A beginner-friendly repository for learning Data Structures and Algorithms in Java through problem-solving patterns.
Most beginners memorize solutions, but still get stuck on new problems.
This repo helps you understand the pattern behind a question, dry run the logic, analyze complexity, avoid mistakes, and practice step by step.
Memorizing solutions ❌
Identifying patterns ✅
Writing clean Java code ✅
Solving with confidence ✅Most DSA repos show:
Problem → Code → OutputThis repo follows:
Concept → Pattern → Example → Dry Run → Complexity → Mistakes → PracticeThe goal is simple:
Learn the pattern behind the problem, not just the final code.
We are starting with Arrays, because arrays are the foundation of many important DSA patterns.
arrays/
├── 00-what-is-array/
├── 01-how-to-approach-array-problems/
├── 02-pattern-01-traversal/
└── 03-pattern-02-linear-search/| Day | Topic | Status |
|---|---|---|
| 1 | What is Array | ✅ Live |
| 2 | How to Approach Array Problems | ✅ Live |
| 3 | Pattern 01: Traversal | ✅ Live |
| 4 | Pattern 02: Linear Search | ✅ Live |
| Day | Topic | Status |
|---|---|---|
| 1 | What is Array | ✅ Live |
| 2 | How to Approach Array Problems | ✅ Live |
| 3 | Pattern 01: Traversal | ✅ Live |
| 4 | Pattern 02: Linear Search | ✅ Live |
| 5 | Pattern 03: Binary Search | Coming soon |
| 6 | Pattern 04: Two Pointers | Coming soon |
| 7 | Pattern 05: In-place Overwrite | Coming soon |
| 8 | Pattern 06: HashSet / Duplicate Detection | Coming soon |
| 9 | Pattern 07: HashMap Frequency | Coming soon |
| 10 | Pattern 08: Sorting-Based Patterns | Coming soon |
| 11 | Pattern 09: Merge Sorted Arrays | Coming soon |
| 12 | Pattern 10: Fixed Size Sliding Window | Coming soon |
| 13 | Pattern 11: Variable Size Sliding Window | Coming soon |
| 14 | Pattern 12: Prefix Sum | Coming soon |
| 15 | Pattern 13: Prefix Sum + HashMap | Coming soon |
| 16 | Pattern 14: Kadane’s Algorithm | Coming soon |
| 17 | Pattern 15: Stock Profit Pattern | Coming soon |
| 18 | Pattern 16: XOR and Math Formula | Coming soon |
| 19 | Pattern 17: Rotate, Dutch Flag, Product Except Self, Difference Array | Coming soon |
| 20 | Pattern 18: Matrix, Spiral Matrix, Final Checklist | Coming soon |
| Skill | Why It Matters |
|---|---|
| Pattern recognition | Know which approach to try |
| Dry run thinking | Understand how code works |
| Complexity analysis | Know if your solution can pass |
| Java implementation | Write clean beginner-friendly code |
| Mistake handling | Avoid common DSA errors |
| Practice flow | Learn in the right order |
Read concept
↓
Understand example
↓
Dry run manually
↓
Check complexity
↓
Read mistakes
↓
Solve practice questionsBefore solving, ask:
What is the pattern?
When should I use it?
Can brute force be optimized?
What is the time complexity?
What mistake should I avoid?Why Pattern-Based DSA?
In real problems, the question will not say:
Use HashMap
Use Sliding Window
Use Two Pointers
Use Prefix SumYou need to identify it.
| Question Signal | Pattern to Think |
|---|---|
| Find sum/count/max/min | Traversal |
| Sorted array + target | Binary Search |
| Sorted array + pair sum | Two Pointers |
| Remove elements in-place | In-place Overwrite |
| Duplicates/frequency | HashSet / HashMap |
Subarray of size k |
Sliding Window |
| Range sum query | Prefix Sum |
| Maximum subarray sum | Kadane’s Algorithm |
Who Is This For?
This repo is useful for:
- beginner DSA students
- Java learners
- placement preparation students
- LeetCode / CodeChef / Codeforces beginners
- students who struggle to identify patterns
- learners who want clean revision notes
Future Roadmap
After Array patterns, this repo can expand into:
| Topic | Focus |
|---|---|
| Strings | Two pointers, frequency, palindrome, substring |
| Linked List | Fast-slow pointer, reversal, cycle detection |
| Stack & Queue | Monotonic stack, next greater element, BFS basics |
| Recursion | Base case, choices, recursion tree |
| Trees | DFS, BFS, height, path problems |
| Graphs | BFS, DFS, shortest path basics |
| Dynamic Programming | Memoization, tabulation, state thinking |
| Java Templates | Reusable problem-solving templates |
Long-term goal:
Build a complete beginner-friendly DSA pattern guide in Java.Contributing
Beginner contributions are welcome.
You can help by adding:
- better examples
- dry runs
- Java solutions
- common mistakes
- edge cases
- practice questions
- formatting improvements
- typo fixes
Before contributing:
- keep explanations simple
- write readable Java code
- mention time and space complexity where needed
- do not add copied paid/protected content
Do not memorize 100 solutions.
Learn the patterns behind them.Pattern recognition is the real skill in DSA.
Built by Idevion as a beginner-friendly open-source learning initiative. Idevion focuses on helping students grow through learning resources, roadmaps, projects, and open-source collaboration.
