Add Digits.java

/*
Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.
Example
Given num = 38.
The process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return 2.
*/

public class Solution {
    /**
     * @param num a non-negative integer
     * @return one digit
     */
    public int addDigits(int num) {
        // Write your code here
// 1    1
// 2    2
// 3    3
// 4    4
// 5    5
// 6    6
// 7    7
// 8    8    
// 9    9    
// 10    1
// 11    2
// 12    3    
// 13    4
// 14    5
// 15    6
// 16    7
// 17    8
// 18    9
// 19    1
// 20    2


        return (num-1)%9+1;
    }
}