Jane Street Software Engineer interview experience

The problem I remember centered on a rooted directed tree built from N uniquely labeled tokens connected by one-way dependency links. The key constraint was structural: exactly one token had no incoming link.

What they asked me to do was think in terms of positions under valid arrangements, and count how many tokens could end up in a particular slot across some topological-valid ordering. I spent most of the time reasoning through the tree structure and how it affected what placements were possible, rather than trying to brute-force anything. It was the kind of question where getting the