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?

public class Solution {

    public int addDigits(int num) {

        if (num < 10) {

            return num;

        }

        int ret = 0;

        while (num != 0) {

            ret += num % 10;

            num /= 10;

        }

        return addDigits(ret);

    }

}

public class Solution {
    public int addDigits(int num) {
        if (num == 0) {
            return 0;
        }
        int ret = num % 9;
        if (ret == 0) {
            return 9;
        }
        return ret;
    }

}