Dany Valevsky gave me this very cool riddle.

You are given a vector of size N, the elements of which are numbers in the range 1, …, N-1. I.e. there is at least one repeating element. Give an algorithm that finds a repeating element (it does not matter which one, in case there are several) with O(N) time complexity and O(1) memory complexity.

NOTE - the time and memory complexities are calculated in integers. I.e. the input is of size N, not N*log(N).