leetcode- 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?

看题目意思，肯定是有规律可循了，写下几个数字 找出规律

class Solution {
public:
    int addDigits(int num) {
        if(num<10)return num; 
	num%=9;
	if(num ==0 )return 9;
        return num;
    }
};

知识点是 Digital root   https://en.wikipedia.org/wiki/Digital_root