Years back I did a couple hundred problems on this site. It's very good at making you learn about important results in basic math and algorithm complexity. It f

Years back I did a couple hundred problems on this site. It's very good at making you learn about important results in basic math and algorithm complexity. It forces you to write your own library of efficient functions for factoring, etc... because you have to use them so many times and as the problems get harder anything brute force will take too long.