Project Euler is a site that presents a series of programming/mathematical problems. The Euler part comes from Leonhard Euler, a Swiss mathematician.
All of the problems are designed so that a correct implementation can arrive at a solution in 1 minute or less. Sometimes the obvious (brute force) solution might take a very long time indeed. This means that a “correct” solution needs to be correct in both answer and performance.
I’ve done the first 158 out of 321 problems so far. New problems are added periodically. I’ve been using the problems as a way to learn the Scala language. That’s been working out fairly well.