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:**

*   <code>0 <= num <= 2<sup>31</sup> - 1</code>

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