I've been in about half a dozen interviews in the last 6 months and I've never had to do this problem, but honestly my first reaction would be "this is a trick problem" out of paranoia that it's too easy. I wonder if more anxious people fall into that trap and overthink it to the point of overwhelming themselves; kind of like freezing up during an exam that you studied extensively for.

My biggest issue in interviews is remembering the recursion formula with some of the sorting algorithms. I eventually get it, but I end up burning a good chunk of time figuring it out, so it looks bad. In my entire career (almost 15 years now?), I've never had to use recursion or sorting algorithms believe it or not. That's what I get for working strictly on enterprise business applications.