https://leetcode.com/problems/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?
Credits:Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

# time O(1)
# space O(1)
# 直接0-n等差求和，减去sum（nums）就是缺失值
class Solution(object):
    def missingNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """

        return sum(range(len(nums) + 1)) - sum(nums)