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?