Java · DSA · LeetCode
LeetCode Problem Sets
191 Problems across 22 categories
Curated, interview-focused problems organised by topic. Each category maps to a core DSA concept. Pick a category, solve the problems in order, and cross-reference the concept page for theory.
Arrays, Strings & Hashing
11 problems
Arrays & Hashing → ▸ Preview problems
- E#1 Two Sum
- E#217 Contains Duplicate
- E#242 Valid Anagram
- M#49 Group Anagrams
- M#238 Product of Array Except Self
- M#347 Top K Frequent Elements
- M#128 Longest Consecutive Sequence
- M#36 Valid Sudoku
- M#271 Encode and Decode Strings
- M#3 Longest Substring Without Repeating Characters
- M#189 Rotate Array
6 problems
HashMap → ▸ Preview problems
- E#1 Two Sum
- M#49 Group Anagrams
- M#347 Top K Frequent Elements
- M#560 Subarray Sum Equals K
- M#128 Longest Consecutive Sequence
- M#146 LRU Cache
6 problems
HashSet → ▸ Preview problems
- E#217 Contains Duplicate
- E#349 Intersection of Two Arrays
- E#202 Happy Number
- M#128 Longest Consecutive Sequence
- M#127 Word Ladder
- M#36 Valid Sudoku
9 problems
Two Pointers → ▸ Preview problems
- E#125 Valid Palindrome
- E#283 Move Zeroes
- E#344 Reverse String
- E#977 Squares of a Sorted Array
- M#142 Linked List Cycle II
- M#167 Two Sum II
- M#11 Container With Most Water
- M#15 3Sum
- H#42 Trapping Rain Water
11 problems
Sliding Window → ▸ Preview problems
- E#121 Best Time to Buy and Sell Stock
- E#643 Maximum Average Subarray I
- M#3 Longest Substring Without Repeating Characters
- M#209 Minimum Size Subarray Sum
- M#424 Longest Repeating Character Replacement
- M#438 Find All Anagrams in a String
- M#567 Permutation in String
- M#904 Fruit Into Baskets
- M#1004 Max Consecutive Ones III
- H#76 Minimum Window Substring
- H#239 Sliding Window Maximum
9 problems
Stack → ▸ Preview problems
- E#20 Valid Parentheses
- M#155 Min Stack
- M#150 Evaluate Reverse Polish Notation
- M#22 Generate Parentheses
- M#394 Decode String
- M#739 Daily Temperatures
- M#853 Car Fleet
- H#84 Largest Rectangle in Histogram
- M#227 Basic Calculator II
11 problems
Binary Search → ▸ Preview problems
- E#704 Binary Search
- E#35 Search Insert Position
- M#34 Find First and Last Position
- M#33 Search in Rotated Sorted Array
- M#153 Find Minimum in Rotated Sorted Array
- M#162 Find Peak Element
- M#74 Search a 2D Matrix
- M#875 Koko Eating Bananas
- M#1011 Capacity To Ship Packages Within D Days
- M#981 Time Based Key-Value Store
- H#4 Median of Two Sorted Arrays
Data Structures
10 problems
Linked List → ▸ Preview problems
- E#206 Reverse Linked List
- E#21 Merge Two Sorted Lists
- E#141 Linked List Cycle
- M#142 Linked List Cycle II
- M#143 Reorder List
- M#19 Remove Nth Node From End of List
- M#2 Add Two Numbers
- M#138 Copy List with Random Pointer
- M#287 Find the Duplicate Number
- H#23 Merge k Sorted Lists
6 problems
Queue → ▸ Preview problems
- M#102 Binary Tree Level Order Traversal
- M#200 Number of Islands
- M#994 Rotting Oranges
- M#622 Design Circular Queue
- M#621 Task Scheduler
- H#239 Sliding Window Maximum
13 problems
Trees → ▸ Preview problems
- E#226 Invert Binary Tree
- E#104 Maximum Depth of Binary Tree
- E#101 Symmetric Tree
- E#100 Same Tree
- E#572 Subtree of Another Tree
- E#94 Binary Tree Inorder Traversal
- M#98 Validate Binary Search Tree
- M#102 Binary Tree Level Order Traversal
- M#105 Construct from Preorder and Inorder
- M#236 Lowest Common Ancestor
- M#230 Kth Smallest Element in a BST
- M#235 Lowest Common Ancestor of a BST
- H#124 Binary Tree Maximum Path Sum
6 problems
BST → ▸ Preview problems
- M#98 Validate Binary Search Tree
- E#700 Search in a Binary Search Tree
- M#701 Insert into a Binary Search Tree
- M#450 Delete Node in a BST
- M#230 Kth Smallest Element in a BST
- M#235 Lowest Common Ancestor of a BST
10 problems
Heap / Priority Queue → ▸ Preview problems
- E#703 Kth Largest Element in a Stream
- E#1046 Last Stone Weight
- M#215 Kth Largest Element in an Array
- M#347 Top K Frequent Elements
- M#973 K Closest Points to Origin
- M#355 Design Twitter
- M#621 Task Scheduler
- M#743 Network Delay Time
- H#23 Merge k Sorted Lists
- H#295 Find Median from Data Stream
6 problems
Trie → ▸ Preview problems
- M#208 Implement Trie (Prefix Tree)
- M#211 Design Add and Search Words
- H#212 Word Search II
- M#421 Maximum XOR of Two Numbers
- M#648 Replace Words
- H#642 Design Search Autocomplete System
12 problems
Graphs → ▸ Preview problems
- M#200 Number of Islands
- M#994 Rotting Oranges
- M#542 01 Matrix
- M#133 Clone Graph
- M#695 Max Area of Island
- M#417 Pacific Atlantic Water Flow
- M#207 Course Schedule
- M#210 Course Schedule II
- M#323 Number of Connected Components
- M#684 Redundant Connection
- M#743 Network Delay Time
- H#127 Word Ladder
6 problems
BFS → ▸ Preview problems
- M#200 Number of Islands
- M#994 Rotting Oranges
- M#542 01 Matrix
- M#1091 Shortest Path in Binary Matrix
- H#127 Word Ladder
- H#773 Sliding Puzzle
6 problems
DFS → ▸ Preview problems
- M#200 Number of Islands
- M#207 Course Schedule
- M#210 Course Schedule II
- M#133 Clone Graph
- M#417 Pacific Atlantic Water Flow
- M#797 All Paths From Source to Target
Algorithms
9 problems
Backtracking → ▸ Preview problems
- M#78 Subsets
- M#90 Subsets II
- M#39 Combination Sum
- M#40 Combination Sum II
- M#46 Permutations
- M#17 Letter Combinations of a Phone Number
- M#79 Word Search
- M#131 Palindrome Partitioning
- H#51 N-Queens
9 problems
Greedy → ▸ Preview problems
- M#55 Jump Game
- M#45 Jump Game II
- M#53 Maximum Subarray
- M#134 Gas Station
- H#135 Candy
- M#678 Valid Parenthesis String
- M#763 Partition Labels
- M#846 Hand of Straights
- M#1899 Merge Triplets to Form Target
8 problems
Intervals → ▸ Preview problems
- M#56 Merge Intervals
- M#57 Insert Interval
- M#435 Non-overlapping Intervals
- E#252 Meeting Rooms
- M#253 Meeting Rooms II
- M#986 Interval List Intersections
- M#1288 Remove Covered Intervals
- H#2402 Meeting Rooms III
9 problems
DP — 1D → ▸ Preview problems
- E#70 Climbing Stairs
- M#198 House Robber
- M#213 House Robber II
- M#91 Decode Ways
- M#322 Coin Change
- M#152 Maximum Product Subarray
- M#139 Word Break
- M#300 Longest Increasing Subsequence
- M#647 Palindromic Substrings
9 problems
DP — 2D / Advanced → ▸ Preview problems
- M#62 Unique Paths
- M#5 Longest Palindromic Substring
- M#1143 Longest Common Subsequence
- M#494 Target Sum
- M#518 Coin Change II
- M#309 Best Time to Buy and Sell Stock with Cooldown
- M#97 Interleaving String
- H#72 Edit Distance
- H#10 Regular Expression Matching
9 problems
Math & Bit Manipulation → ▸ Preview problems
- E#136 Single Number
- E#191 Number of 1 Bits
- E#190 Reverse Bits
- E#268 Missing Number
- E#338 Counting Bits
- M#7 Reverse Integer
- M#48 Rotate Image
- M#73 Set Matrix Zeroes
- M#371 Sum of Two Integers
How to use this:
- Start: Arrays & Hashing → branch into HashMap and HashSet
- Core techniques: Two Pointers → Sliding Window → Stack → Binary Search
- Data structures: Linked List → Queue → Trees → BST → Heap → Trie
- Graphs: Graphs (umbrella) → split into BFS and DFS
- Algorithms: Backtracking → Greedy → Intervals → DP → Math
Each problem links back to the concept page for theory.