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