Traversing trees recursively is so trivial. I have to do this kind of stuff all the time. Just last week actually (in some frontend code no less).

Graph search and B-trees I haven't done professionally since I left college though. But it is still good to know the theory when dealing with databases.

A lot of these algorithms is more about knowing their characteristics than knowing how to implement them. For example cryptographic algorithms can be complex, but having a good lib and knowing each crypto algorithm characteristics is usually good enough for almost everyone.