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:
     def addDigits(self, num):
         """
         :type num: int
         :rtype: int
         """
         while True:
             if num<10:
                 return num
             else:
                 num=sum([int(i) for i in str(num)])
class Solution:
     def addDigits(self, num):
         """
         :type num: int
         :rtype: int
         """
         while True:
             if num<10:
                 return num
             else:
                 num=sum([int(i) for i in str(num)])