LeetCode Add Digits

LeetCode Add Digits

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?

Hint:

A naive implementation of the above process is trivial. Could you come
up with other methods? What are all the possible results? How do they
occur, periodically or randomly? You may find this Wikipedia article
useful.

代码

// https://en.wikipedia.org/wiki/Digital_root
class Solution {
public:
    int addDigits(int num) {
        return num - 9 * ((num - 1) / 9);
    }
};