DSA Revision — Java

A structured DSA revision repository in Java, covering core data structures and algorithms. Built for backend interview prep as a Java Developer (Spring Boot, Kafka).

Why This Exists

I'm systematically revising Data Structures & Algorithms in Java to align my problem-solving with my backend development stack and prepare for product-based company interviews.

Every solution is written with production-style clarity — commented with approach, intuition, edge cases, and complexity analysis.

🗂️ Master Index

1. Sorting Algorithms

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
Selection Sort	Sorting / Basics	Easy	`O(N^2)`	`O(1)`	SelectionSort.java
Bubble Sort	Sorting / Basics	Easy	`O(N^2)`	`O(1)`	BubbleSort.java
Insertion Sort	Sorting / Basics	Easy	`O(N^2)`	`O(1)`	InsertionSort.java
Merge Sort	Sorting / Divide & Conquer	Medium	`O(N log N)`	`O(N)`	MergeSort.java
Quick Sort	Sorting / Divide & Conquer	Medium	`O(N log N)`	`O(log N)`	QuickSort.java

2. Array Challenges

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC1] Two Sum	Arrays / Hashing	Easy	`O(N)`	`O(N)`	LC1TwoSum.java
[LC26] Remove Duplicates	Arrays / Two Pointers	Easy	`O(N)`	`O(1)`	LC26RemoveDuplicates.java
[LC88] Merge Sorted Array	Arrays / Two Pointers	Easy	`O(M+N)`	`O(1)`	LC88MergeSortedArraysInOne.java
[LC118] Pascal's Triangle	Arrays / Math	Easy	`O(N^2)`	`O(N^2)`	LC118PascalsTriangle.java
[LC136] Single Number	Arrays / Bit Manipulation	Easy	`O(N)`	`O(1)`	LC136SingleNumber.java
[LC169] Majority Element	Arrays / Boyer-Moore	Easy	`O(N)`	`O(1)`	LC169MajorityElement.java
[LC268] Missing Number	Arrays / Math	Easy	`O(N)`	`O(1)`	LC268MissingNumber.java
[LC283] Move Zeroes	Arrays / Two Pointers	Easy	`O(N)`	`O(1)`	LC283MoveZeroes.java
[LC485] Max Consecutive Ones	Arrays / Traversal	Easy	`O(N)`	`O(1)`	LC485MaxConsecutiveOnes.java
[GFG] Leaders in an Array	Arrays / Traversal	Easy	`O(N)`	`O(N)`	LeadersInArray.java
[GFG] Union of Two Sorted Arrays	Arrays / Two Pointers	Easy	`O(N+M)`	`O(N+M)`	UnionOfTwoSortedArraysWithDuplicates.java
[LC15] 3Sum	Arrays / Two Pointers	Medium	`O(N^2)`	`O(1)`	LC15ThreeSum.java
[LC18] 4Sum	Arrays / Two Pointers	Medium	`O(N^3)`	`O(1)`	LC18FourSum.java
[LC48] Rotate Image	Arrays / Matrix	Medium	`O(N^2)`	`O(1)`	LC48RotateMatrix.java
[LC53] Maximum Subarray (Kadane)	Arrays / DP	Medium	`O(N)`	`O(1)`	LC59MaximumSubarrayKadanesAlgo.java
[LC54] Spiral Matrix	Arrays / Matrix	Medium	`O(M*N)`	`O(1)`	LC54SpriralMatrix.java
[LC56] Merge Intervals	Arrays / Sorting	Medium	`O(N log N)`	`O(N)`	LC56MergeIntervals.java
[LC73] Set Matrix Zeroes	Arrays / Matrix	Medium	`O(M*N)`	`O(1)`	LC73SetMatrixZeroes.java
[LC75] Sort Colors	Arrays / Dutch National Flag	Medium	`O(N)`	`O(1)`	LC75SortColors.java
[LC128] Longest Consecutive Sequence	Arrays / Hashing	Medium	`O(N)`	`O(N)`	LC128LongestConsecutiveSequence.java
[LC189] Rotate Array	Arrays / Math	Medium	`O(N)`	`O(1)`	LC189RotateArrayByK.java
[LC2149] Rearrange Array by Sign	Arrays / Two Pointers	Medium	`O(N)`	`O(N)`	LC2149RearrangeArrayBySign.java
[LC229] Majority Element II	Arrays / Boyer-Moore	Medium	`O(N)`	`O(1)`	LC229MajorityElements2.java
[IB/LC] Count Subarrays with XOR K	Arrays / Prefix XOR	Medium	`O(N)`	`O(N)`	CountSubarrayXOREqualsK.java
[GFG] Find Missing & Repeating	Arrays / Math	Medium	`O(N)`	`O(1)`	GFG_FindMissingAndRepeatingNumber.java
[GFG] Longest Subarray with Sum K	Arrays / Prefix Sum	Medium	`O(N)`	`O(N)`	GFG_LongestSubarrayWithSumK.java
[LC493] Reverse Pairs	Arrays / Divide & Conquer	Hard	`O(N log N)`	`O(N)`	LC493ReversePairs.java
[GFG] Count Inversions	Arrays / Divide & Conquer	Hard	`O(N log N)`	`O(N)`	GFG_CountInversion.java

3. Binary Search

On 1D Arrays

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[Basic] Find Index of Target	BS on 1D	Easy	`O(log N)`	`O(1)`	FindIndexOfTarget.java
[LC35] Search Insert Position	BS on 1D	Easy	`O(log N)`	`O(1)`	LC35SearchInsertPosition.java
[GFG] Count Occurrences	BS on 1D	Easy	`O(log N)`	`O(1)`	GFG_CountOccurrencesInSortedArray.java
[LC34] Find First & Last Position	BS on 1D	Medium	`O(log N)`	`O(1)`	LC34FindFirstAndLastOccurrences.java
[LC33] Search in Rotated Array	BS on 1D	Medium	`O(log N)`	`O(1)`	LC33RotatedSortedArraySearch.java
[LC81] Search in Rotated Array II	BS on 1D	Medium	`O(log N)*`	`O(1)`	LC81RotatedSortedArraySearch2.java
[LC153] Min in Rotated Array	BS on 1D	Medium	`O(log N)`	`O(1)`	LC153MinOfRotatedSortedArray.java
[GFG] Find Rotation Count	BS on 1D	Medium	`O(log N)`	`O(1)`	GFG_FindRotationCountInSortedArray.java
[LC162] Find Peak Element	BS on 1D	Medium	`O(log N)`	`O(1)`	LC162PeakElementInSortedArray.java
[LC540] Single Element in Sorted Array	BS on 1D	Medium	`O(log N)`	`O(1)`	LC540SingleElementInSortedArray.java

On Answers

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC69] Sqrt(x)	BS on Answers	Easy	`O(log N)`	`O(1)`	LC69SquareRoot.java
[GFG] Nth Root of M	BS on Answers	Easy	`O(log M)`	`O(1)`	GFG_nthRoot.java
[LC875] Koko Eating Bananas	BS on Answers	Medium	`O(N log M)`	`O(1)`	LC875BananaEating.java
[SPOJ/GFG] Aggressive Cows	BS on Answers	Medium	`O(N log M)`	`O(1)`	GFG_AggressiveCows.java
[GFG] Allocate Minimum Pages	BS on Answers	Hard	`O(N log M)`	`O(1)`	GFG_BookAllocation.java
[LC410] Split Array Largest Sum	BS on Answers	Hard	`O(N log M)`	`O(1)`	LC410SplitArrayLargestSum.java

On 2D Matrix

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC74] Search a 2D Matrix	BS on 2D	Medium	`O(log(M*N))`	`O(1)`	LC74SortedMatrixSearch.java
[LC240] Search a 2D Matrix II	BS on 2D	Medium	`O(M + N)`	`O(1)`	LC240MatrixSearch2.java
[GFG] Row with Max 1s	BS on 2D	Medium	`O(M + N)`	`O(1)`	GFG_RowWithMaxOne.java
[LC1901] Find a Peak Element II	BS on 2D	Medium	`O(M log N)`	`O(1)`	LC1901PeakElement2.java
[GFG] Median in a Row-Wise Sorted Matrix	BS on 2D	Hard	`O(M log(MAX) * log N)`	`O(1)`	GFG_MatrixMedian.java

4. Strings

Basics & Easy

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC151] Reverse Words in a String	Strings / Two Pointers	Medium*	`O(N)`	`O(N)`	LC151ReverseWords.java
[LC242] Valid Anagram	Strings / Hashing	Easy	`O(N)`	`O(1)`	LC242Anagram.java
[LC796] Rotate String	Strings / Matching	Easy	`O(N)`	`O(N)`	LC796RotateString.java
[LC1021] Remove Outermost Parentheses	Strings / Stack	Easy	`O(N)`	`O(N)`	LC1021RemoveOutermostParenthesis.java
[LC1903] Largest Odd Number in String	Strings / Math	Easy	`O(N)`	`O(1)`	LC1903LargestOddNumInString.java

Medium

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC5] Longest Palindromic Substring	Strings / Expand Center	Medium	`O(N^2)`	`O(1)`	LC5LongestPalindromicSubstring.java
[LC1781] Sum of Beauty of All Substrings	Strings / Hashing	Medium	`O(N^2)`	`O(1)`	LC1781BeautyOfString.java

5. Linked List

Singly Linked List (Basics)

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
Convert Array to Linked List	Linked List / Basics	Easy	`O(N)`	`O(N)`	ArrayToLinkedList.java
Print Linked List	Linked List / Basics	Easy	`O(N)`	`O(1)`	PrintLinkedList.java
Find Length of Linked List	Linked List / Basics	Easy	`O(N)`	`O(1)`	LinkedListLength.java
Search in Linked List	Linked List / Basics	Easy	`O(N)`	`O(1)`	CheckIfTargetExistsInLL.java
Insert Node in Singly LL	Linked List / Basics	Easy	`O(N)`*	`O(1)`	InsertionInSinglyLinkedList.java
Delete Node in Singly LL	Linked List / Basics	Easy	`O(N)`*	`O(1)`	DeleteInSinglyLinkedList.java

Singly Linked List (Medium)

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC876] Middle of the Linked List	Linked List / Fast & Slow	Easy*	`O(N)`	`O(1)`	LC876MidOfLL.java
[LC141] Linked List Cycle	Linked List / Fast & Slow	Easy*	`O(N)`	`O(1)`	LC141CycleInSinglyLL.java
[LC142] Linked List Cycle II	Linked List / Fast & Slow	Medium	`O(N)`	`O(1)`	LC142StartOfCycleLengthSinglyLinkedList.java
[GFG] Length of Loop in LL	Linked List / Fast & Slow	Medium	`O(N)`	`O(1)`	GFG_LengthOfLoopOfLinkedList.java
[LC19] Remove Nth Node From End	Linked List / Two Pointers	Medium	`O(N)`	`O(1)`	LC19RemoveNthNode.java
[LC160] Intersection of Two LLs	Linked List / Two Pointers	Easy*	`O(N+M)`	`O(1)`	LC160IntersectionInLinkedList.java
[LC234] Palindrome Linked List	Linked List / Two Pointers	Easy*	`O(N)`	`O(1)`	LC234PalindromicLinkedList.java
[LC328] Odd Even Linked List	Linked List / Traversal	Medium	`O(N)`	`O(1)`	LC_328OddEvenLinkedList.java
[LC148] Sort List	Linked List / Merge Sort	Medium	`O(N log N)`	`O(1)`	LC148SortSinglyLinkedList.java
[GFG] Add 1 to a Number as LL	Linked List / Math	Medium	`O(N)`	`O(1)`	GFG_AddOneToSinglyLinkedList.java

Doubly Linked List (Medium)

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[GFG] Find Pairs with Given Sum	Doubly LL / Two Pointers	Medium	`O(N)`	`O(1)`	GFG_PairSumDoublyLinkedList.java
[GFG] Remove All Occurrences of Key	Doubly LL / Traversal	Medium	`O(N)`	`O(1)`	GFG_RemoveAllOccurrencesOfKey.java
[GFG] Remove Duplicates from Sorted DLL	Doubly LL / Traversal	Easy*	`O(N)`	`O(1)`	GFG_RemoveDuplicates.java

Hard & Advanced

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC25] Reverse Nodes in k-Group	Linked List / Reversal	Hard	`O(N)`	`O(1)`	LC25ReverseLinkedListInKGroups.java
[LC61] Rotate List	Linked List / Two Pointers	Medium*	`O(N)`	`O(1)`	LC61RotateSinglyListToRightByK.java
[LC138] Copy List with Random Pointer	Linked List / Deep Copy	Medium*	`O(N)`	`O(1)`	LC138DeepCopyRandomPointers.java
[GFG] Flattening a Linked List	Linked List / Merge	Hard	`O(N * M)`	`O(1)`	GFG_FlattenLinkedList.java

6. Recursion

Basics

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC8] String to Integer (atoi)	Recursion / Strings	Medium	`O(N)`	`O(N)`	LC8_AtoiRecursion.java
[GFG] Sort a Stack	Recursion / Stack	Medium	`O(N^2)`	`O(N)`	GFG_SortStack.java

Subsequence Patterns

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[Code360] Binary Strings w/o Consecutive 1s	Recursion / Backtracking	Medium	`O(2^N)`	`O(N)`	Code360_GenerateBinaryStringWithoutConsecutive1.java
[GFG] Subsequence Sum Target	Recursion / Subsequences	Medium	`O(2^N)`	`O(N)`	GFG_SubsequenceSumTarget.java
[GFG] Count Subsequences with Sum K	Recursion / Subsequences	Medium	`O(2^N)`	`O(N)`	CountSubsequenceWithSumK.java
[GFG] Subset Sums	Recursion / Subsequences	Medium	`O(2^N)`	`O(2^N)`	GFG_SubsetSums.java
[LC90] Subsets II	Recursion / Backtracking	Medium	`O(2^N * N)`	`O(N)`*	LC90SubSetsSum2.java
[LC39] Combination Sum	Recursion / Backtracking	Medium	`O(2^T)`	`O(T)`	LC39CombinationSum.java
[LC40] Combination Sum II	Recursion / Backtracking	Medium	`O(2^N)`	`O(N)`*	LC40CombinationSum_2.java
[LC216] Combination Sum III	Recursion / Backtracking	Medium	`O(9! / (K!*(9-K)!))`	`O(K)`	LC216CombinationSum3.java
[LC17] Letter Combinations of a Phone Number	Recursion / Backtracking	Medium	`O(4^N * N)`	`O(N)`	LC17_LetterCombinations.java
[LC22] Generate Parentheses	Recursion / Backtracking	Medium	`O(4^N / sqrt(N))`	`O(N)`	LC22GenerateParenthesis.java

Hard Backtracking Problems

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC51] N-Queens	Backtracking	Hard	`O(N!)`	`O(N²)`	LC51NQueens.java
[LC131] Palindrome Partitioning	Backtracking / Strings	Medium	`O(N · 2^N)`	`O(N)`	LC131PalindromePartitioning.java
[LC139] Word Break	Backtracking + DP	Medium	`O(N^2 * M)`	`O(N)`	LC139WordBreak.java
[GFG] Rat In A Maze	Backtracking / Strings	Medium	`O(3^(N ^ 2))`	`O(N^2)`	GFG_RatInMaze.java
[LC37] Sudoku Solver	Backtracking	Hard	`O(9^(N^2))`	`O(N^2)`	LC37SudokuSolver

7. Stack and Queue

Implementation

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
Stack Using Array	Stack/Array	Easy	`O(1)`	`O(N)`	StackUsingArray.java
Stack Using Queue	Stack /Queue	Easy	`O(N)`	`O(N)`	StackUsingQueue.java
Queue Using Array	Queue /Array	Easy	`O(1)`	`O(N)`	QueueUsingArray.java

Basic

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[LC20] Valid Parentheses	Stack	Easy	`O(N)`	`O(N)`	LC20ValidParentheses.java
[LC155] MinStack	Stack	Medium	`O(1)`	`O(N)`	LC155MinStack.java.java

Monotonic Stack

Problem Name	Topic	Difficulty	Time Complexity	Space Complexity	Solution Link
[GFG] Next Smaller Element	Stack / Monotonic Stack	Easy	`O(N)`	`O(N)`	GFG_NextSmallerElement.java
[LC42] Trapping Rain Water	Stack / Monotonic Stack	Hard	`O(N)`	`O(N)`	LC42TrappingRainWater.java
[LC496] Next Greater Element I	Stack / Monotonic Stack	Easy	`O(N + M)`	`O(M)`	LC496NextGreterElement.java
[LC503] Next Greater Element II	Stack / Monotonic Stack	Medium	`O(N)`	`O(N)`	LC503NextGreterElement2.java

Repository Structure

DSA-Revision-Java/
├── arrays/
├── sorting/
├── binarysearch/
├── strings/
├── linkedlist/      
├── recursion/       ✅ Done
├── stack&queue/       ← current focus
└── ...more topics added as I progress

File naming: [Platform + ProblemCountIfAny]_[ProblemName].java Example: LC1TwoSum.java (LC = LeetCode), GFG_CountOccurrencesInSortedArray.java (GFG = GeeksForGeeks) Each file includes:

Link to the original problem
Pattern & key insight
Step-by-step approach explanation
Clean, optimized Java implementation
Time & space complexity

Progress

Topic	Status
Basics	✅ Done
Arrays	✅ Done
Sorting	✅ Done
Binary Search	✅ Done
Strings	✅ Done
Linked Lists	✅ Done
Recursion	✅ Done
Stack & Queue	🔄 In Progress
Trees	⏳ Pending
Graphs	⏳ Pending
Dynamic Programming	⏳ Pending

Current Focus — Stack and Queue

Having completed revision of Basics, Arrays, Sorting, Binary Search, Strings, Linked Lists and Recursion, the focus now shifts to Stack & Queue. Topics to be covered:

Basics of stack and queue
Conversion problem
Monotonic stack
Queue problems
Implementation problems

Goal

Build strong DSA fundamentals and problem-solving speed to target backend-focused and product-based company roles.

Name		Name	Last commit message	Last commit date
Latest commit History 100 Commits
.idea		.idea
arrays		arrays
binarysearch		binarysearch
exception		exception
linkedlist		linkedlist
out/production/DSA-Revision-JAVA		out/production/DSA-Revision-JAVA
queue/implementation		queue/implementation
recursion		recursion
sorting		sorting
stack		stack
strings		strings
utilities		utilities
.gitignore		.gitignore
DSA-Revision-JAVA.iml		DSA-Revision-JAVA.iml
FileStructure.txt		FileStructure.txt
Main.java		Main.java
README.md		README.md
TestingCode.java		TestingCode.java

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DSA Revision — Java

Why This Exists

🗂️ Master Index

1. Sorting Algorithms

2. Array Challenges

3. Binary Search

On 1D Arrays

On Answers

On 2D Matrix

4. Strings

Basics & Easy

Medium

5. Linked List

Singly Linked List (Basics)

Singly Linked List (Medium)

Doubly Linked List (Medium)

Hard & Advanced

6. Recursion

Basics

Subsequence Patterns

Hard Backtracking Problems

7. Stack and Queue

Implementation

Basic

Monotonic Stack

Repository Structure

Progress

Current Focus — Stack and Queue

Goal

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