Here you go! http://en.wikipedia.org/wiki/XOR_swap_algorithm

Given an array of positive integers. Let's assume that every value except one repeat an even number of times. The question is: find the only value that repeats an odd number of times.

For example, if the array is: 6 8 8 6 3 9 3 9 3 the answer is 3, because it repeats an odd number of times.

For the solution, see "Rev. 1" of this comment :)

Here you can find some tricks for bit operations http://graphics.stanford.edu/~seander/bithacks.html