Finding a number that repeats even no of times where all the other numbers repeat odd no of times

Question

Given is an array of integers. Each number in the array repeats an ODD number of times, but only 1 number is repeated for an EVEN number of times. Find that number.

Answer 1

Apparently there is a solution in O(n) time and O(1) space, since it was asked at a software engineer company with this constraint explictly. See here : Bing interview question -- it seems to be doable using XOR over numbers in the array. Good luck ! :)