题目：

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?

思路：

直接排序或对比，时间复杂度一定在nlogn以上。利用Hash表或者Bitmap可以满足时间复杂度但是空间复杂度不能满足。O(n)时间类能采用的办法只有异或。

代码：

class Solution {
public:
    int singleNumber(int A[], int n) {
        while (--n!=0) A[n-1]^=A[n];
        return A[0];
    }
};