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

 1 public class Solution {
 2     /**
 3      * @param num a non-negative integer
 4      * @return one digit
 5      */
 6     public int addDigits(int num) {
 7         // Write your code here
 8         while(num>=10){
 9             int tempSum = 0;
10             int temp = num;
11             while(temp>=10){
12                 tempSum += temp%10;
13                 temp/=10;
14             }
15             num = temp+tempSum;
16         }
17         return num;
18     }
19 }