This is often an exercise for beginning programmers, it's a very simple task that's easy to understand, but leaves enough room in the implementation to make it a good exercise.

Sometimes it's used as a test on job applications, which is total bullshit, it isn't a good test of someones actual skills as a software developer. Because of this it's become a bit of a joke on the internet.

[–] barsoap@lemm.ee 4 points 1 day ago* (last edited 1 day ago)

This is often an exercise for beginning programmers

And non-beginners have to weigh the skill of the interviewer to figure out whether "To invert, I'll just keep the tree as it is" is an approach they can grasp. Enlightenment is realising that the tree is already in the perfect shape.