LeetCode：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?

class Solution {
public:
    int floor(int x) {
        return (x - 1) / 9;
    }
    int addDigits(int num) {
        return num - 9 * floor(num);
    }
};