[E].[J].258.AddDigits

class Solution {
    public int addDigits(int num) {
        if (num < 10){
            return num;
        } else {
            int sum = 0;
            while (num != 0){
                int lastDigit = num%10;
                sum += lastDigit;
                num = num/10;
            }
            if (sum > 9){
                return addDigits(sum);
            } else {
                return sum;
            }
        }
    }
}