Practice

Every problem here runs against a real compiler, not a guess.

Actually compiled and executed — not pattern-matched against expected output — in JavaScript, TypeScript, Python, C#, Java, Go, Rust and C++, plus live SQL queries against a real database. If it would fail in a real interview, it fails here too. Looking for front-end/CSS practice instead of algorithms? Try UI Challenges.

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EasycpprustgojavatypescriptJSPYC#

FizzBuzz

Fundamentals

Illustration with sample data

EasycpprustgojavatypescriptJSPYC#

Reverse a String

Strings

EasycpprustgojavatypescriptJSPYC#

Valid Palindrome

Strings

EasycpprustgojavatypescriptJSPYC#

Find All Duplicates

Arrays · Hash Table

MediumcpprustgojavatypescriptJSPYC#

Flatten Array One Level

Arrays

EasycpprustgojavatypescriptJSPYC#

Count Vowels

Strings

MediumcpprustgojavatypescriptJSPYC#

Count Duplicate Characters

Strings · Hash Table

MediumcpprustgojavatypescriptJSPYC#

Count Duplicate Array Members

Arrays · Hash Table

MediumcpprustgojavatypescriptJSPYC#

Find Duplicate Character Positions

Strings · Hash Table

MediumcpprustgojavatypescriptJSPYC#

Find Duplicate Array Positions

Arrays · Hash Table

EasycpprustgojavatypescriptJSPYC#

Two Sum

Arrays · Hash Table

EasycpprustgojavatypescriptJSPYC#

Valid Parentheses

Strings · Stacks

MediumcpprustgojavatypescriptJSPYC#

Maximum Subarray

Arrays · Dynamic Programming

EasycpprustgojavatypescriptJSPYC#

Best Time to Buy and Sell Stock

Arrays · Dynamic Programming

EasycpprustgojavatypescriptJSPYC#

Climbing Stairs

Dynamic Programming

EasycpprustgojavatypescriptJSPYC#

Binary Search

EasycpprustgojavatypescriptJS

Missing Number

Arrays · Math

MediumcpprustgojavatypescriptJS

Longest Substring Without Repeating Characters

Strings · Sliding Window

EasycpprustgojavatypescriptJS

Contains Duplicate

Arrays · Hash Table

EasycpprustgojavatypescriptJS

Valid Anagram

Strings · Hash Table

MediumcpprustgojavatypescriptJS

Product of Array Except Self

Arrays

MediumcpprustgojavatypescriptJS

Merge Intervals

Arrays · Sorting

MediumcpprustgojavatypescriptJS

Number of Islands

Graphs · BFS · DFS · Matrix

EasycpprustgojavatypescriptJS

Majority Element

Arrays

EasycpprustgojavatypescriptJS

Move Zeroes

Arrays · Two Pointers

EasycpprustgojavatypescriptJS

Single Number

Arrays · Bit Manipulation

MediumcpprustgojavatypescriptJS

Kth Largest Element in an Array

Arrays · Sorting · Heaps & Priority Queues

MediumcpprustgojavatypescriptJS

Rotate Array

Arrays

EasycpprustgojavatypescriptJSPYC#

Invert Binary Tree

Trees · Recursion

EasycpprustgojavatypescriptJSPYC#

Reverse Linked List

Linked Lists

EasycpprustgojavatypescriptJSPYC#

Linked List Cycle

Linked Lists · Two Pointers

MediumcpprustgojavatypescriptJSPYC#

Coin Change

Dynamic Programming

MediumcpprustgojavatypescriptJSPYC#

House Robber

Dynamic Programming

MediumcpprustgojavatypescriptJSPYC#

House Robber II

Dynamic Programming

MediumcpprustgojavatypescriptJSPYC#

Word Break

Dynamic Programming · Strings

MediumcpprustgojavatypescriptJSPYC#

Longest Increasing Subsequence

Dynamic Programming

MediumcpprustgojavatypescriptJSPYC#

Unique Paths

Dynamic Programming

MediumcpprustgojavatypescriptJSPYC#

Jump Game

Dynamic Programming · Greedy

MediumcpprustgojavatypescriptJSPYC#

Longest Common Subsequence

Dynamic Programming · Strings

EasycpprustgojavatypescriptJSPYC#

Maximum Depth of Binary Tree

Trees

EasycpprustgojavatypescriptJSPYC#

Same Tree

Trees

EasycpprustgojavatypescriptJSPYC#

Subtree of Another Tree

Trees

MediumcpprustgojavatypescriptJSPYC#

Validate Binary Search Tree

Trees · Binary Search Tree

MediumcpprustgojavatypescriptJSPYC#

Binary Tree Level Order Traversal

Trees · BFS

MediumcpprustgojavatypescriptJSPYC#

Kth Smallest Element in a BST

Trees · Binary Search Tree

MediumcpprustgojavatypescriptJSPYC#

Lowest Common Ancestor of a BST

Trees · Binary Search Tree

EasycpprustgojavatypescriptJSPYC#

Merge Two Sorted Lists

Linked Lists

MediumcpprustgojavatypescriptJSPYC#

Remove Nth Node From End of List

Linked Lists · Two Pointers

EasycpprustgojavatypescriptJSPYC#

Middle of the Linked List

Linked Lists · Two Pointers

EasycpprustgojavatypescriptJSPYC#

Palindrome Linked List

Linked Lists · Two Pointers

MediumcpprustgojavatypescriptJSPYC#

Course Schedule

Graphs · Topological Sort

MediumcpprustgojavatypescriptJSPYC#

Number of Connected Components in an Undirected Graph

Graphs · Union-Find

EasycpprustgojavatypescriptJSPYC#

Flood Fill

Graphs · DFS · Matrix

MediumcpprustgojavatypescriptJSPYC#

Rotting Oranges

Graphs · BFS · Matrix

MediumcpprustgojavatypescriptJSPYC#

Rotate Image

Matrix

MediumcpprustgojavatypescriptJSPYC#

Spiral Matrix

Matrix

MediumcpprustgojavatypescriptJSPYC#

Set Matrix Zeroes

Matrix

EasycpprustgojavatypescriptJSPYC#

Number of 1 Bits

Bit Manipulation

EasycpprustgojavatypescriptJSPYC#

Counting Bits

Bit Manipulation · Dynamic Programming

EasycpprustgojavatypescriptJSPYC#

Reverse Bits

Bit Manipulation

MediumcpprustgojavatypescriptJSPYC#

Sum of Two Integers

Bit Manipulation

MediumcpprustgojavatypescriptJSPYC#

3Sum

Arrays · Two Pointers

MediumcpprustgojavatypescriptJSPYC#

Container With Most Water

Arrays · Two Pointers

MediumcpprustgojavatypescriptJSPYC#

Group Anagrams

Arrays · Strings · Hash Table

MediumcpprustgojavatypescriptJSPYC#

Longest Palindromic Substring

Strings

MediumcpprustgojavatypescriptJSPYC#

Search in Rotated Sorted Array

Arrays · Binary Search

MediumcpprustgojavatypescriptJSPYC#

Top K Frequent Elements

Arrays · Hash Table · Heaps & Priority Queues

EasySQL

Select Every Sales Employee

SQL

EasySQL

Employee Count Per Department

SQL

EasySQL

Employees With No Salary

SQL

MediumSQL

Average Salary Per Department

SQL

MediumSQL

Employees Above Company Average

SQL

MediumSQL

Rank Employees By Department Salary

SQL · Window Functions

HardSQL

Employees Above Their Own Department's Average

SQL · Subqueries

EasySQL

Union Employee and Department Names

SQL

MediumSQL

Departments With More Than One Employee

SQL

MediumSQL

Department With Highest Average Salary

SQL