leetcode 136、Single Number

Given a non-empty 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?

Example 1:

Input: [2,2,1]
Output: 1

Example 2:

Input: [4,1,2,1,2]
Output: 4

两个相同的数异或为0，一个数和0异或不变，因此将所有的数异或即只出现一次的数。

 class Solution {
 public:
     int singleNumber(vector<int>& nums) {
         int ans = ;
         for (int i : nums) {
             ans ^= i;
         }
         return ans;
     }
 };