1. Description

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

2. Example

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

3. Code

public class LeetCode0258 {
    public int addDigits(int num) {
        if (num < 10) {
            return num;
        }

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

    public static void main(String[] args) {
        LeetCode0258 leetcode = new LeetCode0258();
        System.out.println(leetcode.addDigits(38));
    }
}