Ask HN: Which is the most useful CS/math topic that you have learnt?

When studying any subject we learn a lot of things. Most of the things we forget because we never get to use them repeatedly. But there are some topics which we use almost everywhere in our professional lives and those become our strong points.

In CS/math which topic would you place in that category?

For me in math it was linear algebra and for CS it was learning to write an interpreter for a language (or learning about programming language theory in general using an incremental interpreter).