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?

算法思路：

抑或^满足交换律和结合律

交换律： a ^ b ^ c ^ d = a ^ d ^ b ^ c;

结合律： a ^ b ^ c ^ d = a ^ （d ^ b） ^ c;

相同数字 a ^ a = 0

代码如下：

1 public class Solution {2     public int singleNumber(int[] a) {3         int res = 0;4         for(int tem : a){5             res ^= tem;6         }7         return res;8     }9 }