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258. Add Digits (Easy)

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?

Hint 1

A naive implementation of the above process is trivial. Could you come up with other methods?

Hint 2

What are all the possible results?

Hint 3

How do they occur, periodically or randomly?

Hint 4

You may find this Wikipedia article useful.

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