LeetCode - Add Digits

Question

Link: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?

Code

一个比较挫的用到了循环和递归的算法。（C++ : 8ms）

class Solution {
public:
    int addDigits(int num) {
        if(num < 10)
            return num;

        int digit = 0;
        while (num > 0){
            digit += (num % 10);
            num /= 10;
        }

        return addDigits(digit);
    }
};

What the fuck!这玩意一行代码就可以解决了！
看个高级版本的。。。原理百度你就知道了。。。

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