找到单独的数字 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?

class Solution
{
    int FindSingleNumber(int num[], int n)
    {
         if(NULL == num || n <= 0)
        {
             return -1;
        }
        for(int i = 1; i < n; i++)
        {
             num[0] ^= num[i];
        }
        return num[0]
    }
}       

这道题虽然很简单，在各种面试题中也经常可以见到，值得深入挖掘的是这种思考问题并运用已有知识解决问题的思路。