136. Single Number, leetcode

题目：

Given an array of integers, every element appears twice except for one. Find that single one.

解法1，hash map 如代码中注释掉的部分；

解法2，bit manipulation 异或法

class Solution {
public:
    int singleNumber(vector<int>& nums) {
        /*unordered_map<int, int> m;
        for(int num : nums) m[num]++;
        for(auto it : m) if(it.second == 1) return it.first;
        return 0;*/
        int ans = 0;
        for(int i = 0; i < nums.size(); i++)
        ans ^= nums[i];
        return ans;
    }
};