java.lang.management.ManagementFactory;
+import java.util.List;
+
+import com.sun.management.HotSpotDiagnosticMXBean;
+
+/**
+ * Builds a 1,000,000-node left-skewed tree and traverses it recursively until the JVM throws
+ * StackOverflowError, then traverses the same tree iteratively. The depth reached depends on the
+ * thread stack size (-Xss), the JIT state and the frame layout, so it varies per machine and per run:
+ * this demo prints it, it does not assert it.
+ */
+public final class DeepRecursionDemo {
+    private static final int NODES = 1_000_000;
+    private static int deepestEntered;
+
+    private DeepRecursionDemo() {}
+
+    public static void main(String[] args) {
+        HotSpotDiagnosticMXBean vm = ManagementFactory.getPlatformMXBean(HotSpotDiagnosticMXBean.class);
+        System.out.println("JVM: " + System.getProperty("java.vendor") + " " + System.getProperty("java.version")
+                + ", ThreadStackSize=" + vm.getVMOption("ThreadStackSize").getValue() + " KB, cores="
+                + Runtime.getRuntime().availableProcessors());
+        TreeNode root = Trees.leftSkewed(NODES);
+        System.out.println("Built a left-skewed tree with " + NODES + " nodes using a loop (no recursion).");
+        System.out.println();
+
+        System.out.println("Depth reached by a recursive probe before StackOverflowError (3 attempts in one JVM):");
+        for (int attempt = 1; attempt <= 3; attempt++) {
+            deepestEntered = 0;
+            try {
+                probe(root, 1);
+                System.out.println("  attempt " + attempt + ": finished without error, depth " + deepestEntered);
+            } catch (StackOverflowError e) {
+                System.out.println("  attempt " + attempt + ": StackOverflowError after reaching depth " + deepestEntered);
+            }
+        }
+        System.out.println();
+
+        System.out.println("RecursiveTraversals.inorder on the same tree:");
+        try {
+            RecursiveTraversals.inorder(root);
+            System.out.println("  finished without error");
+        } catch (StackOverflowError e) {
+            StackTraceElement[] trace = e.getStackTrace();
+            System.out.println("  caught " + e);
+            System.out.println("  stack trace length: " + trace.length + " frames (the JVM truncates traces; the real depth is larger)");
+            for (int i = 0; i < Math.min(4, trace.length); i++) {
+                System.out.println("      at " + trace[i]);
+            }
+            System.out.println("      ...");
+        }
+        System.out.println();
+
+        System.out.println("IterativeTraversals.inorder on the same tree:");
+        List<Integer> result = IterativeTraversals.inorder(root);
+        System.out.println("  visited " + result.size() + " nodes, first=" + result.get(0) + ", last="
+                + result.get(result.size() - 1) + ", no error");
+    }
+
+    /** Same shape as the recursive inorder, but remembers how deep it got. */
+    private static void probe(TreeNode node, int depth) {
+        if (node == null) {
+            return;
+        }
+        if (depth > deepestEntered) {
+            deepestEntered = depth;
+        }
+        probe(node.left, depth + 1);
+        probe(node.right, depth + 1);
+    }
+}
@@ -0,0 +1,230 @@
+package com.ankurm.interviewlab.bintree;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Deque;
+import java.util.List;
+
+/**
+ * Depth-first traversals with an explicit stack instead of the JVM call stack, plus Morris
+ * traversal, which needs no stack at all. Every method has an overload taking a
+ * {@link StepListener} so the article's step tables come from the real code.
+ */
+public final class IterativeTraversals {
+    private IterativeTraversals() {}
+
+    // ---------------------------------------------------------------- inorder
+
+    public static List<Integer> inorder(TreeNode root) {
+        return inorder(root, null);
+    }
+
+    public static List<Integer> inorder(TreeNode root, StepListener listener) {
+        List<Integer> out = new ArrayList<>();
+        Deque<TreeNode> stack = new ArrayDeque<>();
+        TreeNode current = root;
+        while (current != null || !stack.isEmpty()) {
+            while (current != null) {          // go as far left as possible, remembering the way back
+                stack.push(current);
+                emit(listener, "push " + current.val, stack, null, out);
+                current = current.left;
+            }
+            current = stack.pop();             // nothing further left: this node is next
+            out.add(current.val);
+            emit(listener, "pop " + current.val + ", visit", stack, null, out);
+            current = current.right;           // then the whole right subtree
+        }
+        return out;
+    }
+
+    // --------------------------------------------------------------- preorder
+
+    public static List<Integer> preorder(TreeNode root) {
+        return preorder(root, null);
+    }
+
+    public static List<Integer> preorder(TreeNode root, StepListener listener) {
+        List<Integer> out = new ArrayList<>();
+        if (root == null) {
+            return out;
+        }
+        Deque<TreeNode> stack = new ArrayDeque<>();
+        stack.push(root);
+        while (!stack.isEmpty()) {
+            TreeNode node = stack.pop();
+            out.add(node.val);
+            if (node.right != null) {
+                stack.push(node.right);        // pushed first so it is popped last
+            }
+            if (node.left != null) {
+                stack.push(node.left);
+            }
+            emit(listener, "pop " + node.val + ", visit, push its children", stack, null, out);
+        }
+        return out;
+    }
+
+    // ------------------------------------------------------- postorder, 2 stacks
+
+    public static List<Integer> postorderTwoStacks(TreeNode root) {
+        return postorderTwoStacks(root, null);
+    }
+
+    public static List<Integer> postorderTwoStacks(TreeNode root, StepListener listener) {
+        List<Integer> out = new ArrayList<>();
+        if (root == null) {
+            return out;
+        }
+        Deque<TreeNode> first = new ArrayDeque<>();
+        Deque<TreeNode> second = new ArrayDeque<>();
+        first.push(root);
+        while (!first.isEmpty()) {             // phase 1: root-right-left order lands on `second`
+            TreeNode node = first.pop();
+            second.push(node);
+            if (node.left != null) {
+                first.push(node.left);
+            }
+            if (node.right != null) {
+                first.push(node.right);
+            }
+            emit(listener, "move " + node.val + " to s2, push its children on s1", first, second, out);
+        }
+        while (!second.isEmpty()) {            // phase 2: popping reverses it into left-right-root
+            TreeNode node = second.pop();
+            out.add(node.val);
+            emit(listener, "pop " + node.val + " from s2 to output", first, second, out);
+        }
+        return out;
+    }
+
+    // ------------------------------------------------------- postorder, 1 stack
+
+    public static List<Integer> postorderOneStack(TreeNode root) {
+        return postorderOneStack(root, null);
+    }
+
+    public static List<Integer> postorderOneStack(TreeNode root, StepListener listener) {
+        List<Integer> out = new ArrayList<>();
+        Deque<TreeNode> stack = new ArrayDeque<>();
+        TreeNode current = root;
+        TreeNode lastVisited = null;           // the node we most recently wrote to the output
+        while (current != null || !stack.isEmpty()) {
+            while (current != null) {
+                stack.push(current);
+                emit(listener, "push " + current.val, stack, null, out);
+                current = current.left;
+            }
+            TreeNode top = stack.peek();
+            if (top.right != null && top.right != lastVisited) {
+                emit(listener, "peek " + top.val + ": right child " + top.right.val + " not done, go right",
+                        stack, null, out);
+                current = top.right;           // right subtree still unvisited
+            } else {
+                stack.pop();                   // both children finished (or absent)
+                out.add(top.val);
+                lastVisited = top;
+                emit(listener, "pop " + top.val + ", visit", stack, null, out);
+            }
+        }
+        return out;
+    }
+
+    // ------------------------------------------------------------------ Morris
+
+    public static List<Integer> morrisInorder(TreeNode root) {
+        return morrisInorder(root, null);
+    }
+
+    /**
+     * Inorder in O(1) extra space by temporarily threading each node's inorder predecessor
+     * back to it. The tree is restored before the method returns. For this method the listener's
+     * {@code containerA} holds the nodes whose right pointer is currently a temporary thread.
+     */
+    public static List<Integer> morrisInorder(TreeNode root, StepListener listener) {
+        List<Integer> out = new ArrayList<>();
+        List<Integer> threads = new ArrayList<>();
+        TreeNode current = root;
+        while (current != null) {
+            if (current.left == null) {
+                out.add(current.val);
+                emitThreads(listener, "at " + current.val + ": no left child, visit, go right", threads, out);
+                current = current.right;
+            } else {
+                TreeNode predecessor = current.left;
+                while (predecessor.right != null && predecessor.right != current) {
+                    predecessor = predecessor.right;
+                }
+                if (predecessor.right == null) {
+                    predecessor.right = current;   // create the thread
+                    threads.add(predecessor.val);
+                    emitThreads(listener, "at " + current.val + ": thread " + predecessor.val + " -> " + current.val
+                            + ", go left", threads, out);
+                    current = current.left;
+                } else {
+                    predecessor.right = null;      // thread found: left side finished, remove it
+                    threads.remove(Integer.valueOf(predecessor.val));
+                    out.add(current.val);
+                    emitThreads(listener, "at " + current.val + ": thread from " + predecessor.val
+                            + " found, remove it, visit, go right", threads, out);
+                    current = current.right;
+                }
+            }
+        }
+        return out;
+    }
+
+    public static List<Integer> morrisPreorder(TreeNode root) {
+        List<Integer> out = new ArrayList<>();
+        TreeNode current = root;
+        while (current != null) {
+            if (current.left == null) {
+                out.add(current.val);
+                current = current.right;
+            } else {
+                TreeNode predecessor = current.left;
+                while (predecessor.right != null && predecessor.right != current) {
+                    predecessor = predecessor.right;
+                }
+                if (predecessor.right == null) {
+                    out.add(current.val);          // preorder visits on the FIRST arrival
+                    predecessor.right = current;
+                    current = current.left;
+                } else {
+                    predecessor.right = null;
+                    current = current.right;
+                }
+            }
+        }
+        return out;
+    }
+
+    // ---------------------------------------------------------------- helpers
+
+    private static void emit(StepListener listener, String action, Iterable<TreeNode> a,
+                               Iterable<TreeNode> b, List<Integer> out) {
+        if (listener == null) {
+            return;
+        }
+        listener.step(action, valuesBottomFirst(a), valuesBottomFirst(b), List.copyOf(out));
+    }
+
+    private static void emitThreads(StepListener listener, String action, List<Integer> threads,
+                                      List<Integer> out) {
+        if (listener == null) {
+            return;
+        }
+        listener.step(action, List.copyOf(threads), List.of(), List.copyOf(out));
+    }
+
+    /** ArrayDeque iterates top-of-stack first when used with push; we print bottom first. */
+    private static List<Integer> valuesBottomFirst(Iterable<TreeNode> stack) {
+        List<Integer> values = new ArrayList<>();
+        if (stack != null) {
+            for (TreeNode n : stack) {
+                values.add(n.val);
+            }
+        }
+        java.util.Collections.reverse(values);
+        return values;
+    }
+}
@@ -0,0 +1,101 @@
+package com.ankurm.interviewlab.bintree;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Deque;
+import java.util.List;
+
+/** Breadth-first traversals: a queue instead of a stack. */
+public final class LevelOrder {
+    private LevelOrder() {}
+
+    public static List<Integer> levelOrder(TreeNode root) {
+        return levelOrder(root, null);
+    }
+
+    public static List<Integer> levelOrder(TreeNode root, StepListener listener) {
+        List<Integer> out = new ArrayList<>();
+        if (root == null) {
+            return out;
+        }
+        Deque<TreeNode> queue = new ArrayDeque<>();
+        queue.offer(root);
+        while (!queue.isEmpty()) {
+            TreeNode node = queue.poll();
+            out.add(node.val);
+            if (node.left != null) {
+                queue.offer(node.left);
+            }
+            if (node.right != null) {
+                queue.offer(node.right);
+            }
+            if (listener != null) {
+                List<Integer> snapshot = new ArrayList<>();
+                for (TreeNode n : queue) {
+                    snapshot.add(n.val);       // ArrayDeque iterates front to back
+                }
+                listener.step("poll " + node.val + ", visit, offer its children", snapshot, List.of(),
+                        List.copyOf(out));
+            }
+        }
+        return out;
+    }
+
+    /** One list per level: the trick is to freeze {@code queue.size()} before the inner loop. */
+    public static List<List<Integer>> byLevel(TreeNode root) {
+        List<List<Integer>> levels = new ArrayList<>();
+        if (root == null) {
+            return levels;
+        }
+        Deque<TreeNode> queue = new ArrayDeque<>();
+        queue.offer(root);
+        while (!queue.isEmpty()) {
+            int sizeOfThisLevel = queue.size();
+            List<Integer> level = new ArrayList<>(sizeOfThisLevel);
+            for (int i = 0; i < sizeOfThisLevel; i++) {
+                TreeNode node = queue.poll();
+                level.add(node.val);
+                if (node.left != null) {
+                    queue.offer(node.left);
+                }
+                if (node.right != null) {
+                    queue.offer(node.right);
+                }
+            }
+            levels.add(level);
+        }
+        return levels;
+    }
+
+    /** Left-to-right on level 0, right-to-left on level 1, and so on. */
+    public static List<List<Integer>> zigzag(TreeNode root) {
+        List<List<Integer>> levels = new ArrayList<>();
+        if (root == null) {
+            return levels;
+        }
+        Deque<TreeNode> queue = new ArrayDeque<>();
+        queue.offer(root);
+        boolean leftToRight = true;
+        while (!queue.isEmpty()) {
+            int sizeOfThisLevel = queue.size();
+            Deque<Integer> level = new ArrayDeque<>();
+            for (int i = 0; i < sizeOfThisLevel; i++) {
+                TreeNode node = queue.poll();
+                if (leftToRight) {
+                    level.addLast(node.val);
+                } else {
+                    level.addFirst(node.val);
+                }
+                if (node.left != null) {
+                    queue.offer(node.left);
+                }
+                if (node.right != null) {
+                    queue.offer(node.right);
+                }
+            }
+            levels.add(new ArrayList<>(level));
+            leftToRight = !leftToRight;
+        }
+        return levels;
+    }
+}
@@ -0,0 +1,54 @@
+package com.ankurm.interviewlab.bintree;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/** The three depth-first orders, written the way textbooks write them. */
+public final class RecursiveTraversals {
+    private RecursiveTraversals() {}
+
+    public static List<Integer> inorder(TreeNode root) {
+        List<Integer> out = new ArrayList<>();
+        inorder(root, out);
+        return out;
+    }
+
+    private static void inorder(TreeNode node, List<Integer> out) {
+        if (node == null) {
+            return;
+        }
+        inorder(node.left, out);
+        out.add(node.val);
+        inorder(node.right, out);
+    }
+
+    public static List<Integer> preorder(TreeNode root) {
+        List<Integer> out = new ArrayList<>();
+        preorder(root, out);
+        return out;
+    }
+
+    private static void preorder(TreeNode node, List<Integer> out) {
+        if (node == null) {
+            return;
+        }
+        out.add(node.val);
+        preorder(node.left, out);
+        preorder(node.right, out);
+    }
+
+    public static List<Integer> postorder(TreeNode root) {
+        List<Integer> out = new ArrayList<>();
+        postorder(root, out);
+        return out;
+    }
+
+    private static void postorder(TreeNode node, List<Integer> out) {
+        if (node == null) {
+            return;
+        }
+        postorder(node.left, out);
+        postorder(node.right, out);
+        out.add(node.val);
+    }
+}
@@ -0,0 +1,16 @@
+package com.ankurm.interviewlab.bintree;
+
+import java.util.List;
+
+/**
+ * Optional observer that the iterative traversals call after every push, pop or queue operation.
+ * The tables in the article are produced by this hook on the real implementations, not by a
+ * separate "teaching" copy of the algorithm.
+ *
+ * <p>{@code containerA} is the main stack (bottom first, top last) or the queue (front first).
+ * {@code containerB} is the second stack of the two-stack postorder; it is empty for the others.
+ */
+@FunctionalInterface
+public interface StepListener {
+    void step(String action, List<Integer> containerA, List<Integer> containerB, List<Integer> output);
+}
@@ -0,0 +1,141 @@
+package com.ankurm.interviewlab.bintree;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Deque;
+import java.util.List;
+
+/**
+ * Size, leaves, height and diameter. Height is counted in NODES on the longest root-to-leaf path
+ * (empty tree = 0, single node = 1). Many textbooks count EDGES instead (single node = 0): subtract one.
+ * The same applies to diameter below.
+ */
+public final class TreeMetrics {
+    private TreeMetrics() {}
+
+    public static int countNodes(TreeNode node) {
+        return node == null ? 0 : 1 + countNodes(node.left) + countNodes(node.right);
+    }
+
+    public static int countLeaves(TreeNode node) {
+        if (node == null) {
+            return 0;
+        }
+        if (node.left == null && node.right == null) {
+            return 1;
+        }
+        return countLeaves(node.left) + countLeaves(node.right);
+    }
+
+    public static int height(TreeNode node) {
+        if (node == null) {
+            return 0;
+        }
+        return 1 + Math.max(height(node.left), height(node.right));
+    }
+
+    /** Height without recursion: the number of levels a queue-based traversal walks through. */
+    public static int heightIterative(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        int levels = 0;
+        Deque<TreeNode> queue = new ArrayDeque<>();
+        queue.offer(root);
+        while (!queue.isEmpty()) {
+            levels++;
+            for (int i = queue.size(); i > 0; i--) {
+                TreeNode node = queue.poll();
+                if (node.left != null) {
+                    queue.offer(node.left);
+                }
+                if (node.right != null) {
+                    queue.offer(node.right);
+                }
+            }
+        }
+        return levels;
+    }
+
+    // ---------------------------------------------------------------- diameter
+
+    /**
+     * The obvious solution: for every node, ask for the height of both sides (each a full walk of
+     * that subtree), then recurse into the children. O(n^2) in the worst case.
+     * {@code visits[0]} counts every node entered by either {@code height} or this method.
+     */
+    public static int diameterNaive(TreeNode node, long[] visits) {
+        if (node == null) {
+            return 0;
+        }
+        visits[0]++;
+        int through = heightCounting(node.left, visits) + heightCounting(node.right, visits) + 1;
+        int inLeft = diameterNaive(node.left, visits);
+        int inRight = diameterNaive(node.right, visits);
+        return Math.max(through, Math.max(inLeft, inRight));
+    }
+
+    private static int heightCounting(TreeNode node, long[] visits) {
+        if (node == null) {
+            return 0;
+        }
+        visits[0]++;
+        return 1 + Math.max(heightCounting(node.left, visits), heightCounting(node.right, visits));
+    }
+
+    /** One post-order pass: each node returns its height and updates the best path through itself. */
+    public static int diameterOnePass(TreeNode root, long[] visits) {
+        int[] best = {0};
+        heightAndDiameter(root, best, visits);
+        return best[0];
+    }
+
+    private static int heightAndDiameter(TreeNode node, int[] best, long[] visits) {
+        if (node == null) {
+            return 0;
+        }
+        visits[0]++;
+        int left = heightAndDiameter(node.left, best, visits);
+        int right = heightAndDiameter(node.right, best, visits);
+        best[0] = Math.max(best[0], left + right + 1);
+        return 1 + Math.max(left, right);
+    }
+
+    /** The values along one longest path (ties: prefers the left child), for drawing the diagram. */
+    public static List<Integer> diameterPath(TreeNode root) {
+        TreeNode[] bestNode = {null};
+        int[] bestLen = {0};
+        locateBest(root, bestNode, bestLen);
+        if (bestNode[0] == null) {
+            return List.of();
+        }
+        List<Integer> path = new ArrayList<>(deepestPath(bestNode[0].left));
+        Collections.reverse(path);
+        path.add(bestNode[0].val);
+        path.addAll(deepestPath(bestNode[0].right));
+        return path;
+    }
+
+    private static int locateBest(TreeNode node, TreeNode[] bestNode, int[] bestLen) {
+        if (node == null) {
+            return 0;
+        }
+        int left = locateBest(node.left, bestNode, bestLen);
+        int right = locateBest(node.right, bestNode, bestLen);
+        if (left + right + 1 > bestLen[0]) {
+            bestLen[0] = left + right + 1;
+            bestNode[0] = node;
+        }
+        return 1 + Math.max(left, right);
+    }
+
+    private static List<Integer> deepestPath(TreeNode node) {
+        List<Integer> path = new ArrayList<>();
+        while (node != null) {
+            path.add(node.val);
+            node = height(node.left) >= height(node.right) ? node.left : node.right;
+        }
+        return path;
+    }
+}
@@ -0,0 +1,18 @@
+package com.ankurm.interviewlab.bintree;
+
+/** A binary tree node. Public fields on purpose: this is teaching code, not a library. */
+public final class TreeNode {
+    public int val;
+    public TreeNode left;
+    public TreeNode right;
+
+    public TreeNode(int val) {
+        this.val = val;
+    }
+
+    public TreeNode(int val, TreeNode left, TreeNode right) {
+        this.val = val;
+        this.left = left;
+        this.right = right;
+    }
+}
@@ -0,0 +1,186 @@
+package com.ankurm.interviewlab.bintree;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Random;
+import java.util.function.Consumer;
+
+/** Tree builders used by the demos and tests, plus tree construction from traversals. */
+public final class Trees {
+    private Trees() {}
+
+    /**
+     * The sample tree used by every diagram in the article (values 1..9):
+     * <pre>
+     *           1
+     *         /   \
+     *        2     3
+     *       / \     \
+     *      4   5     6
+     *         /     / \
+     *        7     8   9
+     * </pre>
+     */
+    public static TreeNode sample() {
+        return new TreeNode(1,
+                new TreeNode(2,
+                        new TreeNode(4),
+                        new TreeNode(5, new TreeNode(7), null)),
+                new TreeNode(3,
+                        null,
+                        new TreeNode(6, new TreeNode(8), new TreeNode(9))));
+    }
+
+    /** A random binary tree whose values are a random permutation of 1..n (so values are unique). */
+    public static TreeNode random(Random random, int n) {
+        if (n == 0) {
+            return null;
+        }
+        List<Integer> values = new ArrayList<>();
+        for (int i = 1; i <= n; i++) {
+            values.add(i);
+        }
+        Collections.shuffle(values, random);
+        TreeNode root = new TreeNode(values.get(0));
+        List<TreeNode> open = new ArrayList<>();   // nodes that still have a free child slot
+        open.add(root);
+        for (int i = 1; i < n; i++) {
+            int pick = random.nextInt(open.size());
+            TreeNode parent = open.get(pick);
+            TreeNode child = new TreeNode(values.get(i));
+            boolean goLeft = parent.left == null && (parent.right != null || random.nextBoolean());
+            if (goLeft) {
+                parent.left = child;
+            } else {
+                parent.right = child;
+            }
+            if (parent.left != null && parent.right != null) {
+                open.set(pick, open.get(open.size() - 1));
+                open.remove(open.size() - 1);
+            }
+            open.add(child);
+        }
+        return root;
+    }
+
+    /** n nodes, each the LEFT child of the previous one; built with a loop, so building never overflows. */
+    public static TreeNode leftSkewed(int n) {
+        TreeNode root = null;
+        for (int i = n; i >= 1; i--) {
+            root = new TreeNode(i, root, null);
+        }
+        return root;
+    }
+
+    /** A perfect tree of the given number of levels, values 1.. in level order. */
+    public static TreeNode perfect(int levels) {
+        return perfect(1, levels);
+    }
+
+    private static TreeNode perfect(int value, int levels) {
+        if (levels == 0) {
+            return null;
+        }
+        return new TreeNode(value, perfect(2 * value, levels - 1), perfect(2 * value + 1, levels - 1));
+    }
+
+    /** Ordinary binary SEARCH tree insertion (smaller left, larger right, duplicates ignored). */
+    public static TreeNode bst(int... values) {
+        TreeNode root = null;
+        for (int v : values) {
+            root = insert(root, v);
+        }
+        return root;
+    }
+
+    private static TreeNode insert(TreeNode node, int v) {
+        if (node == null) {
+            return new TreeNode(v);
+        }
+        if (v < node.val) {
+            node.left = insert(node.left, v);
+        } else if (v > node.val) {
+            node.right = insert(node.right, v);
+        }
+        return node;
+    }
+
+    // ----------------------------------------------- construction from traversals
+
+    /**
+     * Rebuilds the tree from its inorder and preorder sequences (values must be unique).
+     * {@code trace} (nullable) receives one line per recursive call, which is how the article shows the splitting.
+     */
+    public static TreeNode fromInorderPreorder(int[] inorder, int[] preorder, Consumer<String> trace) {
+        Map<Integer, Integer> position = new HashMap<>();
+        for (int i = 0; i < inorder.length; i++) {
+            position.put(inorder[i], i);
+        }
+        return buildPre(preorder, new int[] {0}, 0, inorder.length - 1, position, trace, 0);
+    }
+
+    private static TreeNode buildPre(int[] pre, int[] next, int lo, int hi, Map<Integer, Integer> position,
+                                     Consumer<String> trace, int depth) {
+        if (lo > hi) {
+            return null;
+        }
+        int rootValue = pre[next[0]++];
+        int mid = position.get(rootValue);
+        if (trace != null) {
+            trace.accept("  ".repeat(depth) + "root " + rootValue + "  inorder slice [" + lo + ".." + hi
+                    + "]  left size " + (mid - lo) + ", right size " + (hi - mid));
+        }
+        TreeNode node = new TreeNode(rootValue);
+        node.left = buildPre(pre, next, lo, mid - 1, position, trace, depth + 1);
+        node.right = buildPre(pre, next, mid + 1, hi, position, trace, depth + 1);
+        return node;
+    }
+
+    /** Rebuilds the tree from inorder and postorder (the root is the LAST postorder element; build right first). */
+    public static TreeNode fromInorderPostorder(int[] inorder, int[] postorder) {
+        Map<Integer, Integer> position = new HashMap<>();
+        for (int i = 0; i < inorder.length; i++) {
+            position.put(inorder[i], i);
+        }
+        return buildPost(postorder, new int[] {postorder.length - 1}, 0, inorder.length - 1, position);
+    }
+
+    private static TreeNode buildPost(int[] post, int[] next, int lo, int hi, Map<Integer, Integer> position) {
+        if (lo > hi) {
+            return null;
+        }
+        int rootValue = post[next[0]--];
+        int mid = position.get(rootValue);
+        TreeNode node = new TreeNode(rootValue);
+        node.right = buildPost(post, next, mid + 1, hi, position);
+        node.left = buildPost(post, next, lo, mid - 1, position);
+        return node;
+    }
+
+    // -------------------------------------------------------------- utilities
+
+    public static boolean sameShapeAndValues(TreeNode a, TreeNode b) {
+        if (a == null || b == null) {
+            return a == b;
+        }
+        return a.val == b.val && sameShapeAndValues(a.left, b.left) && sameShapeAndValues(a.right, b.right);
+    }
+
+    /** Compact one-line form: value(left,right), with "-" for a missing child, e.g. 1(2(4,5),3(6,-)). */
+    public static String bracket(TreeNode node) {
+        if (node == null) {
+            return "-";
+        }
+        if (node.left == null && node.right == null) {
+            return Integer.toString(node.val);
+        }
+        return node.val + "(" + bracket(node.left) + "," + bracket(node.right) + ")";
+    }
+
+    public static int[] toArray(List<Integer> list) {
+        return list.stream().mapToInt(Integer::intValue).toArray();
+    }
+}
@@ -0,0 +1,64 @@
+package com.ankurm.interviewlab.bintree;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import com.ankurm.interviewlab.bintree.support.Transcript;
+
+/** Outputs 10 and 11: exam staples (rebuild from traversals, BST inorder is sorted). */
+class ConstructTreeTest {
+
+    @Test
+    void rebuildFromInorderAndPreorder() {
+        int[] inorder = {4, 2, 5, 1, 6, 3};
+        int[] preorder = {1, 2, 4, 5, 3, 6};
+        int[] postorder = {4, 5, 2, 6, 3, 1};
+
+        Transcript t = Transcript.start("10-construct-from-traversals.txt", "Constructing a tree from two traversals");
+        t.line("inorder   : " + List.of(4, 2, 5, 1, 6, 3));
+        t.line("preorder  : " + List.of(1, 2, 4, 5, 3, 6)).blank();
+        t.line("from inorder + preorder, one line per recursive call:");
+        TreeNode fromPre = Trees.fromInorderPreorder(inorder, preorder, t::line);
+        t.line("result    : " + Trees.bracket(fromPre));
+        t.line("postorder of the result: " + RecursiveTraversals.postorder(fromPre)).blank();
+
+        assertEquals("1(2(4,5),3(6,-))", Trees.bracket(fromPre));
+        assertEquals(List.of(4, 5, 2, 6, 3, 1), RecursiveTraversals.postorder(fromPre));
+
+        TreeNode fromPost = Trees.fromInorderPostorder(inorder, postorder);
+        assertTrue(Trees.sameShapeAndValues(fromPre, fromPost));
+        t.line("from inorder + postorder the same tree comes back: " + Trees.bracket(fromPost)).blank();
+
+        // Why inorder is needed: preorder + postorder cannot tell a left child from a right child.
+        TreeNode leftOnly = new TreeNode(1, new TreeNode(2), null);
+        TreeNode rightOnly = new TreeNode(1, null, new TreeNode(2));
+        assertEquals(RecursiveTraversals.preorder(leftOnly), RecursiveTraversals.preorder(rightOnly));
+        assertEquals(RecursiveTraversals.postorder(leftOnly), RecursiveTraversals.postorder(rightOnly));
+        t.line("two different trees with the same preorder AND postorder:");
+        t.line("  " + Trees.bracket(leftOnly) + "  preorder " + RecursiveTraversals.preorder(leftOnly)
+                + "  postorder " + RecursiveTraversals.postorder(leftOnly));
+        t.line("  " + Trees.bracket(rightOnly) + "  preorder " + RecursiveTraversals.preorder(rightOnly)
+                + "  postorder " + RecursiveTraversals.postorder(rightOnly));
+        t.flush();
+    }
+
+    @Test
+    void inorderOfABinarySearchTreeIsSorted() {
+        TreeNode bst = Trees.bst(50, 30, 70, 20, 40, 60, 80);
+        List<Integer> inorder = RecursiveTraversals.inorder(bst);
+        assertEquals(List.of(20, 30, 40, 50, 60, 70, 80), inorder);
+        assertEquals(inorder, IterativeTraversals.inorder(bst));
+
+        Transcript t = Transcript.start("11-bst-inorder-sorted.txt", "Inorder of a binary search tree is sorted");
+        t.line("inserted in this order : [50, 30, 70, 20, 40, 60, 80]");
+        t.line("tree                   : " + Trees.bracket(bst));
+        t.line("preorder               : " + RecursiveTraversals.preorder(bst));
+        t.line("inorder                : " + inorder);
+        t.line("postorder              : " + RecursiveTraversals.postorder(bst));
+        t.flush();
+    }
+}
@@ -0,0 +1,61 @@
+package com.ankurm.interviewlab.bintree;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.List;
+import java.util.concurrent.atomic.AtomicReference;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Recursion overflows on a deep skewed tree; the iterative versions do not. The thread is given a
+ * deliberately small 256 KB stack so the outcome does not depend on the machine's default -Xss.
+ */
+class DeepTreeTest {
+
+    private static final int DEPTH = 200_000;
+
+    @Test
+    void recursiveInorderOverflowsButIterativeSurvives() throws Exception {
+        TreeNode root = Trees.leftSkewed(DEPTH);
+        AtomicReference<Throwable> recursiveOutcome = new AtomicReference<>();
+        AtomicReference<List<Integer>> iterative = new AtomicReference<>();
+        AtomicReference<List<Integer>> iterativePost = new AtomicReference<>();
+        AtomicReference<List<List<Integer>>> levels = new AtomicReference<>();
+
+        Thread worker = new Thread(null, () -> {
+            try {
+                RecursiveTraversals.inorder(root);
+            } catch (Throwable t) {
+                recursiveOutcome.set(t);
+            }
+            iterative.set(IterativeTraversals.inorder(root));
+            iterativePost.set(IterativeTraversals.postorderOneStack(root));
+            levels.set(LevelOrder.byLevel(root));
+        }, "small-stack", 256 * 1024);
+        worker.start();
+        worker.join();
+
+        assertEquals(StackOverflowError.class, recursiveOutcome.get().getClass());
+        assertEquals(DEPTH, iterative.get().size());
+        assertEquals(DEPTH, iterativePost.get().size());
+        assertEquals(DEPTH, levels.get().size(), "a skewed tree has one node per level");
+        assertEquals(DEPTH, TreeMetrics.heightIterative(root));
+    }
+
+    @Test
+    void buildingAndMorrisNeedNoRecursion() {
+        TreeNode root = Trees.leftSkewed(DEPTH);
+        assertDoesNotThrow(() -> assertEquals(DEPTH, IterativeTraversals.morrisInorder(root).size()));
+        assertThrows(StackOverflowError.class, () -> {
+            Thread t = new Thread(null, () -> TreeMetrics.height(Trees.leftSkewed(DEPTH)), "h", 256 * 1024);
+            AtomicReference<Throwable> ex = new AtomicReference<>();
+            t.setUncaughtExceptionHandler((th, e) -> ex.set(e));
+            t.start();
+            t.join();
+            throw ex.get();
+        });
+    }
+}
@@ -0,0 +1,47 @@
+package com.ankurm.interviewlab.bintree;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+import com.ankurm.interviewlab.bintree.support.Transcript;
+
+/** Output 09: counted (not timed) node visits, naive O(n^2) diameter versus the one-pass version. */
+class DiameterVisitCountTest {
+
+    @Test
+    void naiveVisitsGrowQuadraticallyOnSkewedTreesAndOnePassStaysLinear() {
+        Transcript t = Transcript.start("09-diameter-visit-counts.txt",
+                "Diameter: node visits, naive vs one pass (counted, not timed)");
+        t.line(String.format("%-22s %8s %14s %12s %10s", "tree", "nodes", "naive visits", "one-pass", "diameter"));
+
+        TreeNode[] trees = {
+            Trees.sample(), Trees.perfect(4), Trees.perfect(7), Trees.perfect(10),
+            Trees.leftSkewed(100), Trees.leftSkewed(1000), Trees.leftSkewed(2000)
+        };
+        String[] names = {
+            "sample (9)", "perfect, 4 levels", "perfect, 7 levels", "perfect, 10 levels",
+            "left-skewed 100", "left-skewed 1000", "left-skewed 2000"
+        };
+        for (int i = 0; i < trees.length; i++) {
+            long[] naive = {0};
+            long[] onePass = {0};
+            int n = TreeMetrics.countNodes(trees[i]);
+            int a = TreeMetrics.diameterNaive(trees[i], naive);
+            int b = TreeMetrics.diameterOnePass(trees[i], onePass);
+            assertEquals(a, b);
+            assertEquals(n, onePass[0], "one pass touches each node exactly once");
+            assertTrue(naive[0] >= onePass[0]);
+            t.line(String.format("%-22s %8d %14d %12d %10d", names[i], n, naive[0], onePass[0], b));
+        }
+
+        // On a skewed tree the naive version visits exactly n(n+1)/2 nodes: the sum 1 + 2 + ... + n.
+        for (int n : new int[] {100, 1000, 2000}) {
+            long[] naive = {0};
+            TreeMetrics.diameterNaive(Trees.leftSkewed(n), naive);
+            assertEquals((long) n * (n + 1) / 2, naive[0]);
+        }
+        t.flush();
+    }
+}
@@ -0,0 +1,147 @@
+package com.ankurm.interviewlab.bintree;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Deque;
+import java.util.IdentityHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Random;
+
+import org.junit.jupiter.api.Test;
+
+/** All implementations must agree with each other on many random trees. */
+class RandomTreesEquivalenceTest {
+
+    @Test
+    void allDepthFirstImplementationsAgreeOnRandomTrees() {
+        Random random = new Random(20261106L);
+        for (int round = 0; round < 1000; round++) {
+            int n = random.nextInt(80);
+            TreeNode root = Trees.random(random, n);
+            String shape = Trees.bracket(root);
+
+            List<Integer> in = RecursiveTraversals.inorder(root);
+            List<Integer> pre = RecursiveTraversals.preorder(root);
+            List<Integer> post = RecursiveTraversals.postorder(root);
+            assertEquals(n, in.size());
+
+            assertEquals(in, IterativeTraversals.inorder(root), shape);
+            assertEquals(pre, IterativeTraversals.preorder(root), shape);
+            assertEquals(post, IterativeTraversals.postorderTwoStacks(root), shape);
+            assertEquals(post, IterativeTraversals.postorderOneStack(root), shape);
+            assertEquals(in, IterativeTraversals.morrisInorder(root), shape);
+            assertEquals(shape, Trees.bracket(root), "Morris inorder must restore the tree");
+            assertEquals(pre, IterativeTraversals.morrisPreorder(root), shape);
+            assertEquals(shape, Trees.bracket(root), "Morris preorder must restore the tree");
+
+            // construction round trips
+            TreeNode a = Trees.fromInorderPreorder(Trees.toArray(in), Trees.toArray(pre), null);
+            TreeNode b = Trees.fromInorderPostorder(Trees.toArray(in), Trees.toArray(post));
+            assertEquals(shape, Trees.bracket(a));
+            assertEquals(shape, Trees.bracket(b));
+        }
+    }
+
+    @Test
+    void levelOrderFamilyAndMetricsAgreeOnRandomTrees() {
+        Random random = new Random(1106L);
+        for (int round = 0; round < 1000; round++) {
+            int n = random.nextInt(80);
+            TreeNode root = Trees.random(random, n);
+            String shape = Trees.bracket(root);
+
+            List<List<Integer>> levels = LevelOrder.byLevel(root);
+            List<Integer> flat = new ArrayList<>();
+            levels.forEach(flat::addAll);
+            assertEquals(LevelOrder.levelOrder(root), flat, shape);
+            assertEquals(levels.size(), TreeMetrics.height(root), shape);
+            assertEquals(TreeMetrics.height(root), TreeMetrics.heightIterative(root), shape);
+
+            List<List<Integer>> zig = LevelOrder.zigzag(root);
+            assertEquals(levels.size(), zig.size());
+            for (int i = 0; i < levels.size(); i++) {
+                List<Integer> expected = new ArrayList<>(levels.get(i));
+                if (i % 2 == 1) {
+                    java.util.Collections.reverse(expected);
+                }
+                assertEquals(expected, zig.get(i), shape);
+            }
+
+            assertEquals(n, TreeMetrics.countNodes(root));
+            int leaves = 0;
+            for (TreeNode node : allNodes(root)) {
+                if (node.left == null && node.right == null) {
+                    leaves++;
+                }
+            }
+            assertEquals(leaves, TreeMetrics.countLeaves(root), shape);
+
+            long[] naive = {0};
+            long[] onePass = {0};
+            int expectedDiameter = bruteForceDiameterInNodes(root);
+            assertEquals(expectedDiameter, TreeMetrics.diameterNaive(root, naive), shape);
+            assertEquals(expectedDiameter, TreeMetrics.diameterOnePass(root, onePass), shape);
+            assertEquals(n, onePass[0]);
+            assertEquals(expectedDiameter, TreeMetrics.diameterPath(root).size(), shape);
+        }
+    }
+
+    // ---- independent oracle: double BFS on the tree seen as an undirected graph ----
+
+    private static List<TreeNode> allNodes(TreeNode root) {
+        List<TreeNode> nodes = new ArrayList<>();
+        Deque<TreeNode> stack = new ArrayDeque<>();
+        if (root != null) {
+            stack.push(root);
+        }
+        while (!stack.isEmpty()) {
+            TreeNode node = stack.pop();
+            nodes.add(node);
+            if (node.left != null) {
+                stack.push(node.left);
+            }
+            if (node.right != null) {
+                stack.push(node.right);
+            }
+        }
+        return nodes;
+    }
+
+    private static int bruteForceDiameterInNodes(TreeNode root) {
+        if (root == null) {
+            return 0;
+        }
+        Map<TreeNode, List<TreeNode>> graph = new IdentityHashMap<>();
+        for (TreeNode node : allNodes(root)) {
+            graph.computeIfAbsent(node, k -> new ArrayList<>());
+            for (TreeNode child : new TreeNode[] {node.left, node.right}) {
+                if (child != null) {
+                    graph.computeIfAbsent(child, k -> new ArrayList<>()).add(node);
+                    graph.get(node).add(child);
+                }
+            }
+        }
+        // longest shortest-path over ALL pairs: BFS from every node (fine for n < 80)
+        int best = 0;
+        for (TreeNode start : graph.keySet()) {
+            Map<TreeNode, Integer> dist = new IdentityHashMap<>();
+            Deque<TreeNode> queue = new ArrayDeque<>();
+            queue.add(start);
+            dist.put(start, 1);
+            while (!queue.isEmpty()) {
+                TreeNode cur = queue.poll();
+                best = Math.max(best, dist.get(cur));
+                for (TreeNode next : graph.get(cur)) {
+                    if (!dist.containsKey(next)) {
+                        dist.put(next, dist.get(cur) + 1);
+                        queue.add(next);
+                    }
+                }
+            }
+        }
+        return best;
+    }
+}
@@ -0,0 +1,106 @@
+package com.ankurm.interviewlab.bintree;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import com.ankurm.interviewlab.bintree.support.Transcript;
+
+/** Output 01 and 08: every traversal of the sample tree, with the numbers the diagrams use. */
+class SampleTreeTranscriptTest {
+
+    static final List<Integer> INORDER = List.of(4, 2, 7, 5, 1, 3, 8, 6, 9);
+    static final List<Integer> PREORDER = List.of(1, 2, 4, 5, 7, 3, 6, 8, 9);
+    static final List<Integer> POSTORDER = List.of(4, 7, 5, 2, 8, 9, 6, 3, 1);
+    static final List<Integer> LEVEL = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9);
+
+    @Test
+    void allTraversalsOfTheSampleTree() {
+        TreeNode root = Trees.sample();
+        String before = Trees.bracket(root);
+
+        Transcript t = Transcript.start("01-sample-tree-all-traversals.txt",
+                "Sample tree: every traversal, recursive and iterative");
+        t.line("tree (value(left,right), - = no child): " + before).blank();
+
+        assertEquals(INORDER, RecursiveTraversals.inorder(root));
+        assertEquals(PREORDER, RecursiveTraversals.preorder(root));
+        assertEquals(POSTORDER, RecursiveTraversals.postorder(root));
+        t.line("recursive inorder        : " + RecursiveTraversals.inorder(root));
+        t.line("recursive preorder       : " + RecursiveTraversals.preorder(root));
+        t.line("recursive postorder      : " + RecursiveTraversals.postorder(root)).blank();
+
+        assertEquals(INORDER, IterativeTraversals.inorder(root));
+        assertEquals(PREORDER, IterativeTraversals.preorder(root));
+        assertEquals(POSTORDER, IterativeTraversals.postorderTwoStacks(root));
+        assertEquals(POSTORDER, IterativeTraversals.postorderOneStack(root));
+        t.line("iterative inorder        : " + IterativeTraversals.inorder(root));
+        t.line("iterative preorder       : " + IterativeTraversals.preorder(root));
+        t.line("iterative postorder (2 stacks): " + IterativeTraversals.postorderTwoStacks(root));
+        t.line("iterative postorder (1 stack) : " + IterativeTraversals.postorderOneStack(root)).blank();
+
+        assertEquals(INORDER, IterativeTraversals.morrisInorder(root));
+        assertEquals(PREORDER, IterativeTraversals.morrisPreorder(root));
+        assertEquals(before, Trees.bracket(root), "Morris must restore every right pointer");
+        t.line("Morris inorder           : " + IterativeTraversals.morrisInorder(root));
+        t.line("Morris preorder          : " + IterativeTraversals.morrisPreorder(root));
+        t.line("tree after Morris        : " + Trees.bracket(root) + "  (unchanged)").blank();
+
+        assertEquals(LEVEL, LevelOrder.levelOrder(root));
+        assertEquals(List.of(List.of(1), List.of(2, 3), List.of(4, 5, 6), List.of(7, 8, 9)), LevelOrder.byLevel(root));
+        assertEquals(List.of(List.of(1), List.of(3, 2), List.of(4, 5, 6), List.of(9, 8, 7)), LevelOrder.zigzag(root));
+        t.line("level order (queue)       : " + LevelOrder.levelOrder(root));
+        t.line("level order by level      : " + LevelOrder.byLevel(root));
+        t.line("zigzag                    : " + LevelOrder.zigzag(root));
+        t.flush();
+    }
+
+    @Test
+    void heightAndDiameterOfTheSampleTree() {
+        TreeNode root = Trees.sample();
+        long[] naive = {0};
+        long[] onePass = {0};
+        int diameterNaive = TreeMetrics.diameterNaive(root, naive);
+        int diameterOnePass = TreeMetrics.diameterOnePass(root, onePass);
+        List<Integer> path = TreeMetrics.diameterPath(root);
+
+        assertEquals(9, TreeMetrics.countNodes(root));
+        assertEquals(4, TreeMetrics.countLeaves(root));
+        assertEquals(4, TreeMetrics.height(root));
+        assertEquals(4, TreeMetrics.heightIterative(root));
+        assertEquals(7, diameterNaive);
+        assertEquals(7, diameterOnePass);
+        assertEquals(9, onePass[0], "one pass visits each node exactly once");
+        assertTrue(naive[0] > onePass[0]);
+        assertEquals(List.of(7, 5, 2, 1, 3, 6, 8), path);
+
+        Transcript t = Transcript.start("08-height-diameter-sample.txt", "Sample tree: size, height and diameter");
+        t.line("nodes                         : " + TreeMetrics.countNodes(root));
+        t.line("leaves                        : " + TreeMetrics.countLeaves(root));
+        t.line("height, counted in nodes      : " + TreeMetrics.height(root));
+        t.line("height, counted in edges      : " + (TreeMetrics.height(root) - 1));
+        t.line("height via level order (BFS)  : " + TreeMetrics.heightIterative(root));
+        t.line("diameter, counted in nodes    : " + diameterOnePass);
+        t.line("diameter, counted in edges    : " + (diameterOnePass - 1));
+        t.line("one longest path              : " + path);
+        java.util.TreeMap<Integer, Integer> heights = new java.util.TreeMap<>();
+        collectHeights(root, heights);
+        assertEquals("{1=4, 2=3, 3=3, 4=1, 5=2, 6=2, 7=1, 8=1, 9=1}", heights.toString());
+        t.line("height of the subtree at node : " + heights);
+        t.line("node visits, naive O(n^2) way : " + naive[0]);
+        t.line("node visits, one-pass way     : " + onePass[0]);
+        t.flush();
+    }
+
+    private static void collectHeights(TreeNode node, java.util.Map<Integer, Integer> into) {
+        if (node == null) {
+            return;
+        }
+        into.put(node.val, TreeMetrics.height(node));
+        collectHeights(node.left, into);
+        collectHeights(node.right, into);
+    }
+}
@@ -0,0 +1,78 @@
+package com.ankurm.interviewlab.bintree;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+import com.ankurm.interviewlab.bintree.support.StepTable;
+import com.ankurm.interviewlab.bintree.support.Transcript;
+
+/** Outputs 02-07: the step-by-step stack, queue and thread tables for the sample tree. */
+class StepTraceTranscriptTest {
+
+    private static final String STACK = "stack (bottom..top)";
+
+    @Test
+    void inorderSteps() {
+        StepTable table = new StepTable(STACK, null);
+        assertEquals(SampleTreeTranscriptTest.INORDER, IterativeTraversals.inorder(Trees.sample(), table));
+        assertEquals(18, table.size(), "9 pushes + 9 pops");
+        Transcript t = Transcript.start("02-inorder-iterative-steps.txt", "Iterative inorder on the sample tree, step by step");
+        table.writeTo(t);
+        t.flush();
+    }
+
+    @Test
+    void preorderSteps() {
+        StepTable table = new StepTable(STACK, null);
+        assertEquals(SampleTreeTranscriptTest.PREORDER, IterativeTraversals.preorder(Trees.sample(), table));
+        assertEquals(9, table.size());
+        Transcript t = Transcript.start("03-preorder-iterative-steps.txt", "Iterative preorder on the sample tree, step by step");
+        table.writeTo(t);
+        t.flush();
+    }
+
+    @Test
+    void postorderTwoStacksSteps() {
+        StepTable table = new StepTable("s1 (bottom..top)", "s2 (bottom..top)");
+        assertEquals(SampleTreeTranscriptTest.POSTORDER, IterativeTraversals.postorderTwoStacks(Trees.sample(), table));
+        assertEquals(18, table.size());
+        Transcript t = Transcript.start("04-postorder-two-stacks-steps.txt", "Iterative postorder with two stacks, step by step");
+        table.writeTo(t);
+        t.flush();
+    }
+
+    @Test
+    void postorderOneStackSteps() {
+        StepTable table = new StepTable(STACK, null);
+        assertEquals(SampleTreeTranscriptTest.POSTORDER, IterativeTraversals.postorderOneStack(Trees.sample(), table));
+        Transcript t = Transcript.start("05-postorder-one-stack-steps.txt", "Iterative postorder with one stack, step by step");
+        table.writeTo(t);
+        t.flush();
+    }
+
+    @Test
+    void levelOrderSteps() {
+        StepTable table = new StepTable("queue (front..back)", null);
+        assertEquals(SampleTreeTranscriptTest.LEVEL, LevelOrder.levelOrder(Trees.sample(), table));
+        assertEquals(9, table.size());
+        Transcript t = Transcript.start("06-level-order-queue-steps.txt", "Level order with a queue, step by step");
+        table.writeTo(t);
+        t.flush();
+    }
+
+    @Test
+    void morrisSteps() {
+        TreeNode root = Trees.sample();
+        String before = Trees.bracket(root);
+        StepTable table = new StepTable("threads (from node)", null);
+        assertEquals(SampleTreeTranscriptTest.INORDER, IterativeTraversals.morrisInorder(root, table));
+        assertEquals(before, Trees.bracket(root));
+        Transcript t = Transcript.start("07-morris-inorder-steps.txt", "Morris inorder on the sample tree, step by step");
+        table.writeTo(t);
+        t.line("");
+        t.line("tree before: " + before);
+        t.line("tree after : " + Trees.bracket(root));
+        t.flush();
+    }
+}
@@ -0,0 +1,51 @@
+package com.ankurm.interviewlab.bintree.support;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+
+import com.ankurm.interviewlab.bintree.StepListener;
+
+/** Collects the steps reported by a traversal and renders them as a fixed-width text table. */
+public final class StepTable implements StepListener {
+    public record Row(int step, String action, String a, String b, String out) {}
+
+    private final List<Row> rows = new ArrayList<>();
+    private final String headerA;
+    private final String headerB; // null for single-container traversals
+
+    public StepTable(String headerA, String headerB) {
+        this.headerA = headerA;
+        this.headerB = headerB;
+    }
+
+    @Override
+    public void step(String action, List<Integer> a, List<Integer> b, List<Integer> output) {
+        rows.add(new Row(rows.size() + 1, action, format(a), format(b), format(output)));
+    }
+
+    public int size() {
+        return rows.size();
+    }
+
+    public List<Row> rows() {
+        return rows;
+    }
+
+    public static String format(List<Integer> values) {
+        return values.stream().map(String::valueOf).collect(Collectors.joining(" ", "[", "]"));
+    }
+
+    public void writeTo(Transcript t) {
+        int wa = Math.max(headerA.length(), rows.stream().mapToInt(r -> r.action().length()).max().orElse(0));
+        int wb = Math.max(headerA.length(), rows.stream().mapToInt(r -> r.a().length()).max().orElse(0));
+        int wc = headerB == null ? 0
+                : Math.max(headerB.length(), rows.stream().mapToInt(r -> r.b().length()).max().orElse(0));
+        t.line(String.format("%-4s | %-" + wa + "s | %-" + wb + "s |%s %s", "step", "action", headerA,
+                headerB == null ? "" : String.format(" %-" + wc + "s |", headerB), "output so far"));
+        for (Row r : rows) {
+            t.line(String.format("%-4d | %-" + wa + "s | %-" + wb + "s |%s %s", r.step(), r.action(), r.a(),
+                    headerB == null ? "" : String.format(" %-" + wc + "s |", r.b()), r.out()));
+        }
+    }
+}
@@ -0,0 +1,43 @@
+package com.ankurm.interviewlab.bintree.support;
+
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+
+/** Writes output/NN-*.txt while the calling test asserts the same numbers it prints. */
+public final class Transcript {
+    private final Path path;
+    private final List<String> lines = new ArrayList<>();
+
+    private Transcript(Path path, String header) {
+        this.path = path;
+        lines.add(header);
+        lines.add("=".repeat(Math.min(header.length(), 100)));
+        lines.add("");
+    }
+
+    public static Transcript start(String fileName, String header) {
+        Path dir = Path.of("output");
+        try {
+            Files.createDirectories(dir);
+        } catch (IOException e) {
+            throw new UncheckedIOException(e);
+        }
+        return new Transcript(dir.resolve(fileName), header);
+    }
+
+    public Transcript line(String text) { lines.add(text); return this; }
+    public Transcript blank() { lines.add(""); return this; }
+
+    public void flush() {
+        try {
+            Files.write(path, (String.join("\n", lines) + "\n").getBytes(StandardCharsets.UTF_8));
+        } catch (IOException e) {
+            throw new UncheckedIOException(e);
+        }
+    }
+}