algorithm - 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.

I was thinking a hash map, with each element's count. It requires O(n) space. Is there a better way?

Answer 1

Hash-map is fine, but all you need to store is each element's count modulo 2. All of those will end up being 1 (odd) except for the 0 (even) -count element.

(As Aleks G says you don't need to use arithmetic (count++ %2), only xor (count ^= 0x1); although any compiler will optimize that anyway.)