🧩 DSA-Decoded

Decode Data Structures & Algorithms — one pattern at a time.

A structured, pattern-based roadmap to crack the DSA round at FAANG and any top tech company — from Easy → Hard. Every solution is clean, commented, and tagged with its complexity.

⭐ Star this repo if it helps your prep — it keeps you accountable and helps others find it!

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🎯 Why this repo?

Most people "grind" 300 random problems and still freeze in interviews. Patterns beat problems. Once you recognize the shape of a problem, the solution writes itself.

This repo is built around 18 core patterns (the NeetCode-style roadmap). Follow the order, and you'll cover ~90% of what interviewers actually ask.

• ✅ Pattern-first — learn the technique, not just the answer
• ✅ Easy → Hard progression within each topic
• ✅ Every solution has intuition + time/space complexity
• ✅ Consistent, readable Java style
• ✅ A progress tracker so you can mark what's done

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🗺️ The Roadmap

Follow the patterns top to bottom. Each builds on the last.

#	Pattern	Folder	Why it matters
01	Arrays & Hashing	01-arrays-and-hashing	The foundation — HashMap/HashSet tricks
02	Two Pointers	02-two-pointers	Sorted arrays, palindromes, pair-finding
03	Sliding Window	03-sliding-window	Subarrays/substrings in O(n)
04	Stack	04-stack	Matching, monotonic stacks
05	Binary Search	05-binary-search	Search space reduction
06	Linked List	06-linked-list	Pointers, cycles, reversal
07	Trees	07-trees	DFS/BFS, BST properties
08	Tries	08-tries	Prefix trees, autocomplete
09	Heap / Priority Queue	09-heap-priority-queue	Top-K, streaming medians
10	Backtracking	10-backtracking	Subsets, permutations, combos
11	Graphs	11-graphs	Islands, BFS/DFS, topo sort
12	Advanced Graphs	12-advanced-graphs	Dijkstra, Union-Find, MST
13	1-D Dynamic Programming	13-1d-dynamic-programming	The classic DP intro
14	2-D Dynamic Programming	14-2d-dynamic-programming	Grids, strings, knapsack
15	Greedy	15-greedy	Local → global optimum
16	Intervals	16-intervals	Merge, overlap, scheduling
17	Math & Geometry	17-math-and-geometry	Rotations, number theory
18	Bit Manipulation	18-bit-manipulation	XOR tricks, bit counting

👉 Full problem checklist with difficulty tags: ROADMAP.md

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📁 Repository Structure

DSA-Decoded/
├── README.md                  ← you are here
├── ROADMAP.md                 ← full 150+ problem checklist
├── SOLUTION_TEMPLATE.java      ← copy this for every new problem
├── CONTRIBUTING.md
├── LICENSE
└── patterns/
    ├── 01-arrays-and-hashing/
    │   ├── README.md           ← pattern intuition + problem list
    │   └── TwoSum.java         ← example solution
    ├── 02-two-pointers/
    │   └── ...
    └── ... (18 patterns)

Each solution file is self-contained and runnable (public class with a header comment carrying problem link, difficulty, and complexity).

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🚀 How to use this repo

1. Pick a pattern from the roadmap (start at 01).
2. Read the pattern's README.md to understand the technique.
3. Attempt the problem yourself first (15–30 min).
4. Compare with the solution here, then re-solve from scratch.
5. Mark it done in the progress tracker.
6. Repeat. Consistency > intensity.

▶️ Running a solution

# Compile and run any single file (Java 11+ supports this directly)
java patterns/01-arrays-and-hashing/TwoSum.java

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📊 Progress Tracker

Tip: copy this into your own fork and check boxes as you go.

Pattern	Easy	Medium	Hard	Status
Arrays & Hashing	⬜	⬜	⬜	🔜
Two Pointers	⬜	⬜	⬜	🔜
Sliding Window	⬜	⬜	⬜	🔜
Stack	⬜	⬜	⬜	🔜
Binary Search	⬜	⬜	⬜	🔜
Linked List	⬜	⬜	⬜	🔜
Trees	⬜	⬜	⬜	🔜
Tries	⬜	⬜	⬜	🔜
Heap / PQ	⬜	⬜	⬜	🔜
Backtracking	⬜	⬜	⬜	🔜
Graphs	⬜	⬜	⬜	🔜
Advanced Graphs	⬜	⬜	⬜	🔜
1-D DP	⬜	⬜	⬜	🔜
2-D DP	⬜	⬜	⬜	🔜
Greedy	⬜	⬜	⬜	🔜
Intervals	⬜	⬜	⬜	🔜
Math & Geometry	⬜	⬜	⬜	🔜
Bit Manipulation	⬜	⬜	⬜	🔜

Legend: ✅ done · 🟡 in progress · 🔜 not started

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🧠 Complexity Cheat Sheet

Big-O	Name	Example
O(1)	Constant	HashMap lookup
O(log n)	Logarithmic	Binary search
O(n)	Linear	Single loop
O(n log n)	Linearithmic	Sorting
O(n²)	Quadratic	Nested loops
O(2ⁿ)	Exponential	Subsets / brute backtracking

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🤝 Contributing

Found a cleaner solution or a typo? PRs welcome! See CONTRIBUTING.md.

📜 License

MIT — free to use, learn, and share.

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If this repo helped you, drop a ⭐ and share it with someone grinding for interviews.

Happy decoding! 🚀