@inex algorithms and math, in Russian:
http://neerc.ifmo.ru/wiki/index.php?title=%D0%97%D0%B0%D0%B3%D0%BB%D0%B0%D0%B2%D0%BD%D0%B0%D1%8F_%D1%81%D1%82%D1%80%D0%B0%D0%BD%D0%B8%D1%86%D0%B0

algorithms, in Russian: http://e-maxx.ru/

algorithmic problems: https://codeforces.com/