public class Solution {
    /*
     * @param num: a non-negative integer
     * @return: one digit
     */
    public int addDigits(int num) {
        // write your code here
        String numString = num+"";
        if(numString.length()==1){
            return num;
        }
        while(numString.length()!=1){
            int result = 0;
        for(int i = 0;i<numString.length();i++){
            result = numString.charAt(i)-'0'+result;
        }
        numString = result+"";
        }
        return Integer.parseInt(numString);
    }
}