DSA Practice (Java)

A single-source-of-truth repo for Data Structures & Algorithms (DSA) practice — solved famous problems, organized by concept, with clear templates, tests, and notes. Made for learning, interviewing, and flexing algorithmic muscles. 💪

---

📌 Project Overview

This repository is a continuous, evolving collection of DSA problems solved in Java. Each problem includes:

• A clear problem statement or link to the original problem (LeetCode / GFG / Codeforces / etc.)
• A clean, well-commented Java implementation
• Time & space complexity analysis
• Edge cases and short explanation
• (Optional) multiple approaches when useful

Goal: cover every core DSA concept through curated, famous problems so you can revise quickly and prepare for interviews.

---

📁 Repository Structure (recommended)

DSA-Practice-Java/
├── README.md
├── CONTRIBUTING.md
├── LICENSE
├── build/                 # compiled classes (gitignored)
├── src/
│   ├── array/
│   │   ├── TwoSum.java
│   │   └── ...
│   ├── linkedlist/
│   ├── stack/
│   ├── queue/
│   ├── tree/
│   ├── graph/
│   ├── dp/
│   ├── greedy/
│   ├── math/
│   └── utils/             # helper classes (IO, Pair, ListNode, TreeNode)
├── tests/                 # simple test runners or JUnit tests
└── notes/                 # short writeups, diagrams, complexity tables

Naming convention: ProblemName.java (use CamelCase). If multiple solutions exist, suffix with Solution1, Solution2 (e.g. KnapsackDP.java, KnapsackGreedy.java).

---

🚀 How to run (simple)

Using javac and java (no build tool):

1. Compile:

javac -d build src/<concept>/ProblemName.java

2. Run:

java -cp build <package?>ProblemName

(If you use packages, compile from repo root and run with the package-qualified class name.)

Using Maven or Gradle (recommended for bigger test suites):

• Add a pom.xml or build.gradle and place sources under src/main/java and tests under src/test/java.

---

🧭 Problem File Template

When adding a new problem, use this header at the top of the Java file (commented):

/*
Problem: Two Sum
Platform: LeetCode #1
Difficulty: Easy
Link: https://leetcode.com/problems/two-sum/
Approach: HashMap single pass
Time: O(n)
Space: O(n)
Notes: Return indices of numbers that add up to target.
*/

public class TwoSum {
    // implementation...
}

This keeps things consistent and searchable.

---

✅ Concepts & Example Problems (Roadmap)

Each bullet is a folder in src/ and should contain a mix of easy → hard problems.

• Arrays & Strings: Two Sum, 3-Sum, Subarray Sum, Sliding Window problems
• Linked List: Reverse Linked List, Cycle Detection, Merge K Lists
• Stack & Queue: Valid Parentheses, Min Stack, Sliding Window Max
• Trees & BSTs: Inorder/Preorder/Postorder, LCA, Serialize/Deserialize
• Graphs: BFS/DFS, Shortest Path (Dijkstra), Topological Sort, SCC
• Dynamic Programming: Knapsack, Longest Increasing Subsequence, Edit Distance
• Greedy: Interval Scheduling, Huffman, Minimum Platforms
• Math & Number Theory: Primes, GCD, Modular Exponentiation
• Backtracking: N-Queens, Permutations, Sudoku Solver
• Bit Manipulation: Single Number, Counting Bits
• Advanced: Segment Trees, Fenwick Trees, Union-Find, Tries

(If a concept is missing — add it. This repo is a living document.)

---

🛠️ Coding Style & Best Practices

• Use descriptive variable names — i is fine for loops, but prefer left, right where applicable.
• Add short comments explaining non-obvious steps.
• Include complexity analysis in the file header.
• Avoid unused imports and dead code.
• If using custom ListNode/TreeNode, keep them in src/utils/.

Sample ListNode (put in src/utils/ListNode.java):

public class ListNode {
    public int val;
    public ListNode next;
    public ListNode(int x) { val = x; }
}

---

🔁 Testing

• Add quick main() based sanity checks inside each problem file for manual testing.
• Prefer to add JUnit tests in tests/ for regression checks.
• Keep test inputs small and focused.

---

🤝 Contributing

Love contributions. Keep it tidy:

1. Fork the repo
2. Create a branch: feature/<concept>-<problem>
3. Add your solution, update notes/ if needed
4. Open a PR with a short description and link to the problem

Checklist for PRs:

• File placed in correct folder
• Header comment with link, difficulty, complexity
• Code compiles and runs
• Tests (if applicable)

---

📚 Resources & References

• LeetCode, GeeksforGeeks, HackerRank, Codeforces
• Introduction to Algorithms — Cormen et al. (for deep theory)
• Competitive Programming — Steven & Felix Halim (for contests)

---

🧾 Roadmap / TODO

• Add JUnit setup and CI (GitHub Actions) for auto test runs
• Add interactive README badges (build, coverage)
• Complete dp/, graph/, and advanced/ folders

---

🧑‍💻 Contact / Author

If something's broken or you want mentoring/feedback on solutions, open an issue or PR.

---

Keep solving. Keep explaining. Code like someone else will read it. (Because they will.) 🧠🔥