LeetCode 258.Add digits

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

(a + b + c + d) % 9
= (999a + 99b + 9*c + a + b + c + d) % 9
= abcd % 9

public int addDigits(int num){
     if(num ==0)return0;
     return num%9==0?9:num%9;
}