human
I found an interesting problem on https://leetcode.com/problems/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?

I tried it in Python, but could not do it. Can you solve it?

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Can you provide a the Python program you requested?
assistant
#Here is the Python program you requested.

class Solution:
     def addDigits(self, num):
         """
         :type num: int
         :rtype: int
         """
         if num == 0:return 0
         return (num-1)%9+1