I'd suggest kattis and CodeForces. I use kattis mostly, but know a number of people who like codeforces better. Here's a few for you to try. Some of these are

I'd suggest kattis and CodeForces. I use kattis mostly, but know a number of people who like codeforces better. Here's a few for you to try. Some of these are pretty hard, but you should be able to find solution sketches online if you google the contests they are from. https://open.kattis.com/problems/increasingsubsequence https://open.kattis.com/problems/increasingsubsequence https://open.kattis.com/problems/maximumsubarrays https://open.kattis.com/problems/maximumsubarrays https://open.kattis.com/problems/tray https://open.kattis.com/problems/tray https://open.kattis.com/problems/dinnerbet https://open.kattis.com/problems/dinnerbet https://open.kattis.com/problems/hyperpyramids https://open.kattis.com/problems/hyperpyramids I've solved all of these as well, so if you get really stuck feel free to reply here and I'll try to guide you through them.