258. Add Digits (Easy)

https://leetcode.com/problems/add-digits/

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

Example:

Input: 38
Output: 2 
Explanation: 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?

Solutions

• Java

https://www.cnblogs.com/grandyang/p/4741028.html

class Solution {
    // write done the numbers and you are able to discover the pattern readily.
    public int addDigits(int num) {
        return (num == 0) ? 0 : (num - 1) % 9 + 1;
    }
}

Incorrect Solutions

References