[leetcode] 258. Add Digits

题目：

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?

---

代码：

class Solution {
    public int addDigits(int num) {
        while(num / 10 != 0){
            int nnum = num;
            num = 0;
            while(nnum != 0){
                num += nnum % 10;
                nnum /= 10;
            }
        }
        return num;
    }
}