"""
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):
        return (num - 1) % 9 + 1 if num > 0 else 0
