[LeetCode] 268. 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) {        const int n = nums.size();        int perfect = (1 + n) * n / 2, actual = 0;        for (int i = 0; i < n; i++)            actual += nums[i];        return perfect - actual;    }};