LeetCode--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?

class Solution {public:    int singleNumber(int A[], int n)     {        if(n==0)            return -1;        int res = A[0];        for(int i=1;i<n;i++)            res = res^A[i];        return res;    }};