Problem Forge
Problem Forge is one corner of a large world of programming puzzles and contests. If you enjoy it, you may enjoy these too. Most are free to use; some offer paid extras.
Puzzle events and puzzle series
- Advent of Code: a puzzle a day in December, since 2015 (12 days a year since 2025). Each puzzle has a personal input and two parts.
- Everybody Codes: a yearly puzzle event in the same spirit, held each autumn, with extra stories during the year.
- Hanukkah of Data: data puzzles for the eight nights of Hanukkah; the 2022 and 2023 sets stay online.
- The Weekly Challenge: two small tasks a week, in any language (it began as the Perl Weekly Challenge).
- i18n puzzles: puzzles about text encodings, time zones and other traps of international software; a one-time 2025 event, kept online.
- IBM Research: Ponder This: a monthly puzzle, many of which need a program.
- Jane Street puzzles: a monthly logic or math puzzle.
- Advent.js: a December series of JavaScript, TypeScript and Python challenges, in Spanish and English.
Math, science and craft
- Project Euler: over a thousand problems where math and a short program meet. The classic of the genre.
- Rosalind: bioinformatics problems, from counting DNA bases to genome assembly.
- Cryptopals: eight sets of exercises in breaking real cryptography.
- CodeAbbey: graded problems for learners.
- Rosetta Code: the same tasks solved in hundreds of languages; good for comparing ideas.
- Pytudes: Peter Norvig's notebooks of puzzles solved in Python, as worked examples.
Practice and interview sites
- LeetCode: a large problem set with weekly contests; widely used to prepare for interviews.
- HackerRank: practice tracks by topic and language.
- Codewars: short "kata" written by the community, with ranks.
- Exercism: free exercises in dozens of languages, with human mentoring.
- HackerEarth, InterviewBit, GeeksforGeeks: more practice sets, many aimed at interviews.
- Codility lessons: free lessons and practice tasks.
- NeetCode: a guided roadmap through the classic interview problems, with videos.
- Edabit and CheckiO: gentle starts, with games and puzzles for beginners.
Contests and online judges
- Codeforces: frequent timed contests and a huge archive; its Gym holds past contests.
- AtCoder: weekly contests from Japan, known for clean problems.
- CodeChef: a long-running contest site with weekly rounds.
- Kattis, SPOJ, UVa Online Judge, DMOJ, Timus, beecrowd, Aizu Online Judge, LightOJ and Toph: archives of contest problems you can solve at any time.
- Luogu (Chinese): a large judge with a big community.
- CSES Problem Set: a well-ordered set of classic algorithm problems.
- USACO and the USACO Guide: the USA Computing Olympiad and a free guide built around it.
- ICPC: the International Collegiate Programming Contest, for university teams of three. Regional contests lead to a yearly World Finals. The ICPC Problem Archive on Kattis holds the problems of recent World Finals; older finals and many regionals are on Kattis and in the Codeforces Gym.
- IOI: the International Olympiad in Informatics, for secondary-school students, held every year since 1989; IOI 2026 was held in Tashkent in August. Its site has the syllabus and the yearly IOI Journal, and IOI statistics has every result since the first olympiad.
- Past olympiad tasks: oj.uz holds IOI tasks from 2007 on and many national olympiads; Yandex Contest has IOI 2003 to 2018; DMOJ has a selection going further back. The USACO Guide's olympiads page maps these archives and the national olympiads (CEOI, Baltic OI, JOI, APIO and others).
- Meta Hacker Cup: a yearly open contest.
Hard problems and optimisation
The closest relatives of Problem Forge: contests where the problems are NP-hard and the best answer wins.
- AtCoder Heuristic Contest: regular contests on optimisation problems with no known fast exact method.
- Al Zimmermann's Programming Contests: months-long contests to find the best answers anyone can to hard number and puzzle problems; one contest at a time, with gaps between.
- PACE: a yearly challenge to implement algorithms for hard graph problems; the 2019 task was vertex cover, and recent years brought dominating set (2025) and agreement forests (2026).
- SAT Competition and MaxSAT Evaluations: where SAT solvers are compared each year.
- MiniZinc Challenge: a yearly comparison of constraint solvers.
- DIMACS Implementation Challenges: challenges on cliques, colouring, satisfiability, the travelling salesman problem and more; the 13th, on network flows, runs in 2026-27.
- ROADEF/EURO Challenge: industrial optimisation problems set by companies.
- OPTIL.io: optimisation problems with an online judge.
- Kaggle competitions: mostly data science, but the yearly Santa contests are large optimisation puzzles.
Code golf
- code.golf: solve each hole in as few bytes as possible, in many languages.
- Code Golf Stack Exchange: golf and other programming puzzles, set by the community.
- Anarchy Golf: a long-running golf site.
Programming games
- CodinGame: puzzles and bot battles in the browser.
- Battlesnake: write a snake that plays against others.
- Screeps: a strategy game you play by writing JavaScript.
- Robocode: program tanks to battle; its newer version is Robocode Tank Royale.
- Elevator Saga: program the elevators of a building.
- Untrusted: edit the game's own code to escape each level.
- Bitburner: an idle game you play by writing scripts.
- Lux AI: bot contests held on Kaggle, 2021-2025.
- Zachtronics games (TIS-100, Shenzhen I/O, Opus Magnum, Exapunks), Human Resource Machine and The Farmer Was Replaced: puzzle games built on programming. Zachtronics no longer makes new games, but its games are still sold.
Puzzles without a computer
- Simon Tatham's Portable Puzzle Collection: small logic puzzles to play in the browser; several of its puzzle types are NP-complete in general.
- KenKen: arithmetic grid puzzles.
Security challenges
Legal practice grounds for security skills; play only where you are invited.
- picoCTF: Carnegie Mellon's free security practice site (now called CyLab Security Academy) and its yearly student contest, picoCTF.
- OverTheWire: "wargames" played over SSH, from beginner to hard.
- CTFtime: a calendar and rankings for capture-the-flag contests.
- Hack The Box and TryHackMe: guided labs and challenges; some free, most paid.
Communities
Places to talk about hard problems, including forums and Discord servers, are listed on the Discuss page.
No longer active
Sites that were once well known for challenges but have closed or moved on. They are listed so you know what became of them.
- Topcoder: retired as a contest site. It ran algorithm contests from 2001 (the last in May 2024) and Marathon Matches until September 2025; it is now a freelance marketplace.
- Baekjoon Online Judge: closed. A large Korean judge from 2010 until 28 April 2026; its home page now shows a closure notice.
- CodeSignal: inactive as a challenge site. It is now mainly a hiring and assessment tool; its learning side is CodeSignal Learn.
- Coderbyte: inactive as a challenge site. It is now mainly a hiring tool; some challenges remain on its developer pages.