0258. Add Digits

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

Description

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

Example 1:

**Input:** num = 38
**Output:** 2
**Explanation:** The process is
38 --> 3 + 8 --> 11
11 --> 1 + 1 --> 2 
Since 2 has only one digit, return it.

Example 2:

**Input:** num = 0
**Output:** 0

Constraints:

• 0 <= num <= 231 - 1

Follow up: Could you do it without any loop/recursion in O(1) runtime?

ac

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

/*
digit root law: 1 + (num - 1) % 9;
*/