DSA Java

A comprehensive collection of Data Structures & Algorithms implemented in Java — covering core concepts, LeetCode problems, HackerRank challenges, and interview prep organized by topic.

280+ Java files · 40+ topic directories

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Table of Contents

• Sorting
• Recursion
• Binary Search
• Hashing & HashMap
• Linked List
• Stack
• Queue
• Heap / Priority Queue
• Greedy
• Backtracking
• Binary Tree
• BST
• Graphs
• Dynamic Programming
• Segment Tree
• Trie
• Bit Manipulation
• LastRun Series (Recent Practice)
• HackerRank
• LeetCode Scratch / Misc

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Sorting

qustions/sorting/

File	Algorithm
bubble.java	Bubble Sort
insertion.java	Insertion Sort
merge.java	Merge Sort
selection.java	Selection Sort

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Recursion

Recursion/

File	Problem
ReverseString.java	Reverse a string
StringSubSequence.java	All subsequences of a string
StringSubSequenceWithoutRepeatedAnswer.java	Unique subsequences
arrayIsSorted.java	Check if array is sorted
checkDuplicate.java	Check duplicates recursively
findDuplicateInString.java	Find duplicate in string
firstAndLastOccur.java	First and last occurrence
keyPadCombinations.java	Keypad letter combinations
moveXtoLast.java	Move element X to end
towerOfHanoi_.java	Tower of Hanoi

Hard Questions — Recursion/Hard Questions/

File	Problem
Place1XMTilesInNXM.java	Tile placement
expressionAddOperator.java	Expression add operators
inviteNpeopleEitherSingleOrInPair.java	Invitation combinations
pathInMazeFrom0_0ToM_N.java	Maze path finding
permutationOfLetters.java	Letter permutations
subsetOfNNatualNum.java	Subsets of N natural numbers

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Binary Search

BinarySearch/

File	Problem
FindFloor.java	Floor in sorted array
MinPairSum.java	Minimum pair sum

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Hashing & HashMap

Hashing/

File	Concept
hasingBasics.java	Hashing fundamentals
iterationInHashMap.java	HashMap iteration patterns
hash methods implementations/hashimplementation.java	Custom hash implementation

HashMap/

File	Problem
UnionOfTwoArr.java	Union of two arrays
hashImplimentation.java	HashMap implementation
intersection.java	Intersection of arrays
iternaryFromTickets.java	Itinerary from tickets
majorityElement.java	Majority element
sumOfSubArray.java	Subarray sum

Hash Set/ — directory available for future problems

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Linked List

LinkedList/

File	Problem
removeZeroSumSublists.java	Remove zero-sum sublists

leetcode/ll/

File	Problem
One.java	LL problem 1
double.java	Doubly linked list
example/one_one.java	LL example

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Stack

26Stack/

File	Concept
Implementation.java	Stack from scratch
reverseStack.java	Reverse a stack
stackCollFrameWork.java	Stack via Java Collections
stackInLL.java	Stack using linked list
validparanthsis.java	Valid parentheses

26Stack/Hard Question/

File	Problem
findMaxAreaInAHistogram.java	Largest rectangle in histogram

26Stack/findNextGreatestNum/

File	Problem
StacknextGreatestNum.java	Next greater element (stack)
nextGreatestNum.java	Next greater element

26Stack/amazon/

File	Problem
pushAtTheBottom.java	Push element to bottom

26Stack/google and microsoft/

File	Problem
duplicateParanthesis.java	Duplicate parentheses

microsoft/

File	Problem
stockSpanProb.java	Stock span problem

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Queue

Queue/

File	Concept
queueJavaCollectionFrameWork.java	Queue via Java Collections
reverse.java	Reverse a queue
interleave2halvesInQueue.java	Interleave two halves

Queue/array/

File	Concept
buildQueue.java	Queue using array
circularQueue.java	Circular queue

Queue/linkedlist/

File	Concept
generalQueue.java	Queue using linked list

Queue/deque/

File	Concept
deque_1.java	Deque basics
StackUsingdeque.java	Stack using deque
queueUsingDeque.java	Queue using deque

Queue/Intuit/

File	Problem
stackUsingTwoQueue.java	Stack using two queues

Queue/microsoftandgoogle/

File	Problem
queueUsingTwoStacks.java	Queue using two stacks

Queue/flipkart/

File	Problem
firstNonRepeatingLetter.java	First non-repeating character

Queue/problems/

File	Problem
binaryNums.java	Binary numbers using queue

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Heap / Priority Queue

Heap/

File	Concept / Problem
insert.java	Heap insertion
heapSort.java	Heap sort
priorityQueue.java	PriorityQueue basics
PQ_implement_comparable.java	PQ with Comparable
connectNRopesWithMinCost.java	Connect ropes minimum cost
nearByCar.java	K nearest cars
slidingWindow.java	Sliding window maximum
weakestSoldier.java	K weakest rows

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Greedy

greedy/

File	Problem
MaximumChainLength.java	Maximum chain length
chocolaProblem.java	Chocola problem
fractionalKnapsack.java	Fractional knapsack
indianCoin.java	Indian coin change
jobSequenceProblem.java	Job sequencing
minAbsoluteDiffPairs.java	Minimum absolute diff pairs

greedy/morganStan_MIcrosoft/

File	Problem
activitySelection.java	Activity selection
activitySelectionUsing2DArray.java	Activity selection (2D)

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Backtracking

BackTracking/

File	Problem
NQueen.java	N-Queens problem
sudokusolver.java	Sudoku solver

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Binary Tree

BinaryTree/

File	Concept / Problem
bulidTree.java	Build binary tree
bfs.java	BFS traversal
heightOfTree.java	Height of tree
diameterOfATree.java	Diameter of tree
subTreeOfAnotherTree.java	Subtree check
NaryTreeLevelOrder.java	N-ary tree level order
narraytreenode.java	N-ary tree node
topOfTheTree.java	Top view of tree

BinaryTree2/

File	Problem
KthAncestor.java	Kth ancestor of a node
minimumDistance.java	Minimum distance between nodes
sumTree.java	Sum tree check

BinaryTree2/KthElement/

File	Approach
KthElementRecursivly.java	Recursive
KthElementInTheTreeIterativly.java	Iterative

BinaryTree2/LCA/

File	Approach
lowestCommonAncestor.java	Basic LCA
lowestCommonAncestorOptimized.java	Optimized LCA

BinaryTree2/Additional Basic Questions/

File	Problem
deleteNode.java	Delete node
univalidBinaryTreeOrNot.java	Univalued tree check
mirrorOfAGivenTree.java	Mirror of a tree

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BST

BST1/

File	Concept
build.java	Build a BST
mirrorOfBst.java	Mirror of BST

BST2/

File	Problem
BstToBalancedBST.java	BST to balanced BST
RecoverBST.java	Recover BST
SortedArrToBalancedTree.java	Sorted array to balanced BST
TwoSumBST.java	Two sum in BST
maxSumBST.java	Maximum sum BST
merge2BST.java	Merge two BSTs
validBST.java	Validate BST

leetcode/BST/

File	Problem
topoftree.java	Top of BST
morise_traversal/morisPreorder.java	Morris preorder traversal

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Graphs

Graphs/

File	Concept / Problem
createGraph.java	Graph creation
graph.java	Graph basics
bfs.java	BFS traversal
dfs.java (in Tree traversal/)	DFS traversal
hasPath.java	Has path check
allPathToTarget.java	All paths to target
topologicalSort.java	Topological sort
dijkstras.java	Dijkstra's algorithm

Graphs/Tree traversal/

File	Traversal
bfs.java	BFS
dfs.java	DFS
Solution.java	Combined solution

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Dynamic Programming

DP/

File	Problem
fibonacci.java	Fibonacci
minimumArrayJump.java	Minimum jumps

DP/0_1_knapsack/

File	Approach
knap_recursion.java	Recursion
knap_memoization.java	Memoization
knap_tabulation.java	Tabulation

DP/climbing stair/

File	Approach
climbingStairs.java	Basic
memo_climbingStair.java	Memoization
tabul_climbing.java	Tabulation

DP/Catalan Number/

File	Problem / Approach
Recursive_catalan.java	Recursive
memoization_catalan.java	Memoization
tabulation_catalan.java	Tabulation
CountBST.java	Count unique BSTs
mountainRange.java	Mountain ranges

DP/Matrix multiplication/

File	Approach
Rec_mcm.java	Recursive MCM
memoization_mcm.java	Memoization MCM
tabulation_mcm.java	Tabulation MCM

DP/least_Common_Factor/

File	Problem
lcs.java	LCS basic
lcs_Memoization.java	LCS memoization
lcs_Tab.java	LCS tabulation
editDistance.java	Edit distance
stringConversion.java	String conversion
wildcard.java	Wildcard matching
least largest sequence/longSeq.java	Longest sequence
lstcommonsubstring/lcs.java	Longest common substring

DP/minimum_Partationing/

File	Problem
mpm.java	Minimum partition

DP/rod_cutting/

File	Problem
rod1.java	Rod cutting

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Segment Tree

Segment Tree/

File	Operation
segmentTree.java	Build segment tree
minMaxST.java	Min/max segment tree
get_minMaxST.java	Query min/max
queriesInST.java	Range queries
updateInST.java	Point update
update_MinMaxST.java	Update min/max

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Trie

LastRun10_trie/

File	Problem
LongestCommonSuffixQueries.java	Longest common suffix queries

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Bit Manipulation

LastRun3/BitManipulation/

File	Problem
BinaryGap.java	Binary gap

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LastRun Series (Recent Practice)

These directories represent the most recent problem-solving sessions, organized by topic.

LastRun — Binary Tree

LastRun/Tree/

File	Problem
BinaryTreeInorderIterative.java	Inorder traversal (iterative)
BinaryTreePreorderIterative.java	Preorder traversal (iterative)
BinaryTreePostorderIterative.java	Postorder traversal (iterative)
BinaryTreeLevelOrderRecursive.java	Level order (recursive)
BuildTreeFromPreorderInorder.java	Build tree from preorder + inorder
BuildTreeFromInorderPostorder.java	Build tree from inorder + postorder
DiameterOfBinaryTree .java	Diameter of binary tree
BalancedBinaryTree.java	Check balanced tree
MaxPathSumBinaryTree.java	Maximum path sum
LowestCommonAncestorBinaryTree.java	LCA in binary tree
FlattenBinaryTree.java	Flatten to linked list
SerializeDeserializeBinaryTree.java	Serialize/Deserialize
CountNodesBinaryTree.java	Count nodes
DistanceKBinaryTree.java	Nodes at distance K
AmountOfTimeToInfectTree.java	Time to infect tree
FindBottomLeftValue.java	Bottom-left tree value
SameTree.java	Same tree check
ChildrenSumProperty.java	Children sum property
MinRemoval.java	Min removal
MinimumCost.java	Minimum cost
MinimumDeletionsBalancedString.java	Min deletions for balanced string

LastRun2 — BST

LastRun2/BST/

File	Problem
SearchInBST.java	Search in BST
InsertIntoBST.java	Insert into BST
DeleteNodeInBst.java	Delete node in BST
ValidateBST.java	Validate BST
ValidateBSTInorder.java	Validate BST (inorder)
KthSmallestInBST.java	Kth smallest element
FloorInBST.java	Floor in BST
InorderSuccessorBST.java	Inorder successor
LCAinBST.java	LCA in BST
BSTIteratorDemo.java	BST iterator
CountNodesBinaryTree.java	Count nodes
FlipEquivalentBinaryTrees.java	Flip equivalent trees

LastRun2/Tree/

File	Problem
Create_Binary_Tree_From_Descriptions.java	Build tree from descriptions
Number of Ways to Assign Edge Weights/assignEdgeWeights_I.java	Edge weights I
Number of Ways to Assign Edge Weights/assignEdgeWeights_II.java	Edge weights II

LastRun4 — Permutations

LastRun4/

File	Problem
DigitorialPermutation.java	Digitorial permutation

LastRun5 — Strings

LastRun5/Strings/

File	Problem
EvaluateBracketPairsOfString.java	1807. Evaluate the Bracket Pairs of a String (Medium)
FindTargetIndicesAfterSortingArray.java	2089. Find Target Indices After Sorting Array (Easy)
MaxDepthAfterSplit.java	1111. Maximum Nesting Depth of Two Valid Parentheses Strings (Medium)
MaximumActiveSectionsAfterTrade.java	Maximum Active Sections After Trade
MaximumNestingDepthOfParentheses.java	1614. Maximum Nesting Depth of the Parentheses (Easy)
MaxAlternatingSumOfSquares.java	Maximum alternating sum of squares
MaxSumOfSquares.java	3723. Maximize Sum of Squares of Digits (Easy)
MinAddToMakeValid.java	921. Minimum Add to Make Parentheses Valid (Medium)
MinDeletionsAdjacentXorNonZero.java	Minimum Deletions to Make Adjacent Elements Bitwise XOR Non-Zero (Easy)
MinInsertionsToBalance.java	1541. Minimum Insertions to Balance a Parentheses String (Medium)
MinOperationsAlternatingString.java	1758. Minimum Changes To Make Alternating Binary String (Easy)
MinOperationsToSortAPermutation.java	3942. Minimum Operations to Sort a Permutation (Medium)
MinSwapsSortByDigitSum.java	3551. Minimum Swaps to Sort by Digit Sum (Medium)
MinSumSquareDiff.java	2333. Minimum Sum of Squared Difference (Medium)
PalindromicSubstrings.java	647. Palindromic Substrings (Medium)
passwordStrengthCheck.java	Password strength check
PermutationInString.java	567. Permutation in String (Medium)
ProcessString.java	Process string
ReverseParentheses.java	1190. Reverse Substrings Between Each Pair of Parentheses (Medium)
RemoveInvalidParentheses.java	301. Remove Invalid Parentheses (Hard)
ScoreOfParentheses.java	856. Score of Parentheses (Medium)
SmallestPalindrome.java	2697. Lexicographically Smallest Palindrome (Easy)
SmallestSubsequenceOfDistinctCharacters.java	1081. Smallest Subsequence of Distinct Characters (Medium)
SumOfSquaresSpecial.java	2778. Sum of Squares of Special Elements (Easy)
ValidParentheses.java	20. Valid Parentheses (Easy)
WeightedWordMapping.java	Weighted word mapping
count no of special character Series/count_no_of_specical_char_I.java	Count special chars I
count no of special character Series/count_no_of_specical_char_II.java	Count special chars II

LastRun6 — Graphs

LastRun6/Graph/

File	Problem
FloodFill.java	Flood fill
RottingOranges.java	Rotting oranges
NumberofEnclaves.java	Number of enclaves
SurroundedRegion.java	Surrounded regions
MakeNetworkConnected.java	Make network connected
MakingALargeIsland.java	Making a large island
NetworkDelayTime.java	Network delay time
CheapestFlightsWithinKStops.java	Cheapest flights within K stops
FindCityWithSmallestNeighbors.java	Find city with fewest neighbors
NumberofWaystoArriveatDestination.java	Number of ways to destination
CourseScheduleII.java	Course Schedule II
wordLadder.java	Word Ladder
wordLadder2.java	Word Ladder II
shortestPathinAmaze.java	Shortest path in maze
MinCost.java	Minimum cost path
minNumberOfSeconds.java	Minimum number of seconds
mergerAccount_graph.java	Merge accounts
SwimInRisingWater.java	Swim in rising water

LastRun6/LastRun_Array/

File	Problem
HardRotatedArrayMinValue.java	Min value in rotated array
_350.java	Intersection of two arrays II

LastRun7 — Mixed

LastRun7/arrays/

File	Problem
CapacityToShipPackagesWithinDDays.java	Ship packages within D days
LeftRightDifference.java	Left-right difference
PartitionArrayAccordingtoGivenPivot.java	Partition array by pivot
RotatingtheBox.java	Rotating the box
MinimumCostOfBuyingCandiesWithDiscount.java	Min cost buying candies
MinimumIndexCapacity.java	Minimum index capacity
SmallestBalancedIndex.java	Smallest balanced index
Count_Kth_Roots_in_a_Range.java	Count Kth roots in range
longestCommonPrefix.java	Longest common prefix

LastRun7/Land and water park rides complete ques/

File	Problem
earliest_finish_time_for_land_and_water_rides_i.java	Earliest finish time I
earliest_finish_time_for_land_and_water_rides_ii.java	Earliest finish time II

LastRun7/Total Wavieness in a number/

File	Problem
TotalWavinessOfNumbersInRangeI.java	Total waviness in range I
TotalWavinessOfNumbersInRangeII.java	Total waviness in range II

LastRun7/max total subarray value/

File	Problem
MaximumTotalSubarrayValueI.java	Max subarray value I
MaximumTotalSubarrayValueII.java	Max subarray value II

LastRun7/Matrix_And_Score/

File	Problem
Minimum_Cost_to_Make_at_Least_One_Valid_Path_in_a_Grid.java	Min cost valid path in grid

LastRun8 — Dynamic Programming

LastRun8_DP/

File	Problem
HouseRobberII.java	House Robber II
Minimum_Path_Sum.java	Minimum path sum
Triangle.java	Triangle minimum path
UniquePathsII.java	Unique paths II
PartitionEqualSubsetsum.java	Partition equal subset sum
isInterleave.java	Interleaving string

LastRun8_DP/jump games/

File	Problem
JumpGame1.java	Jump Game I
JumpGame2.java	Jump Game II
JumpGame3.java	Jump Game III
JumpGame4.java	Jump Game IV
JumpGame5.java	Jump Game V
jumpGame6.java	Jump Game VI
JumpGame7.java	Jump Game VII
jumpGame9.java	Jump Game IX

LastRun8_DP/DP/

File	Problem
MaximumAmountGrid.java	Maximum amount in grid
maxWallsRobotsCanBreak.java	Max walls robots can break

LastRun9 — Hashing

LastRun9_Hashing/

File	Problem
LongestConsecutiveSequence.java	Longest consecutive sequence
TopKFrequentElements.java	Top K frequent elements

LastRun Stack

LastRun_Stack/

File	Problem
Evaluate_Reverse_Polish_Notation.java	Evaluate Reverse Polish Notation
Evaluate_Reverse_Polish_Notation_optimized.java	RPN (optimized)

LastRun Greedy

LastRun_greedy/

File	Problem
MaximumMatrixSum.java	Maximum matrix sum
MinimumInitialEnergyToFinishTasks.java	Min initial energy for tasks
destroyAsteroids.java	Destroy asteroids

LastRun Linked List

LastRun_linkedList/

File	Problem
DeleteMiddleOfLinkedList.java	Delete middle node
MaximumTwinSumOfALinkedList.java	Maximum twin sum
PartitionLinkedList.java	Partition linked list

LastRun Senior Staff — Math / Strings

LastRun_Senior_Staff_maths/

File	Problem
ProcessStringKthCharacter.java	Process string Kth character

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HackerRank

HackerRank/

Organized by category — directories are ready for more solutions.

Directory	Files
Hashing/	icecreamParlor.java
Strings/	PalindromeCheck.java, palindromeIndex_.java, Solution.java
Arrays/	(empty — ready)
DynamicProgramming/	(empty — ready)
Graphs/	(empty — ready)
Greedy/	(empty — ready)
LinkedList/	(empty — ready)
Recursion/	(empty — ready)
Searching/	(empty — ready)
Sorting/	(empty — ready)
Stack/	(empty — ready)
Trees/	(empty — ready)

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LeetCode Scratch / Misc

leetcode/

File	Notes
bsf.java	BFS scratch
recursion/CountGoodNumber.java	Count good numbers
recursion/pow.java	Power function
BST/morise_traversal/morisPreorder.java	Morris preorder

demo/

File	Notes
Main.java	Entry point demo
doublefdx.java	Misc
leetcode_496_3.java	LC 496 solution
tracelink/Client.java	Tracelink client

alpna plus library/amazon/easy/

File	Problem
mergeSort.java	Merge sort
reverseDuplicate.java	Reverse duplicates

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Directory Structure at a Glance

DSA JAVA/
├── qustions/sorting/          — Sorting algorithms
├── Recursion/                 — Recursion + hard problems
├── BinarySearch/              — Binary search
├── Hashing/                   — Hashing basics
├── HashMap/                   — HashMap problems
├── LinkedList/                — Linked list
├── 26Stack/                   — Stack (impl + problems)
├── microsoft/                 — Stock span (Microsoft)
├── Queue/                     — Queue (impl + problems)
├── Heap/                      — Heap / PriorityQueue
├── greedy/                    — Greedy algorithms
├── BackTracking/              — Backtracking
├── BinaryTree/                — Binary tree basics
├── BinaryTree2/               — Binary tree advanced
├── BST1/                      — BST basics
├── BST2/                      — BST advanced
├── AVL Tree/                  — AVL tree (placeholder)
├── Graphs/                    — Graph algorithms
├── DP/                        — Dynamic programming
├── Segment Tree/              — Segment tree
├── LastRun/Tree/              — Tree problems (recent)
├── LastRun2/BST/              — BST problems (recent)
├── LastRun2/Tree/             — Tree problems (recent)
├── LastRun3/BitManipulation/  — Bit manipulation
├── LastRun4/                  — Permutations
├── LastRun5/Strings/          — String problems
├── LastRun6/Graph/            — Graph problems (recent)
├── LastRun7/                  — Mixed (arrays, special series)
├── LastRun8_DP/               — DP problems (recent)
├── LastRun9_Hashing/          — Hashing (recent)
├── LastRun10_trie/            — Trie
├── LastRun_Stack/             — Stack (recent)
├── LastRun_greedy/            — Greedy (recent)
├── LastRun_linkedList/        — Linked list (recent)
├── LastRun_Senior_Staff_maths/— Senior-level math/strings
├── HackerRank/                — HackerRank solutions
├── leetcode/                  — LeetCode scratch files
├── demo/                      — Demo / entry points
└── alpna plus library/        — Amazon easy problems