leetcode Missing Number 消失的数

Given an array containing n distinct numbers taken from 0, 1, 2, …, n, find the one that is missing from the array.

For example,
Given nums = [0, 1, 3] return 2.

Note:
Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?
题意：给定长度为n的数组，存储0~n这n+1个数字，找到其中丢失的数字，要求时间复杂度O（n），不使用额外的空间。
思路：从0到n相加，然后减去数组中的数，即为消失的数。

class Solution {public:    int missingNumber(vector<int>& nums) {        int len = nums.size();        int i = 0;        for (auto num : nums) {            len += i++ - num;        }        return len;    }};