[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;
}
}