[ulber]

The "reverse a binary tree" problem gets often brought up as one of these tricky algorithm problems that you just have to know. However, the "algorithm" is to just swap all of the left pointers with the right pointers in each node.

The biggest difficulty for most seems to be that they don't know what "reverse a binary tree" actually means. It sounds kind of mathy and opaque, so I get it, but candidates should be able to have a dialog to figure out what the requirements mean. And on the flipside interviewers should be ready to have that dialog and not count not knowing the term by heart against the candidate.

This problem to me feels qualitatively different than the "rabbit and hare" algorithm for finding a loop in a linked list mentioned by another poster. That one needs a non-trivial algorithmic insight that just might not come to you during an interview. The solution to "reverse a binary tree" flows out of the structure of the problem statement as long as you have the fundamental skills for walking and manipulating data structures and the conversational skills to understand the problem, both of which seem fair to test for.