258. Add Digits

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

Example:

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

Follow up:
Could you do it without any loop/recursion in O(1) runtime?

class Solution {
    public int addDigits(int num) {
        while(num >= 10){
            num = num/10 + num%10;
        }
        return num;
    }
}

这题是工作人员上厕所时想出来的吗？

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

而且是拉稀状态