It really does surprise me how a simple programming questions like this does stump the average interviewee. If they need a bit of guidance to get started, I never hold it against the candidate, but if they need a massive hand holding to come to a solution, or if they can't explain how their solution works and how it could be improved, then I know where they stand programming wise.

It is a low bar skills wise, and it tells a lot within a very short amount of time.