🧠 Data Structures & Algorithms (DSA) Playlist Solutions

A curated collection of optimal solutions to Data Structures and Algorithms problems from LeetCode, CSES, and competitive programming sheets. Every solution is written in Java with an emphasis on algorithmic patterns, clean code, edge-case handling, and optimal time & space complexity.

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📌 Repository Overview

This repository documents my structured DSA preparation and problem-solving journey. Rather than memorizing individual solutions, the focus is on identifying reusable patterns (e.g., Two Pointers, Monotonic Stacks, Binary Search on Answer, Cyclic Sort) to build deep intuition for technical interviews and competitive programming.

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📊 Progress & Problem Statistics

Category	Easy	Medium	Hard	Total
🔢 Arrays & Math	3	2	0	5
🔍 Binary Search	0	4	0	4
🔄 Sorting & Indexing	1	0	1	2
🗂️ HashMaps & Strings	4	1	0	5
🔗 Linked Lists	0	2	1	3
📚 Stacks & Queues	1	3	2	6
Total	9	12	4	25

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📂 Repository Structure

DSA/
├── Arrays/
│   ├── Basic Problems.java        # Array manipulation, digit math & number operations
│   ├── Advanced Problems.java     # CSES math, combinatorics & recursion
│   ├── Search.java                # Binary search, rotated search & search-on-answer
│   └── Sorting.java               # Cyclic sort & in-place array hashing
├── HashMaps/
│   └── Questions.java             # Frequency maps, isomorphic mapping & anagram grouping
├── LinkedList/
│   └── Questions.java             # Cycle detection, k-group reversal & list interleaving
└── Stacks and Queues/
    ├── Stack.java                 # Bracket sequence validation & simulation
    └── Monotonic Stack.java       # Next greater element, stock span & histogram areas

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📑 Problem Index by Topic

1. Arrays & Mathematics

#	Problem	Platform	Difficulty	Core Pattern	Solution File
66	Plus One	LeetCode	Easy	Array & Digit Carry	Arrays/Basic Problems.java
754	Reach a Number	LeetCode	Medium	Math & Parity Analysis	Arrays/Basic Problems.java
—	Number Spiral	CSES	Easy	Grid Pattern & Algebra	Arrays/Advanced Problems.java
—	Two Knights	CSES	Medium	Combinatorics & Complementary Counting	Arrays/Advanced Problems.java
—	Tower of Hanoi	CSES	Easy	Divide & Conquer / Recursion	Arrays/Advanced Problems.java

2. Binary Search

#	Problem	Platform	Difficulty	Core Pattern	Solution File
34	Find First and Last Position of Element in Sorted Array	LeetCode	Medium	Boundary Binary Search	Arrays/Search.java
33	Search in Rotated Sorted Array	LeetCode	Medium	Modified Binary Search on Sorted Half	Arrays/Search.java
540	Single Element in a Sorted Array	LeetCode	Medium	Binary Search via Index Parity	Arrays/Search.java
875	Koko Eating Bananas	LeetCode	Medium	Binary Search on Answer Space	Arrays/Search.java

3. Sorting & In-Place Placement

#	Problem	Platform	Difficulty	Core Pattern	Solution File
41	First Missing Positive	LeetCode	Hard	Cyclic Sort / In-place Hashing ($O(n)$ time, $O(1)$ space)	Arrays/Sorting.java
645	Set Mismatch	LeetCode	Easy	Cyclic Sort for Duplicates & Missing	Arrays/Sorting.java

4. HashMaps & Hashing

#	Problem	Platform	Difficulty	Core Pattern	Solution File
1	Two Sum	LeetCode	Easy	One-pass Complement Lookup	HashMaps/Questions.java
350	Intersection of Two Arrays II	LeetCode	Easy	Frequency Counting Map	HashMaps/Questions.java
205	Isomorphic Strings	LeetCode	Easy	Dual Bi-directional HashMaps	HashMaps/Questions.java
290	Word Pattern	LeetCode	Easy	Bi-directional Character-to-String Mapping	HashMaps/Questions.java
49	Group Anagrams	LeetCode	Medium	Canonical String Key Hashing	HashMaps/Questions.java

5. Linked Lists

#	Problem	Platform	Difficulty	Core Pattern	Solution File
142	Linked List Cycle II	LeetCode	Medium	Floyd's Cycle-Finding (Tortoise & Hare)	LinkedList/Questions.java
25	Reverse Nodes in k-Group	LeetCode	Hard	In-place Sublist Reversal & Pointers	LinkedList/Questions.java
143	Reorder List	LeetCode	Medium	Midpoint Detection, Reversal & Interleaving	LinkedList/Questions.java

6. Stacks & Monotonic Stacks

#	Problem	Platform	Difficulty	Core Pattern	Solution File
20	Valid Parentheses	LeetCode	Easy	LIFO Bracket Sequence Matching	Stacks and Queues/Stack.java
32	Longest Valid Parentheses	LeetCode	Hard	Index-tracking Stack for Valid Substrings	Stacks and Queues/Stack.java
735	Asteroid Collision	LeetCode	Medium	Stack-based State Simulation	Stacks and Queues/Stack.java
84	Largest Rectangle in Histogram	LeetCode	Hard	Monotonic Increasing Stack ($O(n)$)	Stacks and Queues/Monotonic Stack.java
503	Next Greater Element II	LeetCode	Medium	Monotonic Stack on Circular Array ($2n$)	Stacks and Queues/Monotonic Stack.java
901	Online Stock Span	LeetCode	Medium	Monotonic Stack with Span Compression	Stacks and Queues/Monotonic Stack.java

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🎯 Key Algorithmic Patterns Mastered

• Monotonic Stack: Efficiently finding the next/previous smaller or greater element in $O(n)$ time without nested $O(n^2)$ loops (applied in Histogram, Stock Span, Next Greater Element).
• Binary Search on Answer: Transforming an optimization problem into a monotonic feasibility check ($T/F$ predicate space), e.g., Koko Eating Bananas.
• Cyclic Sort: In-place array arrangement in $O(n)$ time and $O(1)$ extra space when elements are within $[1, n]$ range (applied in First Missing Positive).
• Two Pointers & Fast-Slow Pointers: Cycle detection, cycle entry finding, and palindrome/midpoint processing in singly-linked lists.
• Bi-directional Hashing: Enforcing strict one-to-one (bijection) mappings for isomorphic strings and patterns.

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🛠️ How to Compile & Run

Each file contains complete, standalone solution classes or runnable mains. You can compile and test any file using standard Java tooling:

# Navigate to the topic folder
cd "Arrays"

# Compile any Java file
javac "Basic Problems.java"

# Or run directly with modern JDK (Java 11+)
java "Advanced Problems.java"

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🚀 Goal & Roadmap

• Arrays, Searching (Binary Search) & Sorting (Cyclic Sort)
• HashMaps, Sets & Frequency Hashing
• Singly Linked Lists, Reversals & Cycle Detection
• Stacks, Queue Simulation & Monotonic Stacks
• Two Pointers & Sliding Window Problems
• Trees (Binary Trees, BST, Traversals & Construction)
• Graphs (BFS, DFS, Dijkstra, Topological Sort)
• Dynamic Programming (1D, 2D, Knapsack & Subsequences)

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Maintained with ❤️ by Nitin Kumar — Continuously updated as new topics and problems are solved!