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?

这题首先看到要求线性时间复杂度，觉得排序肯定没戏，后来自己折腾还是先用的排序，果然超时。最后参考了别人的代码，是用位运算解决的。没用过位运算，翻了《C Primer Plus》第15章位操作，恶补。

 1 class Solution{ 2 public: 3     int singleNumber(int A[],int n){ 4         int i,bit=0; 5         if(n<0) 6         { 7             return -1; 8         } 9         for(i=0;i<n;i++)10         {11             bit^=A[i];12         }13         return bit;14     }15 };