[LeetCode 258] Add Digits (easy)

class Solution {
    public int addDigits(int num) {
        //solution1: digital root formula as following
        /*
        if (num == 0)
            return 0;
        
        if (num % 9 == 0)
            return 9;
        
        return num % 9;
        */
        
        // solution2: recursively
        int result = 0;
        while (true) {
            
            while (num > 0) {
                result += num % 10;
                num = num / 10;
            }
            
            if (result < 10)
                break;
            
            num = result;
            result = 0;
        }
        
        return result;
    }
}