I stumbled across this tree problem and i can't seem to find the name of the data structure for the problem.

It's like this: You are given a tree and a set of nodes. I want to build another tree that contains all the nodes needed and removes all the uneccessary nodes. I believe it's called virtual trees or something, but i just can't find any materials about it. Could anyone tell me the name of it? Some documentations about it helps too.

Apreciate all helps! Thanks