There's the typical interview question about how to create a distributed and scalable URL shortener. One of the issues is to know if a URL was shortened already or not. There's a special data structure that tells you if no or maybe. So it can have negatives or false positives but at least you know it wasn't calculated and you spare the visit do the database. The question is: what's the name of that structure? I forgot it and searched a lot but can't find it. Thanks!