Add Digits leetcode

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) {
        while (num > 9) {
            int newnum = 0;
            while (num > 0) {
                newnum += num % 10;
                num = num / 10;
            }
            num = newnum;
        }
        return num;
    }
}

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