Introduction

In this blog post, we will explore how to solve the LeetCode problem "3260" 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

You are given two positive integers n and k. An integer x is called k-palindromic if: x is a palindrome. x is divisible by k. Return the largest integer having n digits (as a string) that is k-palindromic. Note that the integer must not have leading zeros. Example 1: Input: n = 3, k = 5 Output: "595" Explanation: 595 is the largest k-palindromic integer with 3 digits. Example 2: Input: n = 1, k = 4 Output: "8" Explanation: 4 and 8 are the only k-palindromic integers with 1 digit. Example 3: Input: n = 5, k = 6 Output: "89898" Constraints: 1 <= n <= 105 1 <= k <= 9

Explanation

• Generate Palindromes: Construct palindromes of length n by iterating through possible prefixes/suffixes.
  • Check Divisibility: For each generated palindrome, check if it is divisible by k.
  • Maximize and Return: Maintain the largest k-palindromic number found so far and return it as a string.

• Runtime Complexity: O(10^(n+1)/2), Storage Complexity: O(n)

Code

    def largest_k_palindromic(n: int, k: int) -> str:
    """
    Finds the largest n-digit k-palindromic integer.

    Args:
        n: The number of digits.
        k: The divisor.

    Returns:
        The largest n-digit k-palindromic integer as a string.
    """

    max_palindrome = -1

    start = 10**(n // 2 - (1 if n % 2 == 0 else 0))
    end = 10**(n // 2)

    for i in range(end -1, start - 1, -1):
        prefix = str(i)
        if n % 2 == 0:
            palindrome = prefix + prefix[::-1]
        else:
            palindrome = prefix + prefix[:-1][::-1]

        if int(palindrome) % k == 0:
            if max_palindrome == -1 and len(palindrome) == n and int(palindrome) >= 10**(n-1) :
               max_palindrome = int(palindrome)
            elif len(palindrome) == n and int(palindrome) >= 10**(n-1) and int(palindrome) > max_palindrome:
                max_palindrome = int(palindrome)


    if max_palindrome == -1:

        if n == 1:
           for i in range (9, 0, -1):
             if i % k == 0:
                return str(i)
        return ""
    else:
        return str(max_palindrome)