How about the R-Tree, used for accessing/searching spatial data: https://en.wikipedia.org/wiki/R-tree https://en.wikipedia.org/wiki/R-tree Maybe not so obscure

How about the R-Tree, used for accessing/searching spatial data: https://en.wikipedia.org/wiki/R-tree https://en.wikipedia.org/wiki/R-tree Maybe not so obscure if you've worked in the geospatial realm. I actually had a job interview once that asked me to implement an R-Tree from scratch in C++ as a homework assignment - I didn't get that job :)