Ask HN: What is your favorite exotic data structure?

Most (software) engineers are aware of commonly used data structures such as Arrays, Linked Lists, HashMaps, Binary Search Trees, Min/MaxHeaps etc.

However, I've recently learned about "lesser known" data structures like van Emde Boas Trees and Radix Trees.

This made me wonder about how many cool (and somewhat useful) data structures are out there that are not very well known.