I think it's probably too hard as an interview question, but also that it actually is a somewhat realistic problem that I'd expect a senior programmer to (eventually) puzzle out correctly. (As opposed to, say, the archetypal bad interview question, "How do you swap two integer values without using a temporary?", where many people happen to already know the trick, and if you don't, you're unlikely to figure it out on your own in the space of an interview.)