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  • Course and AI tutor169 plain-English lessons on system design, low-level design and SQL. Free to read, with an AI tutor.
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Problems

Practice problems

Algorithms and data structures, one problem at a time. Free for everyone.

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All 265 coding problems

Easy (66)

  • Two Sum
  • Best Time to Buy and Sell Stock
  • Number of 1 Bits (Count the Set Bits)
  • Missing Number
  • Reverse Bits
  • Climbing Stairs
  • Diameter of Binary Tree
  • Balanced Binary Tree
  • Symmetric Tree
  • Minimum Absolute Difference in BST
  • Convert Sorted Array to Binary Search Tree
  • Merge Sorted Array
  • Find All Numbers Disappeared in an Array
  • Implement Queue using Stacks
  • Implement Stack using Queues
  • Pascal's Triangle
  • Pascal's Triangle II
  • Plus One
  • Add Binary
  • Design HashMap
  • Design HashSet
  • Valid Palindrome II (One Deletion Allowed)
  • Reverse String
  • Length of Last Word
  • Fizz Buzz
  • Greatest Common Divisor of Strings
  • Kids With the Greatest Number of Candies
  • Can Place Flowers (Can We Plant More Flowers)
  • Reverse Linked List
  • Invert Binary Tree
  • Maximum Depth of Binary Tree
  • Same Tree
  • Subtree of Another Tree
  • Valid Palindrome
  • Last Stone Weight
  • Kth Largest Element in a Stream
  • Single Number
  • Valid Parentheses
  • Merge Two Sorted Lists
  • Path Sum
  • Roman to Integer
  • Palindrome Number
  • Sqrt(x)
  • Valid Perfect Square
  • First Bad Version
  • Search Insert Position
  • Majority Element
  • Move Zeroes
  • Remove Duplicates from Sorted Array
  • Power of Two
  • Power of Three
  • Palindrome Linked List
  • Remove Linked List Elements (Remove a Value)
  • Middle of the Linked List
  • Is Subsequence
  • Ransom Note (Build a Note from Magazine Letters)
  • Find Index of the First Occurrence in a String
  • Meeting Rooms
  • Contains Duplicate (Check for Repeated Numbers)
  • Contains Duplicate II (Repeat Within K Steps)
  • Happy Number
  • Isomorphic Strings
  • Word Pattern (Sentence Follows Pattern)
  • Longest Common Prefix
  • Counting Bits
  • Valid Anagram (Check If Two Words Are Anagrams)

Medium (163)

  • Find Minimum in Rotated Sorted Array
  • Search in Rotated Sorted Array
  • 3Sum
  • Container With Most Water (Two Walls)
  • Coin Change
  • Longest Increasing Subsequence
  • Longest Common Subsequence
  • Combination Sum
  • House Robber (Maximum Sales on a Street)
  • House Robber II (Max Sales on a Circular Street)
  • Count Good Nodes in Binary Tree
  • Sum of Two Integers
  • Binary Tree Right Side View
  • Delete Node in a BST
  • Insert into a Binary Search Tree
  • Inorder Successor in BST
  • All Nodes Distance K in Binary Tree
  • Find the Duplicate Number
  • Product of Array Except Self (Product of Others)
  • Maximum Subarray
  • Single Element in a Sorted Array (Lone Element)
  • Capacity to Ship Packages Within D Days
  • Minimum Number of Days to Make m Bouquets
  • Min Deletions to Make Frequencies Unique
  • Design Circular Queue
  • Design Circular Deque
  • Design Browser History
  • Flatten Nested List Iterator (Flatten a List)
  • Insert Delete GetRandom O(1) (Randomized Set)
  • Snapshot Array
  • Max Concatenated String With Unique Characters
  • Minimum Size Subarray Sum (Shortest Subarray)
  • Longest Subarray of 1s After Deleting One Element
  • Max Consecutive Ones III (Longest Run, K Flips)
  • Number of Provinces (Number of City Groups)
  • Maximum Width of Binary Tree
  • Find All Anagrams in a String (Anagram Positions)
  • String Compression
  • Increasing Triplet Subsequence (3 Rising Numbers)
  • Asteroid Collision (Colliding Asteroids)
  • Decode String
  • Online Stock Span (Stock Price Span)
  • Keys and Rooms (Visit All Rooms With Keys)
  • Equal Row and Column Pairs
  • Reorder List
  • Remove Nth Node From End of List
  • Lowest Common Ancestor of a Binary Search Tree
  • Validate Binary Search Tree
  • Kth Smallest Element in a BST
  • Construct Binary Tree from Preorder and Inorder
  • Implement Trie (Prefix Tree)
  • Design Add and Search Words Data Structure
  • Top K Frequent Elements
  • Longest Consecutive Sequence
  • Palindromic Substrings
  • Merge Intervals
  • Insert Interval
  • Non-overlapping Intervals
  • Number of Islands
  • Pacific Atlantic Water Flow (Water to Two Seas)
  • Surrounded Regions
  • Course Schedule
  • Course Schedule II
  • Rotting Oranges (Rotting Fruit in a Crate)
  • Edit Distance
  • LRU Cache
  • Daily Temperatures (Days Until a Warmer Day)
  • Car Fleet (Car Fleets to a Destination)
  • Min Cost to Connect All Points
  • Network Delay Time
  • Cheapest Flights Within K Stops
  • Permutations
  • Subsets
  • Word Search
  • Letter Combinations of a Phone Number
  • Palindrome Partitioning
  • K Closest Points to Origin
  • Task Scheduler
  • Design Twitter
  • Kth Largest Element in an Array
  • Subsets II
  • Combination Sum II
  • Permutations II
  • Min Stack
  • Evaluate Reverse Polish Notation
  • Generate Parentheses
  • Time Based Key-Value Store
  • Search a 2D Matrix
  • Koko Eating Bananas (Minimum Eating Speed)
  • Add Two Numbers
  • Binary Tree Level Order Traversal
  • Longest Substring Without Repeating Characters
  • Longest Palindromic Substring
  • Swap Nodes in Pairs
  • 3Sum Closest
  • Next Permutation
  • Rotate Image (Rotate Matrix 90 Degrees)
  • Spiral Matrix
  • Set Matrix Zeroes
  • Minimum Path Sum
  • Triangle (Triangle Minimum Path Sum)
  • Interleaving String
  • Path Sum II
  • Binary Tree Zigzag Level Order Traversal
  • Recover Binary Search Tree
  • Best Time to Buy and Sell Stock II
  • Rotate Array
  • Gas Station
  • Partition List
  • Reverse Linked List II
  • String to Integer (atoi)
  • Integer to Roman
  • Reverse Integer
  • Pow(x, n)
  • Find First and Last Position in Sorted Array
  • Find Peak Element
  • Peak Index in a Mountain Array
  • H-Index
  • Majority Element II
  • Sort Colors
  • Remove Duplicates from Sorted Array II
  • Boats to Save People (Rescue Boats)
  • Two Sum II - Input Array Is Sorted
  • Longest Repeating Character Replacement
  • Permutation in String
  • Subarray Sum Equals K
  • Maximal Square
  • Perfect Squares
  • Partition Equal Subset Sum
  • Target Sum
  • Graph Valid Tree
  • Number of Connected Components in a Graph
  • Redundant Connection (Find the Extra Link)
  • Accounts Merge (Merge Duplicate Accounts)
  • Valid Sudoku
  • Basic Calculator II
  • Multiply Strings
  • Integer Break (Split for Maximum Product)
  • Single Number II
  • Single Number III
  • Bitwise AND of Numbers Range
  • Odd Even Linked List (Group Odd and Even Nodes)
  • Sort List
  • Insertion Sort List
  • Rotate List
  • Split Linked List in Parts (Equal Parts)
  • Zigzag Conversion (Zigzag String Reading)
  • Reverse Words in a String
  • Simplify Path (Shorten a File Path)
  • Compare Version Numbers
  • Meeting Rooms II
  • Word Break (Can the Text Be Split into Words)
  • Decode Ways (Count Ways to Decode a Message)
  • Unique Paths (Count Robot Paths in a Grid)
  • Unique Paths II (Robot Paths with Blocked Cells)
  • Jump Game (Can You Reach the Last Stone)
  • Jump Game II (Fewest Jumps to the Last Stone)
  • Maximum Product Subarray
  • Remove K Digits (Smallest Number Left)
  • Group Anagrams (Words with Same Letters)
  • Best Time to Buy and Sell Stock with Cooldown
  • House Robber III (Max Sales on a Tree of Houses)
  • Coin Change II (Count the Ways to Make an Amount)

Hard (36)

  • First Missing Positive
  • Split Array Largest Sum (Minimize Largest Sum)
  • Word Ladder II (All Shortest Transformations)
  • LFU Cache
  • Vertical Order Traversal of a Binary Tree
  • Binary Tree Maximum Path Sum
  • Word Search II (Find Words in a Grid)
  • Find Median from Data Stream
  • Word Ladder
  • Merge k Sorted Lists
  • Trapping Rain Water
  • Largest Rectangle in Histogram
  • Median of Two Sorted Arrays
  • Regular Expression Matching
  • Minimum Window Substring
  • Sliding Window Maximum
  • N-Queens
  • Candy (Candy Distribution)
  • Longest Valid Parentheses
  • Distinct Subsequences
  • Burst Balloons (Maximum Coins)
  • Maximal Rectangle
  • Dungeon Game (Minimum Starting Health)
  • Wildcard Matching
  • Alien Dictionary
  • Reconstruct Itinerary (Rebuild the Flight Route)
  • Swim in Rising Water (Cross the Flooding Grid)
  • Longest Increasing Path in a Matrix
  • Sudoku Solver
  • Basic Calculator
  • Expression Add Operators
  • Text Justification
  • Word Break II (All Ways to Split the Text)
  • Contains Duplicate III (Close Numbers)
  • Best Time to Buy and Sell Stock III (Two Trades)
  • Best Time to Buy and Sell Stock IV (K Trades)
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