Oh no, that bit's easy.

But you don't ask them "swap the left and right field in each node". You ask them to "invert a binary tree".

Why. The Fuck. Does "invert a binary tree" mean "swapping the left and right fields"?

To me, it means inverting the relationship so child points to parent, which would end up badly-defined, there would be no way to uniquely specify a root. But if you don't know that knowledge, of what the jargon means, you're screwed. So that's what you're testing for.