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?

Python

 
class Solution(object):
    def addDigits(self, num):
        """
        :type num: int
        :rtype: int
        """
        while num > 9:
            num = self.add(num)
        return num
            
            
    def add(self, a):
        result = 0
        for i in str(a):
            result += int(i)
        return result