LeetCode 258 -Add Digits ( JAVA )

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?

1. You may find this Wikipedia article useful.

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

总结：由题目可知，求一个数的数根，有求数根的公式（PS：别问我怎么来的，我也不知道- - .）