【LeetCode】258_Add Digits

题目

Add Digits

 Total Accepted: 5413 Total Submissions: 11941My Submissions

Question Solution 

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:

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

解析

用for循环和递归很简单，但是题目不允许，只能自己发现规律

没什么讲的，可以看看维基百科https://en.wikipedia.org/wiki/Digital_root

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