🧩 100 Problems to Master a Language

75 algorithm problems (25 easy, 25 medium, 25 hard) plus 25 object-oriented design problems. Every problem is language-neutral: it reads standard input, writes standard output, and comes with exact expected outputs so you can check your own solution — no solutions are given.

1. Pick a problem and read the statement, input/output format and examples.
2. Write a program in any language that reads from stdin and prints to stdout. Templates →
3. Check it: paste your output into the page's checker, or run all tests at once with runner/run-tests.py.