Introduction

In this blog post, we will explore how to solve the LeetCode problem "258" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

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?

Explanation

Here's the approach to solve this problem efficiently:

• Digital Root Concept: The problem is essentially asking for the digital root of a number. The digital root is the single-digit value obtained by repeatedly adding the digits of a number until a single-digit result is achieved.
• Congruence Formula: A key mathematical insight is that the digital root of a number n is equivalent to n mod 9. If the result is 0, it means the number is divisible by 9, and the digital root is 9 (except when the number itself is 0).

• Direct Calculation: This allows us to calculate the digital root directly using modular arithmetic, avoiding loops or recursion.

• Runtime Complexity: O(1), Storage Complexity: O(1)

Code

    class Solution:
    def addDigits(self, num: int) -> int:
        if num == 0:
            return 0
        elif num % 9 == 0:
            return 9
        else:
            return num % 9