LeetCode 136 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 tmp = 0;
        for(int i = 0; i < A.length; i ++) tmp ^= A[i];
        return tmp;
    } 
}