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?

思路：因为所有元素都出现了两次，只有一个元素出现了一次。因此，可以使用异或运算来完成查找。

1 class Solution {2 public:3     int singleNumber(int A[], int n) {4         if( n <= 0 ) { return 0; }5         int val = A[0];6         for( int i = 1; i < n; ++i ) { val ^= A[i]; }7         return val;8     }9 };