I personally love https://www.codingame.com/, but it's geared more towards university students. The challenges get pretty hard, and they all pretty much gravitate around game AI if that's your thing.
The cool thing is that some challenges are multiplayer, in the sense where you need to develop an AI for a given game, and then send your AI on a ladder to compete against other coders' AI.
The cool thing is that some challenges are multiplayer, in the sense where you need to develop an AI for a given game, and then send your AI on a ladder to compete against other coders' AI.