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Current Progress

✅ v1.0 Array Pattern-Wise Module completed
🚧 v2.0 Linked List Pattern-Wise Module started

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DSA Patterns Java

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 students 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 ✅

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Why This Repo?

Most DSA repos follow:

Problem → Code → Output

This repo follows:

Concept → Pattern → Example → Dry Run → Complexity → Mistakes → Practice

The goal is simple:

Learn the pattern behind the problem, not just the final code.

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Completed Modules

Version	Module	Status
v1.0	Array Patterns in Java	✅ Completed

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Current Module

Version	Module	Status
v2.0	Linked List Patterns in Java	🚧 In Progress

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Project Roadmap

Version	Module	Status
v1.0	Array Patterns	✅ Completed
v2.0	Linked List Patterns	🚧 In Progress
v3.0	Stack Patterns	Planned
v4.0	Queue Patterns	Planned
v5.0	String Patterns	Planned
v6.0	Recursion Patterns	Planned
v7.0	Tree Patterns	Planned
v8.0	Graph Patterns	Planned
v9.0	Dynamic Programming Patterns	Planned
v10.0	Mixed Interview Practice	Planned

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Completed: Array Patterns in Java

The first complete module is Array Patterns in Java.

It includes:

22-day structured roadmap
18 core array patterns
Java templates
Dry runs
Common mistakes
Easy mixed practice
Medium mixed practice
Pattern identification approach

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	✅ Live
6	Pattern 04: Two Pointers	✅ Live
7	Pattern 05: In-place Overwrite	✅ Live
8	Pattern 06: HashSet / Duplicate Detection	✅ Live
9	Pattern 07: HashMap Frequency	✅ Live
10	Pattern 08: Sorting-Based Patterns	✅ Live
11	Pattern 09: Merge Sorted Arrays	✅ Live
12	Pattern 10: Fixed Size Sliding Window	✅ Live
13	Pattern 11: Variable Size Sliding Window	✅ Live
14	Pattern 12: Prefix Sum	✅ Live
15	Pattern 13: Prefix Sum + HashMap	✅ Live
16	Pattern 14: Kadane’s Algorithm	✅ Live
17	Pattern 15: Stock Profit Pattern	✅ Live
18	Pattern 16: XOR and Math Formula	✅ Live
19	Pattern 17: Rotate, Dutch Flag, Product Except Self, Difference Array	✅ Live
20	Pattern 18: Matrix and Final Checklist	✅ Live
21	Easy Mixed Array Practice	✅ Live
22	Medium Mixed Array Practice	✅ Live

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Started: Linked List Patterns in Java

The second module is Linked List Patterns in Java.

It follows the same structure as the Array module:

Concept
Pattern recognition
Dry run
Java implementation
Complexity
Common mistakes
Practice

The Linked List module is planned as:

19-day module
15 core linked list patterns
Beginner to interview-ready flow

Day	Topic	Status
1	What is Linked List	Planned
2	How to Approach Linked List Problems	Planned
3	Pattern 01: Traversal, Search and Length	Planned
4	Pattern 02: Pointer Rewiring: Insertion, Deletion and Swapping	Planned
5	Pattern 03: Dummy Node / Sentinel Node	Planned
6	Pattern 04: Two Pointers Gap Pattern	Planned
7	Pattern 05: Fast and Slow Pointers	Planned
8	Pattern 06: Cycle Detection / Floyd’s Algorithm	Planned
9	Pattern 07: Linked List Reversal	Planned
10	Pattern 08: Advanced Reversal: Sublist, Pairs and K-Group	Planned
11	Pattern 09: Merge Pattern	Planned
12	Pattern 10: Split and Merge Sort Pattern	Planned
13	Pattern 11: Palindrome Checking Pattern	Planned
14	Pattern 12: Intersection of Linked Lists	Planned
15	Pattern 13: Add Two Numbers / Carry Pattern	Planned
16	Pattern 14: List Weaving and Reordering Pattern	Planned
17	Pattern 15: Clone List With Random Pointer	Planned
18	Easy Mixed Linked List Practice	Planned
19	Medium Mixed Linked List Practice	Planned

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Standard Folder Structure

Every learning day or pattern folder follows this structure wherever applicable:

folder-name/
├── 01-concept-or-pattern-name.md
├── 02-pattern-name-templates-java.md
└── 03-day-N-practice.md

Example:

linked-list/
└── 08-pattern-07-linked-list-reversal/
    ├── 01-pattern-07-linked-list-reversal.md
    ├── 02-linked-list-reversal-templates-java.md
    └── 03-day-9-practice.md

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What You Will Learn

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

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How to Use This Repo

Read concept
↓
Understand example
↓
Dry run manually
↓
Check complexity
↓
Read mistakes
↓
Solve practice questions

Before 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?

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Why Pattern-Based DSA?

In real problems, the question will not say:

Use HashMap
Use Sliding Window
Use Two Pointers
Use Prefix Sum

You 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
Node movement	Linked List Traversal
Head deletion / edge cases	Dummy Node
Nth node from end	Two Pointers Gap
Middle / split list	Fast and Slow Pointers
Reverse links	Linked List Reversal
Cycle in list	Floyd’s Cycle Detection

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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
• students preparing for internships and coding rounds

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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:

• read the Contribution Guidelines
• follow the standard folder structure
• keep explanations simple
• write readable Java code
• mention time and space complexity where needed
• do not add copied paid/protected content

Please also follow the Code of Conduct.

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Learning Principle

Do not memorize 100 solutions.
Learn the patterns behind them.

Pattern recognition is the real skill in DSA.

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Maintained By

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.

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Connect With Idevion

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Understand Patterns. Write Clean Java. Solve With Confidence.

If this repo helps you, consider starring it and sharing it with beginner DSA learners.