leetcode Missing Number

题目连接

https://leetcode.com/problems/missing-number/  

Missing Number

Description

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 n = nums.size();
		ull res = (n + 1) * n / 2;
		ull tmp = 0;
		for (int i = 0; i < n; i++) tmp += nums[i];
		return (int)(res - tmp);
	}
private:
	typedef unsigned long long ull;
};