Or maybe it’s making the tree upside down, so all leaves become roots, and the root becomes a leaf. But when we do that, do we just invert all the edges or transform the whole tree?
Maybe it’s referring to changing a red-black tree into a green-white tree? Or turning a splay tree into a merge tree?
Howell (the homebrew 'google didn't hire me because of inverting a binary tree' guy) confirmed that they meant swapping right and left children of all nodes.
in fact any candidate that doesnt ask clarifying questions and goes deep into coding right away - has a chance of going into wrong direction and is flagged as red flag.
As an interviewer I always lookout for candidates that dont ask questions, dont start with test cases, assume in their mind what is their understanding of ideal solution - and start coding. These are very inexperienced people who do that