Difficulty: Easy Topics: Math, Simulation, Number Theory Tags:

LeetCode: Problem 258

Given an integer num, repeatedly add all its digits until the result has only one digit, and return it.

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.

Input: num = 0
Output: 0

• 0 <= num <= 2^31 - 1

Follow up: Could you do it without any loop/recursion in O(1) runtime?