Leetcode: Single Number

Given an array of integers, every element appears twice except for one. Find that single one.

Could you implement it without using extra memory?

int singleNumber(int A[], int n) {
        // Note: The Solution object is instantiated only once.
        if(n<0)return -1;
		int res = 0;
		for(int i =0;i<n;i++)
			res ^= A[i];
		return res;
    }