# sliding-window

Companion code for the ankurm.com article **Sliding Window and Two Pointers in Java: 12 Interview Problems Solved**.

Twelve classic interview problems solved with the two patterns, 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 sliding-window
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 twelve problems

| # | Problem | Pattern | Method |
|---|---|---|---|
| 1 | Maximum sum of k consecutive elements | fixed window | `FixedWindow.maxSumSubarray` |
| 2 | Permutation in string | fixed window + counts | `FixedWindow.permutationInString` |
| 3 | Sliding window maximum | fixed window + monotonic deque | `FixedWindow.slidingWindowMaximum` |
| 4 | Longest substring without repeating characters | variable window | `VariableWindow.longestUniqueSubstring` |
| 5 | Longest substring with at most K distinct characters | variable window | `VariableWindow.longestKDistinct` |
| 6 | Longest repeating character replacement | variable window | `VariableWindow.characterReplacement` |
| 7 | Max consecutive ones III | variable window | `VariableWindow.longestOnes` |
| 8 | Minimum size subarray sum | shrinking window | `VariableWindow.minSubArrayLen` |
| 9 | Minimum window substring | shrinking window | `VariableWindow.minWindow` |
| 10 | Two Sum II (sorted input) | opposite-end pointers | `TwoPointers.twoSumSorted` |
| 11 | 3Sum | sort + pointers | `TwoPointers.threeSum` |
| 12 | Container with most water | opposite-end pointers | `TwoPointers.maxArea` |

## Source layout

```
sliding-window/
├── pom.xml
├── scripts/run-all.sh
├── src/main/java/com/ankurm/interviewlab/slidingwindow/
│   ├── FixedWindow.java       problems 1-3
│   ├── VariableWindow.java    problems 4-9
│   ├── TwoPointers.java       problems 10-12
│   ├── BruteForce.java        slow, obviously-correct references
│   ├── DryRuns.java           instrumented copies that record every step
│   ├── Complexity.java        operation counters (counted, not timed)
│   └── Pitfalls.java          four plausible-looking wrong versions
├── src/test/java/.../         SolutionsTest, RandomAgreementTest, DryRunTest, ComplexityTest, PitfallsTest
└── output/*.txt               captured real output
```

## Captured output (`output/`)

| File | What it shows |
|---|---|
| `01-fixed-window-trace.txt` | max-sum window sliding over `[2,1,5,1,3,2]` |
| `02-longest-unique-trace.txt` | left pointer jumps on `abcabcbb` and the `abba` case |
| `03-min-window-trace.txt` | grow until valid, shrink until invalid on `ADOBECODEBANC` |
| `04-sliding-max-deque-trace.txt` | the monotonic deque contents at every step |
| `05-two-pointers-trace.txt` | Two Sum II and Container With Most Water |
| `06-all-twelve-results.txt` | one example per problem beside the brute-force answer |
| `07-random-agreement.txt` | 24,000 seeded comparisons, 0 disagreements |
| `08-operation-counts.txt` | brute force vs window operation counts at n = 1k..8k |
| `09-pitfalls.txt` | four classic mistakes, each reproduced |