[LeetCode] Single Number

使用异或即可，因为

1.a^a = 0;

2.0^a = a;

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