Add digits leetcode July
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?