add-digits

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258. Add Digits (Easy)

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?

Related Topics

[Math] [Simulation] [Number Theory]

Similar Questions

1. Happy Number (Easy)
2. Sum of Digits in the Minimum Number (Easy)
3. Sum of Digits of String After Convert (Easy)

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.