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.

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?

class Solution:
    def missingNumber(self, nums: List[int]) -> int:
        
        n = len(nums)
        
        expectSum = int(n * (n+1) / 2)
        totalSum = 0
        for i in nums:
            totalSum += i
        
        return expectSum - totalSum