I was asking for a function that converted a number (auto incremented id) to a string, such that all possible shortest urls would be iterated through.

E.g. given character set a-z, 0: a 1: b 26: aa 27: ab

etc.

I probably had no idea what the interviewee was suggesting and tried and failed to explain my question any better.

He just said "Yeah, ok, that could work, now do it with a database." :P