Leetcode 上一道简单题的效率问题

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?

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class Solution {
public:
    int singleNumber(vector<int>& nums) {
        int ret = 0;
         for(int i:nums) {
             ret ^= i;
         }
         return ret;
    }
};

上 leetcode 做了一道题，怎么也想不明白为什么这个写法效率这么低，效率最高的那部分代码要怎么写才能达到？