https://a2oj.com
http://projects.csail.mit.edu/films/aifilms/AIFilms.html
https://www.tutorialspoint.com/index.htm
https://www.kaggle.com/
https://www.hackerrank.com
http://codeforces.com/
https://www.topcoder.com/
https://www.codechef.com/
for mathematics
https://projecteuler.net/