leetcode1228

class Solution:
    def missingNumber(self, arr: 'List[int]') -> int:
        n = len(arr)
        common = (arr[n-1] - arr[0]) // n
        if common == 0:
            return 0
        for i in range(n):
            need = arr[0] + common * i
            if arr[i] != need:
                return need