Pattern-first · self-paced · judge-backed

Learn DSA by solving the right problems in the right order.

Build pattern recognition across arrays, hashing, two pointers, and sliding windows. Each live problem gives you a clear statement, progressive hints, examples, and a real grading suite.

12parts
21patterns
163problems
148live now
~14hest. time
Start the course1.1 — How to read Big-O
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Launch curriculum

IFoundationsBeginner → Intermediate1 problems

1Foundations

1 problems
1.1How to read Big-Olesson · soon
1.2Counting operations, amortized costlesson · soon
1.3Recursion: the call stack, visuallylesson · soon
1.4Recursion patterns: reduce, divide, explorelesson · soon
1.5Bubble, insertion, selection — visuallylesson · soon
1.6Merge sortlesson · soon
1.7Quick sort & partitioninglesson · soon
1.8Heap sortlesson · soon
#912Sort an Arraymediumopen →
IIArrays & HashingBeginner → Intermediate15 problems

1Arrays & Hashing

9 problems
1.1Hashing: trade space for speedlesson · soon
#217Contains Duplicateeasyopen →#242Valid Anagrameasyopen →#1Two Sumeasyopen →
DSAGroup Anagramssoon
#347Top K Frequent Elementsmediumopen →#238Product of Array Except Selfmediumopen →
DSAEncode & Decode Stringssoon
#128Longest Consecutive Sequencemediumopen →#36Valid Sudokumediumopen →

2Matrices

3 problems
#54Spiral Matrixmediumopen →#48Rotate Imagemediumopen →#73Set Matrix Zeroesmediumopen →

3Prefix Sums

3 problems
3.1Prefix sums: precompute once, answer fastlesson · soon
#560Subarray Sum Equals Kmediumopen →#2062Count Vowel Substringsmediumopen →
DSARange Sum Query — Immutablesoon
IIITwo Pointers & Sliding WindowIntermediate → Advanced17 problems

1Two Pointers

9 problems
1.1Opposite-ends two pointerslesson · soon
#167Two Sum IImediumopen →#125Valid Palindromeeasyopen →#153Summediumopen →#11Container With Most Watermediumopen →#611Valid Triangle Numbermediumopen →#283Move Zeroeseasyopen →#75Sort Colorsmediumopen →#42Trapping Rain Waterhardopen →#88Merge Sorted Arrayeasyopen →

2Sliding Window

8 problems
2.1Fixed-size windowslesson · soon
2.2Variable-size windowslesson · soon
DSAMaximum Sum Subarray of Size Keasyopen →#2461Maximum Sum of Distinct Subarrays With Length Kmediumopen →#1423Maximum Points From Cardsmediumopen →#3Longest Substring Without Repeating Charactersmediumopen →#424Longest Repeating Character Replacementmediumopen →#567Permutation in Stringmediumopen →#76Minimum Window Substringhardopen →#239Sliding Window Maximumhardopen →
IVBinary SearchBeginner → Advanced9 problems

1Binary Search

9 problems
1.1Halving the search spacelesson · soon
#704Binary Searcheasyopen →#74Search a 2D Matrixmediumopen →#153Find Minimum in Rotated Sorted Arraymediumopen →#33Search in Rotated Sorted Arraymediumopen →#875Koko Eating Bananasmediumopen →#1011Minimum Shipping Capacitymediumopen →#410Split Array Largest Sumhardopen →#378Kth Smallest in a Sorted Matrixmediumopen →
DSAFirst Bad Versionsoon
VStackBeginner → Advanced8 problems

1Stack

8 problems
1.1LIFO thinkinglesson · soon
#20Valid Parentheseseasyopen →
DSAMin Stacksoon
#150Evaluate Reverse Polish Notationmediumopen →#394Decode Stringmediumopen →
1.2Monotonic stackslesson · soon
#739Daily Temperaturesmediumopen →#84Largest Rectangle in Histogramhardopen →#32Longest Valid Parentheseshardopen →#853Car Fleetmediumopen →
VILinked ListsBeginner → Intermediate10 problems

1Linked Lists

10 problems
1.1Pointers on paperlesson · soon
#206Reverse Linked Listeasyopen →#21Merge Two Sorted Listseasyopen →
DSALinked List Cyclesoon
#234Palindrome Linked Listeasyopen →#19Remove Nth Node From Endmediumopen →#24Swap Nodes in Pairsmediumopen →#143Reorder Listmediumopen →#2Add Two Numbersmediumopen →
DSALRU Cachesoon
DSACopy List With Random Pointersoon
VIITrees & TrieBeginner → Advanced17 problems

1Trees

14 problems
1.1Depth-first search: go deep firstlesson · soon
1.2Tree traversal orderslesson · soon
1.3Passing values downlesson · soon
1.4Returning values uplesson · soon
#104Maximum Depth of Binary Treeeasyopen →#100Same Treeeasyopen →#226Invert Binary Treeeasyopen →#112Path Sumeasyopen →#113Path Sum IImediumopen →#543Diameter of Binary Treeeasyopen →#563Binary Tree Tilteasyopen →#687Longest Univalue Pathmediumopen →#236Lowest Common Ancestormediumopen →#98Validate Binary Search Treemediumopen →#124Binary Tree Maximum Path Sumhardopen →
DSASerialize & Deserialize Binary Treesoon
#230Kth Smallest in a BSTmediumopen →#105Construct Tree from Preorder and Inordermediumopen →

2Trie

3 problems
2.1Prefix treeslesson · soon
DSAImplement Triesoon
DSAPrefix Matchingsoon
DSADesign Add & Search Wordssoon
VIIIHeaps & IntervalsIntermediate → Advanced12 problems

1Heaps

7 problems
1.1Priority queueslesson · soon
#215Kth Largest Elementmediumopen →#973K Closest Pointsmediumopen →#658Find K Closest Elementsmediumopen →#621Task Schedulermediumopen →#23Merge K Sorted Listshardopen →
DSAFind Median From Data Streamsoon
#1046Last Stone Weighteasyopen →

2Intervals

5 problems
2.1Sorting by start/endlesson · soon
#56Merge Intervalsmediumopen →#57Insert Intervalmediumopen →#435Non-overlapping Intervalsmediumopen →#252Meeting Roomseasyopen →#253Meeting Rooms IImediumopen →
IXGraphsBeginner → Advanced25 problems

1Graph DFS & Grids

6 problems
1.1Graphs: nodes, edges, representationslesson · soon
1.2Building adjacency listslesson · soon
DSAClone Graphsoon
#261Graph Valid Treemediumopen →
1.3Grids as graphslesson · soon
#733Flood Filleasyopen →#200Number of Islandsmediumopen →#417Pacific Atlantic Water Flowmediumopen →#130Surrounded Regionsmediumopen →

2Breadth-First Search

9 problems
2.1Breadth-first: level by levellesson · soon
DSALevel Order Sumeasyopen →#103Zigzag Level Ordermediumopen →#199Rightmost Nodemediumopen →#662Maximum Widthmediumopen →#1197Minimum Knight Movesmediumopen →#994Rotting Orangesmediumopen →#54201 Matrixmediumopen →#815Bus Routeshardopen →#127Word Ladderhardopen →

3Weighted & Advanced Graphs

10 problems
3.1Union-Find, visuallylesson · soon
#323Connected Componentsmediumopen →#684Redundant Connectionmediumopen →#207Course Schedulemediumopen →#210Course Schedule IImediumopen →
3.2Dijkstra, Bellman-Ford, Floyd-Warshall — the maplesson · soon
#743Network Delay Timemediumopen →#787Cheapest Flights Within K Stopsmediumopen →#1631Path With Minimum Effortmediumopen →#1334Find the City With the Smallest Number of Neighborsmediumopen →#1584Min Cost to Connect All Pointsmediumopen →#269Alien Dictionaryhardopen →
XBacktracking & GreedyIntermediate → Advanced16 problems

1Backtracking

10 problems
1.1Choose, explore, un-chooselesson · soon
1.2Solution-space treeslesson · soon
#78Subsetsmediumopen →#46Permutationsmediumopen →#39Combination Summediumopen →#17Letter Combinationsmediumopen →#22Generate Parenthesesmediumopen →#131Palindrome Partitioningmediumopen →#79Word Searchmediumopen →#51N-Queenshardopen →#90Subsets IImediumopen →
DSAWord Search IIsoon

2Greedy

6 problems
2.1When local choices are globally rightlesson · soon
#121Best Time to Buy and Sell Stockeasyopen →#55Jump Gamemediumopen →#45Jump Game IImediumopen →#134Gas Stationmediumopen →#763Partition Labelsmediumopen →#846Hand of Straightsmediumopen →
XIDynamic ProgrammingIntermediate → Advanced23 problems

1Dynamic Programming

23 problems
1.1Memoization: never solve twicelesson · soon
1.2From recursion to tablelesson · soon
#70Climbing Stairseasyopen →#746Min Cost Climbing Stairseasyopen →#198House Robbermediumopen →#213House Robber IImediumopen →#53Maximum Subarraymediumopen →#338Counting Bitseasyopen →#62Unique Pathsmediumopen →#322Coin Changemediumopen →#518Coin Change IImediumopen →#91Decode Waysmediumopen →#139Word Breakmediumopen →#300Longest Increasing Subsequencemediumopen →#1143Longest Common Subsequencemediumopen →#5Longest Palindromic Substringmediumopen →#647Palindromic Substringsmediumopen →#416Partition Equal Subset Summediumopen →#494Target Summediumopen →#72Edit Distancehardopen →#221Maximal Squaremediumopen →#256Paint Housemediumopen →#265Paint House IIhardopen →#727Minimum Window Subsequencehardopen →#1235Maximum Profit in Job Schedulinghardopen →
XIIBit Manipulation & MathBeginner → Intermediate10 problems

1Bit Manipulation

5 problems
1.1Bits: AND, OR, XOR, shiftslesson · soon
#136Single Numbereasyopen →#191Number of 1 Bitseasyopen →#190Reverse Bitseasyopen →#268Missing Numbereasyopen →#371Sum of Two Integersmediumopen →

2Math & Geometry

5 problems
#202Happy Numbereasyopen →#66Plus Oneeasyopen →#50Pow(x, n)mediumopen →#7Reverse Integermediumopen →#43Multiply Stringsmediumopen →