Like many algorithm interview questions, I'm not sure it's meant to be a practical problem, but rather one that demonstrates your ability to think through problems.
(Out of principle, I prefer to ask interview questions that are based on real problems I've had to solve in my job, but sometimes they're harder to ask coherently than well-defined/concise problems like "invert a binary tree" so I can see the appeal)
> If my whiteboard answer was "keep a flag that says whether or not the tree is inverted" would I be hired?
Personally I'd give you bonus points for recognizing that could be a solution, then clarify that I wanted a solution that actually moved the data around and give you the opportunity to answer it in light of that clarification.
(Out of principle, I prefer to ask interview questions that are based on real problems I've had to solve in my job, but sometimes they're harder to ask coherently than well-defined/concise problems like "invert a binary tree" so I can see the appeal)
> If my whiteboard answer was "keep a flag that says whether or not the tree is inverted" would I be hired?
Personally I'd give you bonus points for recognizing that could be a solution, then clarify that I wanted a solution that actually moved the data around and give you the opportunity to answer it in light of that clarification.