Add Digits

Difficulty

Easy

Question

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 <= 2^31 - 1

Link

Add Digits

Solution

Solution is O(1) using digital root property of specified digit sum problem.