Introduction

In this blog post, we will explore how to solve the LeetCode problem "2484" 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 a string of digits s, return the number of palindromic subsequences of s having length 5. Since the answer may be very large, return it modulo 109 + 7. Note: A string is palindromic if it reads the same forward and backward. A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters. Example 1: Input: s = "103301" Output: 2 Explanation: There are 6 possible subsequences of length 5: "10330","10331","10301","10301","13301","03301". Two of them (both equal to "10301") are palindromic. Example 2: Input: s = "0000000" Output: 21 Explanation: All 21 subsequences are "00000", which is palindromic. Example 3: Input: s = "9999900000" Output: 2 Explanation: The only two palindromic subsequences are "99999" and "00000". Constraints: 1 <= s.length <= 104 s consists of digits.

Explanation

Here's a breakdown of the solution approach, complexity analysis, and the Python code:

• High-Level Approach:

  • Iterate through all possible combinations of indices for a subsequence of length 5.
  • Check if the subsequence formed by these indices is a palindrome.
  • Count the number of palindromic subsequences, taking the modulo at the end.

• Complexity:

  • Runtime Complexity: O(n⁵) due to the five nested loops needed to check all combinations of indices for a subsequence of length 5, where n is the length of the input string.
  • Storage Complexity: O(1) - constant extra space.

Code

    def countPalindromicSubsequences(s: str) -> int:
    """
    Given a string of digits s, return the number of palindromic subsequences of s having length 5.
    Since the answer may be very large, return it modulo 109 + 7.

    Args:
        s (str): The input string of digits.

    Returns:
        int: The number of palindromic subsequences of length 5, modulo 10^9 + 7.
    """

    n = len(s)
    count = 0
    MOD = 10**9 + 7

    for i in range(n):
        for j in range(i + 1, n):
            for k in range(j + 1, n):
                for l in range(k + 1, n):
                    for m in range(l + 1, n):
                        subsequence = s[i] + s[j] + s[k] + s[l] + s[m]
                        if subsequence == subsequence[::-1]:
                            count = (count + 1) % MOD

    return count