258 Add Digits

public int addDigits(int num) {        while (num > 9) {            num = func(num);        }        return num;    }    public static int func(int n) {        int temp = 0;        while (n > 0) {            temp += (n % 10);            n = n / 10;        }        return temp;    }

没有实现：
Follow up:
Could you do it without any loop/recursion in O(1) runtime?