Kattis
Solve graded problems from a large contest archive
Kattis is a problem archive with an online judge. You pick a problem, write a program and submit it, and the judge tells you right away if it passes. Each problem has a difficulty score, from trivial to hard, and solving it adds points to your total.
Free · Login needed · Browser · Intermediate
About Kattis
It trains algorithm and problem-solving skills with many problems from past contests. Ranklists compare users, universities and countries. It suits students preparing for programming contests and anyone who wants a big, well-graded set of problems.
The archive holds thousands of problems and accepts a long list of languages, from C++, Python and Java to older ones like COBOL and Fortran. You need to register to submit. The site also hosts contests.
On Breakpoint since 23 Oct 2026.