Introduction

In this blog post, we will explore how to solve the LeetCode problem "2982" 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 a string s that consists of lowercase English letters. A string is called special if it is made up of only a single character. For example, the string "abc" is not special, whereas the strings "ddd", "zz", and "f" are special. Return the length of the longest special substring of s which occurs at least thrice, or -1 if no special substring occurs at least thrice. A substring is a contiguous non-empty sequence of characters within a string. Example 1: Input: s = "aaaa" Output: 2 Explanation: The longest special substring which occurs thrice is "aa": substrings "aaaa", "aaaa", and "aaaa". It can be shown that the maximum length achievable is 2. Example 2: Input: s = "abcdef" Output: -1 Explanation: There exists no special substring which occurs at least thrice. Hence return -1. Example 3: Input: s = "abcaba" Output: 1 Explanation: The longest special substring which occurs thrice is "a": substrings "abcaba", "abcaba", and "abcaba". It can be shown that the maximum length achievable is 1. Constraints: 3 <= s.length <= 5 * 105 s consists of only lowercase English letters.

Explanation

Here's a breakdown of the solution:

• Identify Special Substrings: Iterate through the string to identify consecutive sequences of the same character. Store the length and character of each such "special" substring.

• Count Occurrences: For each identified special substring length, check how many times substrings of that length appear in the original string.

• Determine Maximum Length: Return the largest length that occurs at least three times. Return -1 if no such length is found.

• Complexity: O(n) time and O(1) space, where n is the length of the string. Although we iterate through the string and check substrings, the number of unique characters is limited to 26 (lowercase English letters), making the space complexity constant.

Code

    def longest_special_substring(s: str) -> int:
    """
    Finds the length of the longest special substring of s which occurs at least thrice.
    """
    n = len(s)
    max_len = -1

    for length in range(1, n // 3 + 1):
        for char in set(s):
            substring = char * length
            count = 0
            for i in range(n - length + 1):
                if s[i:i+length] == substring:
                    count += 1
            if count >= 3:
                max_len = max(max_len, length)

    return max_len