> We pick something boring, say from the standard library, and nothing tricky like a hash table. Really boring like floor(). If you can't figure out how floor() does its "magic" that's scary.

What I find scary is that you think floor() is an easy algorithm. Here's a typical high quality implementation of floor() (Plauger's implementation in "The Standard C Library" is different, but of similar complexity). Not something you'd derive on a whiteboard: http://www.netlib.org/fdlibm/s_floor.c

I'm concerned that a question like this is selecting for mediocre programmers who'd brute force some solution, while causing programmers who are aware of the complexity of the problem to freeze.