Seems pretty harsh to me, and had you successfully answered that question I'm not quite sure what they'd have learnt. If I were put on the spot with those cons

Seems pretty harsh to me, and had you successfully answered that question I'm not quite sure what they'd have learnt. If I were put on the spot with those constraints I'd suggest a lookup table. I'm currently looking for a new job, however I tend to ask about the process upfront, if it sounds like a wanky anal probe type of interview I simply withdraw from the process. Which is sad, as some of the jobs I'd really like.