#1Easy
Two Sum
Return the indices of two numbers whose sum matches a target.
Practice
Coding Challenges
Solve Kotlin interview problems in a focused workspace with a timer and AI-powered review after every submission.
#2Easy
Valid Parentheses
Check whether a string of brackets is balanced.
#3Easy
Merge Two Sorted Lists
Merge two sorted linked lists into one sorted list.
#4Easy
Best Time to Buy and Sell Stock
Find the maximum profit from one buy and one sell.
#5Easy
Contains Duplicate
Detect whether any value appears at least twice.
#6Medium
Product of Array Except Self
Build an output array where each index holds the product of all other elements.
#7Medium
Maximum Subarray
Find the contiguous subarray with the largest sum.
#8Medium
Maximum Product Subarray
Find the contiguous subarray with the maximum product.
#9Medium
Find Minimum in Rotated Sorted Array
Find the minimum value in a rotated sorted array with unique values.
#10Medium
Search in Rotated Sorted Array
Search a target in a rotated sorted array.
#11Medium
3Sum
Find all unique triplets that sum to zero.
#12Medium
Container With Most Water
Choose two lines that hold the most water.
#13Medium
Sum of Two Integers
Compute a + b without using + or -.
#14Easy
Number of 1 Bits
Count how many set bits appear in an integer.
#15Easy
Counting Bits
Return the bit count for every number from 0 to n.
#16Easy
Missing Number
Find the missing number from 0..n.
#17Easy
Reverse Bits
Reverse the bit order of a 32-bit unsigned integer.
#18Easy
Climbing Stairs
Count how many ways to climb to the top taking 1 or 2 steps.
#19Medium
Coin Change
Find the minimum number of coins to reach an amount.
#20Medium
Longest Increasing Subsequence
Return the length of the longest strictly increasing subsequence.
#21Medium
Longest Common Subsequence
Find the length of the longest common subsequence of two strings.
#22Medium
Word Break
Determine if a string can be segmented into dictionary words.
#23Medium
Combination Sum
Return all combinations of candidates that sum to a target.
#24Medium
House Robber
Maximize robbed money without taking adjacent houses.
#25Medium
House Robber II
House Robber with houses arranged in a circle.
#26Medium
Decode Ways
Count how many ways a digit string can be decoded.
#27Medium
Unique Paths
Count robot paths from top-left to bottom-right in a grid.
#28Medium
Jump Game
Determine if you can reach the last index.
#29Medium
Clone Graph
Deep-copy a connected graph.
#30Medium
Course Schedule
Check if all courses can be completed given prerequisites.
#31Medium
Pacific Atlantic Water Flow
Find all cells that can flow to both oceans.
#32Medium
Number of Islands
Count how many connected land masses exist in a grid.
#33Medium
Longest Consecutive Sequence
Find the longest run of consecutive integers.
#34Hard
Alien Dictionary
Infer a character ordering from a sorted alien dictionary.
#35Medium
Graph Valid Tree
Check whether an undirected graph is a tree.
#36Medium
Number of Connected Components in an Undirected Graph
Count connected components in an undirected graph.
#37Medium
Insert Interval
Insert a new interval into sorted non-overlapping intervals.
#38Medium
Merge Intervals
Merge all overlapping intervals.
#39Medium
Non-overlapping Intervals
Remove the minimum number of intervals to eliminate overlaps.
#40Easy
Meeting Rooms
Check if a person can attend all meetings.
#41Medium
Meeting Rooms II
Compute the minimum number of meeting rooms required.
#42Easy
Reverse Linked List
Reverse a singly linked list.
#43Easy
Linked List Cycle
Detect whether a linked list contains a cycle.
#44Hard
Merge k Sorted Lists
Merge many sorted linked lists into one sorted list.
#45Medium
Remove Nth Node From End of List
Remove the nth node from the end in one pass.
#46Medium
Reorder List
Reorder a list by alternating front and back nodes.
#47Medium
Set Matrix Zeroes
Set rows and columns to zero when a cell is zero.
#48Medium
Spiral Matrix
Return matrix elements in spiral order.
#49Medium
Rotate Image
Rotate an n x n matrix 90 degrees clockwise in place.
#50Medium
Word Search
Find whether a word exists in a board by adjacent traversal.
#51Medium
Longest Substring Without Repeating Characters
Find the longest substring with all unique characters.
#52Medium
Longest Repeating Character Replacement
Maximize repeating characters by replacing at most k characters.
#53Hard
Minimum Window Substring
Find the smallest substring containing all target characters.
#54Easy
Valid Anagram
Check whether two strings are anagrams.
#55Medium
Group Anagrams
Group words that are anagrams of each other.
#56Easy
Valid Palindrome
Check if a string is a palindrome ignoring punctuation and case.
#57Medium
Palindromic Substrings
Count all palindromic substrings.
#58Medium
Longest Palindromic Substring
Find the longest palindromic substring.
#59Medium
Encode and Decode Strings
Encode a list of strings to one string and decode it back.
#60Easy
Maximum Depth of Binary Tree
Return the height of a binary tree.
#61Easy
Same Tree
Check whether two binary trees are structurally identical.
#62Easy
Invert Binary Tree
Swap left and right children throughout a tree.
#63Hard
Binary Tree Maximum Path Sum
Find the maximum path sum anywhere in a binary tree.
#64Medium
Binary Tree Level Order Traversal
Traverse a tree level by level.
#65Hard
Serialize and Deserialize Binary Tree
Convert a binary tree to a string and back.
#66Easy
Subtree of Another Tree
Check whether one tree is a subtree of another.
#67Medium
Construct Binary Tree from Preorder and Inorder Traversal
Reconstruct a binary tree from preorder and inorder traversals.
#68Medium
Validate Binary Search Tree
Check whether a binary tree satisfies BST rules.
#69Medium
Kth Smallest Element in a BST
Find the kth smallest value in a BST.
#70Easy
Lowest Common Ancestor of a Binary Search Tree
Find the lowest common ancestor of two nodes in a BST.
#71Medium
Top K Frequent Elements
Return the k most frequent elements.
#72Hard
Find Median from Data Stream
Maintain a running median as numbers are inserted.
#73Medium
K Closest Points to Origin
Return the k points closest to the origin.
#74Medium
Task Scheduler
Schedule tasks with cooling intervals.
#75Medium
Implement Trie (Prefix Tree)
Build a trie supporting insert, search, and prefix queries.
#76Medium
Add and Search Word
Trie with wildcard search.
#77Hard
Word Search II
Find all dictionary words present in a board.
#78Hard
Word Ladder
Find the shortest transformation sequence length between words.
#79Medium
Maximum XOR of Two Numbers in an Array
Find the maximum XOR value of any pair in the array.
#80Medium
Find the Duplicate Number
Find the repeated number without modifying the array.
#81Hard
Sliding Window Maximum
Return the maximum in every window of size k.
#82Medium
Min Stack
Design a stack that can return the minimum in constant time.
#83Medium
Evaluate Reverse Polish Notation
Evaluate an arithmetic expression in postfix notation.
#84Medium
Generate Parentheses
Generate all valid parenthesis strings of length 2n.
#85Medium
Daily Temperatures
Find how many days until a warmer temperature.
#86Medium
Car Fleet
Count how many car fleets reach the target.
#87Hard
Largest Rectangle in Histogram
Find the largest rectangle area in a histogram.
#88Medium
Search a 2D Matrix
Search for a target in a row-wise sorted matrix.
#89Medium
Time Based Key-Value Store
Store values by timestamp and query the latest valid one.
#90Hard
Median of Two Sorted Arrays
Find the median of two sorted arrays in logarithmic time.