DigitalRoot.java

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

Example:

Input: 38
Output: 2 
Explanation: The process is like: 3 + 8 = 11, 1 + 1 = 2. 
             Since 2 has only one digit, return it.
*/

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