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?

Solution1: with while loop

public class Solution {
    public int addDigits(int num) {
        if (num < 10) {
            return num;
        }
        
        int res = 0;
        while (num / 10 >= 1) {
            res = 0;
            res += num % 10;
            num = num / 10;
            res += num;
            num = res;
        }
        return res;
    }
}

Solution2: 观察得出规律，
if (in % 9 != 0) return in % 9;
else return 9;

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

Or:

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