Project Euler

The most fun on this site is solving a problem and then having your mind blown by solutions in Apl/j/k and trying to guess what they mean without knowing anything about those languages

See also Uiua, a newcomer to the "extremely cool but completely incomprehensible language" family!

Even better than the crazy languages, is seeing some fundamental math used to prevent having to do a ridiculously expensive search.

That said, raw brute force often did far better than you'd like to admit.

The biggest thing I learned from PE was that neither elegant theory nor brute force had a monopoly on successful optimization strategies. It's been something I've carried with me ever since and has over and over again proven its value.

A real gem of a resource.