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Java Solution for LeetCode algorithm problems, continually updating
258\. Add Digits
Easy
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?