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?

• 主要考察了逻辑运算中的异或（^）运算，这个是比较常见的题型。C++ 代码如下：

class Solution {
public:
    int singleNumber(int A[], int n) {
        int num = 0;
        if(n<=0)
          return -1;
        for(int i =0; i< n; i++)
        {
            num^=A[i];
        }

        return num;
    }
};