leetcode Question: Add Digits

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
For example:
Given num = 38, the process is like: 3 + 8 = 111 + 1 = 2. Since 2 has only one digit, return it.
Could you do it without any loop/recursion in O(1) runtime?

Analysis:

This is a math problem, which can be easily searched in the web and the only be careful with the python, in which the mod operation always returns positive numbers.

Code(C++):

class Solution {
public:
int addDigits(int num) {
return (num-1)%9 +1;
}
};


Code(Python):

class Solution(object):