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Claude b9aeb78b24 Add stack-queue module: ten stack and queue interview problems, monotonic stack, min stack, queue from stacks and stack from a queue, brute-force cross-checks, dry-run traces, operation counts, pitfalls (11 tests)

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b6nxynun5JFivebVb1

2026-11-12 22:35:43 +05:30

output

Add stack-queue module: ten stack and queue interview problems, monotonic stack, min stack, queue from stacks and stack from a queue, brute-force cross-checks, dry-run traces, operation counts, pitfalls (11 tests)

2026-11-12 22:35:43 +05:30

scripts

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src

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pom.xml

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README.md

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README.md

stack-queue

Companion code for the ankurm.com article Stack and Queue Problems in Java: Balanced Parentheses, Next Greater Element, Min Stack.

Ten classic interview problems on stacks and queues, each cross-checked against an obviously-correct brute-force version on seeded random inputs. Every figure and console line in the article is quoted from a file in output/, which the JUnit tests write while asserting the same numbers. There is no docs/ folder: the explanations live in the article itself.

Versions

Component	Version	Verified against
JDK	Temurin 25.0.5+8 (LTS)	`java -version`
JUnit	6.1.3	`junit-bom` `maven-metadata.xml` on Maven Central (latest GA)
Maven	3.9.x	`mvn -version`

Quickstart

cd stack-queue
export JAVA_HOME=/path/to/jdk-25 PATH="$JAVA_HOME/bin:$PATH"
./scripts/run-all.sh      # runs 11 tests and regenerates output/*.txt

The first Maven run must be online so the plugins land in the local cache.

The ten problems

#	Problem	Idea	Method
1	Balanced parentheses	stack of expected closers	`Parentheses.isBalanced`
2	Evaluate Reverse Polish Notation	stack of operands	`Rpn.evaluate`
3	Min stack	second stack, or one stack of differences	`MinStack`, `EncodedMinStack`
4	Next greater element	monotonic stack	`MonotonicStack.nextGreater`, `nextGreaterSubset`
5	Daily temperatures	monotonic stack of indices	`MonotonicStack.dailyTemperatures`
6	Next greater element, circular	monotonic stack, two laps	`MonotonicStack.nextGreaterCircular`
7	Largest rectangle in a histogram	monotonic stack with a sentinel	`MonotonicStack.largestRectangle`
8	Decode string	stack of parked text and counts	`DecodeString.decode`
9	Queue using two stacks	pour only when the output stack is empty	`QueueViaStacks`
10	Stack using one queue	rotate after every push	`StackViaQueue`

Source layout

stack-queue/
├── pom.xml
├── scripts/run-all.sh
├── src/main/java/com/ankurm/interviewlab/stackqueue/
│   ├── Parentheses.java, Rpn.java, DecodeString.java     problems 1, 2, 8
│   ├── IntMinStack.java, MinStack.java, EncodedMinStack.java   problem 3
│   ├── MonotonicStack.java                               problems 4-7
│   ├── QueueViaStacks.java, StackViaQueue.java           problems 9, 10
│   ├── BruteForce.java        slow, obviously-correct references
│   ├── DryRuns.java           instrumented copies that record every step
│   ├── Complexity.java        operation counters (counted, not timed)
│   ├── Pitfalls.java          plausible-looking wrong versions
│   └── JdkStackDemo.java      java.util.Stack versus ArrayDeque
├── src/test/java/.../         SolutionsTest, DryRunTest, RandomAgreementTest, ComplexityTest, PitfallsTest, JdkStackTest
└── output/*.txt               captured real output