# 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

```bash
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
```