# Day 26 of July LeetCode Challenge

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?