I recently discovered the website Jane Street. They are a quantitative trading firm that hires people with a strong mathematical background. They also publish monthly challenges in the form of puzzles. If you solve the puzzle, you can submit your solution and if correct, they will publish your name on the puzzle’s page.
The first one I tackled was Pent-up Frustration 2.
I also publish the code on GitHub (once the solution is out).
Do you want to take a look at more puzzles? Check out here