258. Add Digits
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Topics
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?
Hints
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.