"""

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

"""

n = len(nums)

s = n * (n + 1) / 2

res = 0

for i in nums:

res += i

return s - res

s = Solution()

a1 = [0, 1, 3]

print(s.missingNumber(a1))