HARD Lab · Blind 75

Interactive algorithm study modules

One module per problem, each with five tabs — a concept crash course, a step-through visualizer you can run on your own input, annotated TypeScript, flashcards, and a quiz that targets the real failure points. Pick a problem and drill it.

0 / 77 attempted
Arrays & Hashing
#1● Easy
Two Sum
Hash MapComplement Trick
#121● Easy
Best Time to Buy and Sell Stock
GreedyOne Pass
#217● Easy
Contains Duplicate
Hash SetSeen-so-far
#238● Medium
Product of Array Except Self
Prefix/Suffix Products
#128● Medium
Longest Consecutive Sequence
Hash Set
Two Pointers
#15● Medium
3Sum
Two PointersSorting
#11● Medium
Container With Most Water
Two PointersGreedy
Binary Search
#153● Medium
Find Minimum in Rotated Sorted Array
Binary Search
#33● Medium
Search in Rotated Sorted Array
Binary Search
Sliding Window
#3● Medium
Longest Substring Without Repeating Characters
Sliding WindowHash Set
#424● Medium
Longest Repeating Character Replacement
Sliding Window
#76● Hard
Minimum Window Substring
Sliding WindowHash Map
#239● Hard
Sliding Window Maximum
Monotonic DequeSliding Window
Strings
#20● Easy
Valid Parentheses
Stack
#242● Easy
Valid Anagram
Hash Map / CountingFrequency
#125● Easy
Valid Palindrome
Two Pointers
#49● Medium
Group Anagrams
Hash MapFrequency Signature
#271● Medium
Encode and Decode Strings
DesignLength Prefix
#5● Medium
Longest Palindromic Substring
Expand Around Center
#647● Medium
Palindromic Substrings
Expand Around Center
Binary
#371● Medium
Sum of Two Integers
Bit Manipulation
#191● Easy
Number of 1 Bits
Bit Manipulation
#338● Easy
Counting Bits
DPBit Manipulation
#268● Easy
Missing Number
Bit ManipulationMath
#190● Easy
Reverse Bits
Bit Manipulation
Dynamic Programming
#70● Easy
Climbing Stairs
DPFibonacci
#322● Medium
Coin Change
DPUnbounded Knapsack
#53● Medium
Maximum Subarray
Kadane
#152● Medium
Maximum Product Subarray
KadaneMin/Max Tracking
#300● Medium
Longest Increasing Subsequence
DPPatience Sorting
#1143● Medium
Longest Common Subsequence
2D DP
#139● Medium
Word Break
DPHash Set
#198● Medium
House Robber
DP
#213● Medium
House Robber II
DPCircular Array
#91● Medium
Decode Ways
DPString
#62● Medium
Unique Paths
2D DPCombinatorics
#55● Medium
Jump Game
Greedy
#72● Hard
Edit Distance
2D Dynamic ProgrammingString Alignment
Backtracking
#39● Medium
Combination Sum
BacktrackingDecision Tree
#79● Medium
Word Search
BacktrackingMatrix DFS
Graphs
#200● Medium
Number of Islands
Flood FillBFS/DFS
#133● Medium
Clone Graph
BFS/DFSHash Map
#417● Medium
Pacific Atlantic Water Flow
Multi-source DFS/BFS
#207● Medium
Course Schedule
Topological SortCycle Detection
#269● Hard
Alien Dictionary
Topological SortBFS (Kahn)
#261● Medium
Graph Valid Tree
Union-FindConnectivity
#323● Medium
Number of Connected Components in an Undirected Graph
Union-Find
Intervals
#57● Medium
Insert Interval
IntervalsMerge
#56● Medium
Merge Intervals
SortingIntervals
#435● Medium
Non-overlapping Intervals
GreedyIntervals
#252● Easy
Meeting Rooms
SortingIntervals
#253● Medium
Meeting Rooms II
Sweep LineTwo Pointers
Linked List
#206● Easy
Reverse Linked List
Linked ListPointer Rewiring
#141● Easy
Linked List Cycle
Fast & Slow Pointers
#21● Easy
Merge Two Sorted Lists
Linked ListMergeDummy Head
#23● Hard
Merge k Sorted Lists
Divide & ConquerHeapMerge
#19● Medium
Remove Nth Node From End of List
Two PointersDummy Head
#143● Medium
Reorder List
Linked ListFast & Slow PointersReverse
Matrix
#73● Medium
Set Matrix Zeroes
MatrixIn-place Markers
#54● Medium
Spiral Matrix
MatrixBoundary Simulation
#48● Medium
Rotate Image
MatrixIn-place Transpose
Trees
#104● Easy
Maximum Depth of Binary Tree
DFSBFS
#100● Easy
Same Tree
DFSTree Comparison
#226● Easy
Invert Binary Tree
DFSBFS
#124● Hard
Binary Tree Maximum Path Sum
DFSPost-order
#102● Medium
Binary Tree Level Order Traversal
BFSQueue
#297● Hard
Serialize and Deserialize Binary Tree
DFSPreorder
#572● Easy
Subtree of Another Tree
DFSTree Matching
#105● Medium
Construct Binary Tree from Preorder and Inorder Traversal
Divide & ConquerHash Map
#98● Medium
Validate Binary Search Tree
DFSValid BST Bounds
#230● Medium
Kth Smallest Element in a BST
Inorder TraversalBST
#235● Medium
Lowest Common Ancestor of a Binary Search Tree
BST PropertyTree Walk
Tries
#208● Medium
Implement Trie (Prefix Tree)
TriePrefix Tree
#211● Medium
Design Add and Search Words Data Structure
TrieDFS Wildcard
#212● Hard
Word Search II
TrieBacktrackingMatrix DFS
Heap / Priority Queue
#347● Medium
Top K Frequent Elements
Bucket SortHash MapHeap
#295● Hard
Find Median from Data Stream
Two HeapsBalanced Partition