(leetcode)Missing Number

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?

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int len = nums.size();
        int sum = len*(len+1)/2;
        for(int i = 0;i<len;++i)
        {
            sum -= nums[i];
        }
        return sum;
    }
};