What's Your Favorite Algorithm?

The product quote unquote family of algorithms, I'm just a huge fan of. In that family exists inner product, otherwise known as transform reduce, a Cartesian product, which is sometimes just known as product in languages like Python and Ruby, and outer product. And I also wish that there was an algorithm that I have yet to discover called triangle product, which I mentioned in my CPP con 2020 talk. So outer product, whoa, why aren't we going to talk about that? I don't know what triangle product is. Well, no, no. You go first. I want to hear your new favorite algorithm.