java 简单的“add digits”算法

题目：

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

For example:

Given num = 38, 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? 

下面是我的代码实现：

public class Solution {
    public int addDigits(int num) {
        int i=1;
        if(num>=10){
            i++;
        }
        int tmp = num;
        while(tmp/10>=10){
            i++;
            tmp/=10;
        }
        int[] array = new int[i];
        int sum = 0;
        for(i=0;isum =sum+num%10;
            num = num/10;
        }
        if(sum>=10){
            sum = addDigits(sum);
        }
        return sum;
    }

}