Solutions to some problems at https://open.kattis.com/
I like to program on my free time when I have time over, and kattis is a good place for me to do that. It's a website that post program/math problems in all different difficulties. From easy hello world to advanced mathematic problems. You can write your solution for the problems in different language, but I mostly use java for these problems. And you can see how you rank with other people /schools etc. check it out!