Coding interview solutions & explanations

Free, worked solutions to 279 coding-interview problems — each with the problem statement, the pattern to recognize, a clear approach, runnable Python and Java code, and complexity analysis. Then practice every one in a real in-browser editor with instant test feedback.

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Easy (94)

EValid Parentheses
EReverse a Linked List
EMerge Two Sorted Lists
EBest Time to Buy and Sell Stock
EClimbing Stairs
EValid Anagram
EBinary Search
EPalindrome Number
ELinked List Cycle
EInvert Binary Tree
EMaximum Depth of Binary Tree
EContains Duplicate
EMissing Number
EPlus One
ESingle Number
EMajority Element
EMove Zeros
EReverse String
EValid Palindrome
EFizz Buzz
EInteger Square Root
ECounting Bits
ENumber of 1 Bits
EReverse Bits
EHappy Number
ERoman to Integer
EMin Cost Climbing Stairs
ESearch Insert Position
EFirst Bad Version
EIntersection of Two Arrays
ELongest Palindrome (Letters)
ESame Tree
ESubtree of Another Tree
EDiameter of Binary Tree
EBalanced Binary Tree
EConvert Sorted Array to BST
EPalindrome Linked List
EMiddle of the Linked List
EPower of Two
EAdd Binary
ERemove Element
ERemove Duplicates from Sorted Array
ELast Stone Weight
ESymmetric Tree
EPath Sum
ESearch in a Binary Search Tree
ERange Sum of BST
EFibonacci Number
EHamming Distance
EPower of Four
ELemonade Change
EMeeting Rooms
EFlood Fill
EIsland Perimeter
ETwo Sum IV — BST
EConvert Sorted Array to BST
EClosest BST Value
EFind Mode in BST
ENext Greater Element I
EBaseball Game
EBackspace String Compare
ERemove Adjacent Duplicate Letters
EGuess Number Higher or Lower
EValid Perfect Square
EArranging Coins
EFind Smallest Letter Greater Than Target
ERemove Linked List Elements
EIntersection of Two Linked Lists
ESum of Left Leaves
EAverage of Levels in Binary Tree
EMinimum Depth of Binary Tree
EBinary Tree Paths
EFind the Town Judge
ESummary Ranges
EPower of Three
EComplement of Base 10 Integer
EAssign Cookies
ECan Place Flowers
EKth Largest Element in a Stream
EDesign HashMap
EDesign HashSet
EDesign Stack (Array-backed)
EDesign Queue Using Stacks
EComparable — Sort Employees by Salary
EDesign Parking System
ENumber of Recent Calls
EFinal Prices With a Special Discount
ESelect All Customers
EDistinct Countries
EActive Users in 2025
ECount Orders Per Customer
ECustomers With No Orders
EEmployees Earning More Than Managers
EDuplicate Emails

Medium (170)

MMaximum Subarray
MContainer With Most Water
M3Sum
MLongest Substring Without Repeating Characters
MGroup Anagrams
MTop K Frequent Words
MSort Array by Increasing Frequency
MDegree of an Array
MFind All Duplicates in an Array
MH-Index
MValid Sudoku
MSubarray Sum Equals K
M4Sum II
MProduct of Array Except Self
MNumber of Islands
MCoin Change
MHouse Robber
MLongest Common Subsequence
MTop K Frequent Elements
MDaily Temperatures
MGenerate Parentheses
MSubsets
MSearch in Rotated Sorted Array
MMin Stack
MTwo Sum II — Sorted
MPow(x, n)
MUnique Paths
MJump Game
MJump Game II
MGas Station
MMerge Intervals
MInsert Interval
MNon-overlapping Intervals
MLongest Increasing Subsequence
MWord Break
MLongest Repeating Character Replacement
MPermutations
MCombination Sum
MLetter Combinations of a Phone Number
MSort Colors (Dutch Flag)
MDecode Ways
MMaximum Product Subarray
MFind the Duplicate Number
MKth Largest Element in an Array
MMinimum Path Sum
MEvaluate Reverse Polish Notation
MWord Search
MValidate Binary Search Tree
MLowest Common Ancestor of a BST
MBinary Tree Level Order Traversal
MBinary Tree Right Side View
MCount Good Nodes in Binary Tree
MKth Smallest in BST
MRemove Nth Node from End
MAdd Two Numbers
MReorder List
MLinked List Cycle II
MClone Graph
MCourse Schedule
MPacific Atlantic Water Flow
MImplement Trie (Prefix Tree)
MPartition Equal Subset Sum
MLongest Palindromic Substring
MString to Integer (atoi)
MReverse Integer
MRotate Array
MPermutation in String
MRotate Image
MSpiral Matrix
MSet Matrix Zeros
MCombinations
MSum of Two Integers
MLongest Consecutive Sequence
MPartition Labels
MFind Minimum in Rotated Sorted Array
MRotting Oranges
MCourse Schedule II
MHouse Robber II
MPalindromic Substrings
MMeeting Rooms II
MK Closest Points to Origin
MSubsets II
MCombination Sum II
MInsert into a BST
MSearch a 2D Matrix
MKoko Eating Bananas
MFind Peak Element
MDecode String
MCar Fleet
MMin Arrows to Burst Balloons
MTask Scheduler
MFind All Anagrams in a String
MGray Code Sequence
MPalindrome Partitioning
MInterval List Intersections
MTwo City Scheduling
MCar Pooling
MQueue Reconstruction by Height
MMax Area of Island
MSurrounded Regions
MNumber of Provinces
MMin Add to Make Parens Valid
MRemove K Digits
MOnline Stock Span
MAsteroid Collision
MBasic Calculator II
MScore of Parentheses
MValid Parenthesis String
MSimplify Path
MPeak Index in a Mountain Array
MSingle Element in Sorted Array
MCapacity to Ship Packages in D Days
MMinimum Days to Make M Bouquets
MFind the Smallest Divisor Given a Threshold
MH-Index II
MFind K Closest Elements
MSwap Nodes in Pairs
MOdd Even Linked List
MLowest Common Ancestor of a Binary Tree
MDelete Node in a BST
MUnique Binary Search Trees
MConvert BST to Greater Tree
MTarget Sum
MCombinations
MPermutations II
MRestore IP Addresses
MKeys and Rooms
MWalls and Gates
MRemove Covered Intervals
MFind Right Interval
MTotal Hamming Distance
MWiggle Subsequence
MHand of Straights
MSort Characters by Frequency
MReorganize String
MMinimum Size Subarray Sum
MFruit Into Baskets
MDesign Linked List
MDesign Circular Queue
Mequals/hashCode — Fraction Class
MDesign Time-Based Key-Value Store
MSum of Subarray Minimums
MNext Greater Element II
MDesign Browser History
MDesign Underground System
MLRU Cache
MDesign Twitter
MMax Consecutive Ones III
MSubarray Product Less Than K
MMaximum Sum Circular Subarray
MNumber of Closed Islands
MShortest Path in Binary Matrix
MCount K-Digit Combinations
MPartition to K Equal Sum Subsets
MSecond Highest Salary
MNth Highest Salary
MRank Scores (Dense Rank)
MThree Consecutive Logins
MDelete Duplicate Emails
MDepartment Top Salary
MRunning Total of Daily Sales
M7-Day Moving Average
MProduct Sales by Year
MExchange Seats
MTree Node Types
MFriend Requests Acceptance Rate
MPivot Daily Sales by Region
MMonth-Over-Month Revenue Change
MFind Symmetric Pairs
MProducts Frequently Bought Together

Hard (15)

HMinimum Window Substring
HTrapping Rain Water
HMerge K Sorted Lists
HBasic Calculator
HLargest Rectangle in Histogram
HSplit Array Largest Sum
HN-Queens
HFind Median from Data Stream
HTop 3 Salaries Per Department
HTrips and Cancellation Rate
HMedian Employee Salary Per Department
HCumulative Distinct Customers
HFind the Longest Streak Per User
HOrg Chart Depth
HP90 Response Time
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