I’m also not sure if this is obscure, but Bloom Filters! It’s a structure that you can add elements to then ask it if it has seen the element before with the answer being either “no” or “probably yes”. There’s a trade-off between confidence of a “probably yes”, how many elements you expect to add, and how big the Bloom Filter is, but it’s very space and time efficient. And it uses hash functions which always make for a fun time.

Benn Jordan tends to make longer videos, but they are always worthwhile. If he releases a 40 minute video about a topic I don’t care about, I’ll still jump to watch it because he’ll make it interesting, and I’ll probably end up caring about it by the end.