Ditto on this. Someone who is passionate about algorithms outside of just memorizing interview problems i.e - they find problems like this -
http://codeforces.com/contest/724/problem/G - interesting, usually have high problem solving and mathematical aptitude.
Perhaps I'd like to draw a distinction between genuine algorithms problems that you haven't seen before, and the really bad interview questions like "Find a cycle in a linked list". You'd be really hard pressed to find anyone who could solve the problem above.
Edit: To clarify, I would not remotely consider the linked Codeforces problem as a good interview problem.
