Single Number

Given an array of integers, every element appears twice except for one. Find that single one. 
Note: Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

public class Solution {
    public int singleNumber(int[] A) {
        int res = 0;
        for(int i = 0; i < A.length; i++) {
            res = res^A[i];
        }
        return res;
    }
}