leetcode Question: Add Digits

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 = 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?

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