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)