[leetcode] Add Digits

from : https://leetcode.com/problems/add-digits/

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?

思路：

任意x，x为n位数，则

那么，

又因为x%9 < 9， 所以，x%9即为所求。

public class Solution {
    public int addDigits(int num) {
        return (num-1)%9+1;
    }
}