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(object):    def missingNumber(self, nums):        """        :type nums: List[int]        :rtype: int        """        """        nums.sort()        n = sum(nums)        if nums[0] != 0:            return 0        elif nums[-1] != len(nums):            return len(nums)        else:            return nums[-1]*(len(nums) + 1)/2 - n        """        max_num = max(nums)        if max_num != len(nums):            return len(nums)        return max_num*(len(nums) + 1)/2 - sum(nums)