来看这个题，https://oj.leetcode.com/proble…：

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?

第一反应就是异或，唯一需要确认的就是异或运算是否支持交换律，通过这里，http://longzxr.blog.sohu.com/1…，确认了支持，那么答案就很简单了：

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

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