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) {
        int sum = 0;
        while(num!=0) {
            int digit = num%10;
            num = num/10;
            sum = sum+digit;
        }
        if(sum<10) return sum;
        else return addDigits(sum);
    }
}