(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+)/;
         for(int i = ;i<len;++i)
         {
             sum -= nums[i];
         }
         return sum;
     }
 };