[LeetCode&Python] Problem 258. Add Digits

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

Example:

Input: 38
Output: 2
Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2.
             Since 2 has only one digit, return it.

Follow up:
Could you do it without any loop/recursion in O(1) runtime?

class Solution(object):
    def addDigits(self, num):
        """
        :type num: int
        :rtype: int
        """
        s=str(num)
        while len(s)>1:
            n=0
            for i in s:
               n+=int(i)
            s=str(n)

        return int(s)