Question:

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Example 1:
    Input: num = 38
    Output: 2
    Explanation: The process is
    38 --> 3 + 8 --> 11
    11 --> 1 + 1 --> 2 
    Since 2 has only one digit, return it.

Example 2:
    Input: num = 0
    Output: 0

My thoughts

Another trick question, solvable only if you know what digital root is, as well as enumerate all the items.
    0=0
    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,....
there is a cycle from 9. every time. 

Initial Solution:

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