Quesiton

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

Example 1:

Input: [3,0,1] Output: 2

Example 2:

Input: [9,6,4,2,3,5,7,0,1] Output: 8

Note: Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?

Solution

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