LeetCode DSA Java Playbook

Learn the pattern. Understand the invariant. Build the solution.

This repository is a pattern-first learning playbook for Data Structures and Algorithms practice using Java. It is not meant to be a collection of copied LeetCode answers. Each note is written to explain how to recognize the problem, choose the right pattern, reason about the invariant, and implement the solution independently.

Repository description: Pattern-first LeetCode solutions in Java, with DSA explanations, invariants, complexity analysis, mistakes, and revision notes.

Philosophy

The goal is to learn reusable problem-solving patterns before moving into broader LeetCode practice. A solution is useful only when the reasoning behind it is clear enough to repeat later without memorizing the exact code.

The learning workflow is:

Clues -> Pattern -> Invariant -> Implementation

For each problem, the focus is to:

• Read the problem carefully.
• Identify important clues.
• Recognize the applicable DSA pattern.
• Explain why the pattern applies.
• State the invariant or condition that remains true.
• Develop the brute-force approach.
• Implement the optimized Java solution.
• Analyze time and space complexity.
• Record mistakes and revision insights.

Current Status

• Completed pattern: Hashing
• Problem index: LeetCode problems
• Full learning plan: ROADMAP.md

Repository Structure

.
├── patterns/              # Pattern notes and revision guides
├── problems/leetcode/     # LeetCode problem notes and Java solutions
├── docs/                  # Minimal note templates and supporting docs
├── ROADMAP.md             # Planned learning order
└── README.md

Naming Conventions

• Use lowercase kebab-case for directories.
• Use two-digit numeric prefixes for pattern folders, such as 01-hashing.
• Use four-digit LeetCode problem numbers.
• Use the problem folder format 0001-two-sum.
• Each problem folder contains README.md and Solution.java.
• Java solutions use class Solution for LeetCode compatibility.
• Keep one canonical solution folder per problem.
• Pattern notes link to problems instead of duplicating full problem explanations.

Problem Notes

Each problem note follows a consistent structure:

• Important clues
• Brute-force approach
• Pattern decision
• Core invariant
• Step-by-step reasoning
• Edge cases
• Mistakes and lessons
• Final complexity
• Revision prompts

Use docs/problem-template.md when adding a new LeetCode problem.

Contributing

Corrections, clearer explanations, alternative approaches, missing edge cases, and better revision prompts are welcome. Please keep contributions simple, readable, and focused on understanding the pattern rather than collecting many variations of the same solution.

Disclaimer

This project is unofficial and is not affiliated with, endorsed by, or sponsored by LeetCode.

License

This repository is licensed under the MIT License.