Here's a GitHub repository with solutions to a lot of IOI, APIO, IOITC problems. If you solve a problem and there is no solution in the GitHub repository (or if yours is better / cleaner), let me know and I'll add it to the repository.
Word of caution: Good problems are in limited supply. Don't waste good problems by reading the solution too soon. Make sure you try to solve a problem for at least a few hours before reading the solution.
If you need access to the repository to contribute, just let reply to this discussion and let me know what you need to do, and also include your GitHub username.