Introduction

In this blog post, we will explore how to solve the LeetCode problem "3504" 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 strings, s and t. You can create a new string by selecting a substring from s (possibly empty) and a substring from t (possibly empty), then concatenating them in order. Return the length of the longest palindrome that can be formed this way. Example 1: Input: s = "a", t = "a" Output: 2 Explanation: Concatenating "a" from s and "a" from t results in "aa", which is a palindrome of length 2. Example 2: Input: s = "abc", t = "def" Output: 1 Explanation: Since all characters are different, the longest palindrome is any single character, so the answer is 1. Example 3: Input: s = "b", t = "aaaa" Output: 4 Explanation: Selecting "aaaa" from t is the longest palindrome, so the answer is 4. Example 4: Input: s = "abcde", t = "ecdba" Output: 5 Explanation: Concatenating "abc" from s and "ba" from t results in "abcba", which is a palindrome of length 5. Constraints: 1 <= s.length, t.length <= 1000 s and t consist of lowercase English letters.

Explanation

Here's a breakdown of the approach, complexities, and the Python code:

• High-Level Approach:

  • Iterate through all possible center positions in the combined string space formed by s and t.
  • For each center, expand outwards to find the longest palindrome centered there. The center could be within s, within t, or straddling the boundary between s and t.
  • Keep track of the maximum palindrome length found.

• Complexity:

  • Runtime: O((m+n)^2), where n is the length of s, and m is the length of t.
  • Storage: O(1)

Code

    def longestPalindrome(s: str, t: str) -> int:
    """
    Finds the length of the longest palindrome that can be formed by concatenating a substring of s and a substring of t.
    """

    def expand_around_center(left: int, right: int) -> int:
        """
        Expands outwards from a center (or center pair) to find the longest palindrome.
        """
        count = 0
        while left >= 0 and right < len(s) + len(t) and get_char(left) == get_char(right):
            count += 1
            left -= 1
            right += 1
        return count

    def get_char(index: int) -> str:
        """
        Helper function to retrieve a character from the combined string (s + t).
        """
        if index < len(s):
            return s[index]
        else:
            return t[index - len(s)]

    max_len = 0
    combined_len = len(s) + len(t)

    # Iterate through all possible center positions
    for center in range(combined_len):
        # Odd length palindromes
        max_len = max(max_len, 2 * expand_around_center(center, center) - 1)

        # Even length palindromes
        max_len = max(max_len, 2 * expand_around_center(center, center + 1))

    return max_len