🚀 DSA Java — Placement Preparation

Pattern-based DSA practice in Java for product-based company interviews

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📌 About

This repo documents my consistent DSA journey in Java — structured around patterns, not just problems.

Every solution is:

• Written in clean, readable Java
• Commented with approach + time/space complexity
• Grouped by pattern for fast interview revision

💡 Core philosophy: Don't grind problems blindly. Recognize the pattern in 30 seconds, solve in 5 minutes.

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📂 Repository Structure

Folder	Status	Problems Solved	Patterns Covered
📁 Arrays	🟢 Active	57	HashMap, Two Pointers, Greedy, Prefix Sum, Kadane's, Cyclic Sort, Floyd Cycle, Intervals, Binary Search, Binary Search on Answer, Heap, Sliding Window, Partition, LCM+GCD, Frequency Array, Matrix Traversal, Array Traversal, Modular Arithmetic, Conditional Math
📁 Strings	🟢 Active	24	Two Pointers, Traverse from End, Sliding Window, Fixed Window, HashMap Grouping, String Matching, Expand Around Center, Simulation, Stack, DP on Strings, String Manipulation, Greedy
📁 LinkedList	🟢 Active	6	Reversal, Two Pointer Merge, Fast & Slow Pointers, Dummy Node, Two Pointer Sync
📁 HashMap	⚪ Upcoming	—	—
📁 Stack	⚪ Upcoming	—	—
📁 Queue	⚪ Upcoming	—	—
📁 Trees	⚪ Upcoming	—	—
📁 Graphs	⚪ Upcoming	—	—
📁 Dynamic Programming	⚪ Upcoming	—	—

📌 Each folder has its own README with pattern-grouped problems, approach explanations, key templates, and common mistakes.

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📊 Overall Progress

Arrays           ████████████████████  57/--   🟢 Active
Strings          ████████████████░░░░  24/30+  🟢 Active
LinkedList       ██░░░░░░░░░░░░░░░░░░   6/50+  🟢 Active
HashMap          ░░░░░░░░░░░░░░░░░░░░   0/--   ⚪ Upcoming
Stack            ░░░░░░░░░░░░░░░░░░░░   0/--   ⚪ Upcoming
Queue            ░░░░░░░░░░░░░░░░░░░░   0/--   ⚪ Upcoming
Trees            ░░░░░░░░░░░░░░░░░░░░   0/--   ⚪ Upcoming
Graphs           ░░░░░░░░░░░░░░░░░░░░   0/--   ⚪ Upcoming
DP               ░░░░░░░░░░░░░░░░░░░░   0/--   ⚪ Upcoming

Total: 87 / 300+ problems  ██████░░░░░░░░░░░░░░  29%

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🧠 DSA Patterns Covered

Pattern	Covered In	Status
HashMap & HashSet	Arrays, Strings	✅
Two Pointers	Arrays, Strings	✅
Greedy	Arrays, Strings	✅
Prefix Sum	Arrays	✅
Kadane's Algorithm	Arrays	✅
Cyclic Sort	Arrays	✅
Floyd's Cycle Detection	Arrays	✅
Interval Merge	Arrays	✅
Binary Search	Arrays	✅
Binary Search on Answer	Arrays	✅
Heap / Top K	Arrays	✅
Sliding Window	Arrays, Strings	✅
Partition (QuickSort)	Arrays	✅
LCM & GCD	Arrays	✅
Frequency Array	Arrays	✅
Matrix Traversal	Arrays	✅
Array Traversal — Max + Count	Arrays	✅
Modular Arithmetic	Arrays	✅
Conditional Math / Formula Derivation	Arrays	✅
Fixed-Size Sliding Window	Strings	✅
Expand Around Center	Strings	✅
Simulation / Parsing	Strings	✅
Stack (bracket matching, chain removal)	Strings	✅
DP on Strings	Strings	✅
String Manipulation	Strings	✅
Reversal (Linked List)	LinkedList	✅
Fast & Slow Pointers	LinkedList	✅
Two Pointer Merge	LinkedList	✅
Dummy Node	LinkedList	✅
Two Pointer Sync	LinkedList	✅
Recursion & Backtracking	—	🔜
BFS / DFS on Trees	—	🔜
Graph Algorithms	—	🔜
Advanced DP (2D, Knapsack)	—	🔜

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🗺️ Roadmap

✅ Phase 1 — Arrays
   └── HashMap · Two Pointers · Greedy · Prefix Sum · Kadane's
       Cyclic Sort · Floyd Cycle · Binary Search · Intervals · Heap
       Sliding Window · Partition · LCM+GCD · Frequency Array
       Matrix Traversal · Array Traversal · Modular Arithmetic · Conditional Math

✅ Phase 2 — Strings
   └── Two Pointers · Traverse from End · Sliding Window · Fixed Window
       HashMap Grouping · String Matching · Expand Around Center
       Simulation/Parsing · Stack · DP on Strings · String Manipulation · Greedy

🟢 Phase 3 — LinkedList (In Progress)
   └── ✅ Reversal · ✅ Two Pointer Merge · ✅ Fast/Slow Pointers
       ✅ Dummy Node · ✅ Two Pointer Sync
       ⬜ Reversal II · ⬜ Fast/Slow Advanced · ⬜ Merge Advanced · ⬜ Recursion on Lists

⬜ Phase 4 — Stack + Queue
   └── Monotonic Stack · Deque · Expression Evaluation · Sliding Window Maximum

⬜ Phase 5 — Trees + Recursion
   └── DFS · BFS · Level Order · Path Sum · LCA · BST operations

⬜ Phase 6 — Graphs + BFS/DFS
   └── Topological Sort · Union-Find · Dijkstra · Cycle Detection

⬜ Phase 7 — Dynamic Programming
   └── 1D DP · 2D DP · Knapsack · LCS · Edit Distance · Palindrome DP

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⚡ Master Pattern Recognition Cheat Sheet

🔑 In an interview, identify the pattern first — then code.

Arrays & General

Trigger (What You See)	Pattern
Pair / target sum	HashMap
Frequency count, fast lookup	HashMap
Sorted array + pair or triplet	Two Pointers
Two sorted arrays, closest sum	Two Pointers (opposite ends)
Maximize profit, track running min/max	Greedy
Maximum contiguous subarray	Kadane's Algorithm
Subarray sum = k	Prefix Sum + HashMap
Zero-sum subarray	Prefix Sum + HashMap
Missing / duplicate in [1..n]	Cyclic Sort
Find duplicate, no extra space, no modify	Floyd's Cycle Detection
Overlapping ranges	Interval Merge
Sorted array, find element or peak	Binary Search
Minimize the maximum / threshold	Binary Search on Answer
Top K / Kth smallest / largest	Heap
Contiguous subarray, fixed window size	Sliding Window
Rearrange around pivot	Partition (QuickSort)
All of A divides X, X divides all of B	LCM + GCD
Count occurrences in small fixed range	Frequency Array
Square matrix, main + secondary diagonal	Matrix Traversal (index formula)
Largest element + count of occurrences	Array Traversal — Max + Count
Sum of all except one, min/max contribution	Total Sum ± min/max
Count pairs with sum divisible by k	Modular Arithmetic (remainders)
Round to nearest multiple, conditional rounding	Conditional Math (formula)
Minimum turns, can start from either end	Compute both directions, take min

Strings

Trigger (What You See)	Pattern
Reverse string in-place	Two Pointers (swap from ends)
Check palindrome, ignore special chars	Two Pointers + skip invalid
Last word / reverse words / trailing spaces	Traverse from End
Longest substring, no repeating chars	Sliding Window (variable)
Smallest window with all chars of t	Sliding Window + Frequency Map
Permutation of pattern / all anagrams	Fixed-Size Window + int[26]
Concatenation of equal-length words	Fixed-Size Window (chunked)
Group strings by same characters	HashMap Grouping (signature key)
Find first occurrence of pattern	String Matching (i <= n - m)
Longest palindromic substring	Expand Around Center
String to integer / validate format	Simulation / Parsing
Track movement up/down, count transitions	Simulation with state variable
Convert one format to another, edge cases	String Parsing + Conditional Handling
Shift/rotate characters cyclically	Character Arithmetic + Modulo
Longest valid bracket substring	Stack of Indices
Character number system with subtraction rules	Greedy (largest chunk first)
Repeating pattern, compare expected vs actual	Modulo-based Pattern Matching
Matching brackets / balanced pairs	Stack
Remove adjacent duplicates, chain reactions	Stack (push-pop)
Pattern with * or . wildcards	DP on Strings

Linked Lists

Trigger (What You See)	Pattern
Reverse entire list / modify next pointers	Three-Pointer Reversal
Merge two sorted lists, pick smaller each step	Two Pointer Merge + Dummy Node
Remove duplicates from sorted list	Adjacent Comparison (no extra space)
Detect cycle / find middle / kth from end	Fast & Slow Pointers
Check if linked list is a palindrome	Fast & Slow + Reversal
Remove Nth node from end	Fast & Slow (gap of N+1) + Dummy
Delete nodes matching a value, head may change	Dummy Node + prev/curr pointers
Swap adjacent pairs without modifying values	Pointer Rewiring + Dummy Node
Find intersection of two lists, no extra space	Two Pointers + Head Switching

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🏢 Company Pattern Mapping

Company	Primary Patterns	Must-Know Problems
Amazon	HashMap, Greedy, Sliding Window	Two Sum, Best Time to Buy Stock, Longest Substring
Google	Prefix Sum, Binary Search, DP	Subarray Sum = K, Min Window Substring, Regex Matching
Microsoft	Arrays, Two Pointers, Intervals	Merge Intervals, Valid Palindrome, 3Sum
Meta	Two Pointers, HashMap, Graphs	Remove Duplicates, Group Anagrams, Find All Anagrams
Adobe	Sorting, Strings, Simulation	Valid Number, Longest Palindrome, atoi
Flipkart	Greedy, Binary Search, Kadane's	Product Except Self, Koko Bananas, Max Subarray

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📈 LeetCode Stats

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🔁 How to Use This Repo

Mode	What to Do
📖 Learn	Open the topic folder → read the pattern README → solve problems group by group
🔁 Revise	Read only the cheat sheet above + key templates in each folder README
⚡ Interview Prep	Cheat sheet only — identify pattern in ≤ 30 seconds, then code from template

Daily target: 3 problems/day — one pattern group at a time. Don't jump between topics.

Weekly review: Re-attempt hard problems without notes. Timed: 20 minutes each.

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🙌 Connect

 

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⭐ Star this repo if it helps you. Keep solving. Keep growing. 🚀