Introduction

In this blog post, we will explore how to solve the LeetCode problem "1297" 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 s, return the maximum number of occurrences of any substring under the following rules: The number of unique characters in the substring must be less than or equal to maxLetters. The substring size must be between minSize and maxSize inclusive. Example 1: Input: s = "aababcaab", maxLetters = 2, minSize = 3, maxSize = 4 Output: 2 Explanation: Substring "aab" has 2 occurrences in the original string. It satisfies the conditions, 2 unique letters and size 3 (between minSize and maxSize). Example 2: Input: s = "aaaa", maxLetters = 1, minSize = 3, maxSize = 3 Output: 2 Explanation: Substring "aaa" occur 2 times in the string. It can overlap. Constraints: 1 <= s.length <= 105 1 <= maxLetters <= 26 1 <= minSize <= maxSize <= min(26, s.length) s consists of only lowercase English letters.

Explanation

Here's a breakdown of the approach, followed by the Python code:

• Focus on minSize: Since we are looking for the maximum number of occurrences, and the substring length must be between minSize and maxSize, we can optimize by only considering substrings of length minSize. Any substring longer than minSize that satisfies the unique character constraint must contain a substring of length minSize that also satisfies the constraint. If the longer string occurs more often than its shorter substrings, then the shorter substrings are less important.

• Sliding Window and Frequency Counting: Iterate through the string using a sliding window of size minSize. For each substring, check if the number of unique characters is within the allowed limit (maxLetters). If it is, update the frequency count of that substring.

• Maximize Frequency: After iterating through all possible substrings, find the substring with the maximum frequency.

• Time & Space Complexity: O(n), O(n) where n is the length of string s.

Code

    def maxRepOpt1(s: str, maxLetters: int, minSize: int, maxSize: int) -> int:
    """
    Finds the maximum number of occurrences of any substring under given rules.
    """
    n = len(s)
    substring_counts = {}

    for i in range(n - minSize + 1):
        substring = s[i:i + minSize]
        unique_chars = set(substring)

        if len(unique_chars) <= maxLetters:
            substring_counts[substring] = substring_counts.get(substring, 0) + 1

    max_count = 0
    for count in substring_counts.values():
        max_count = max(max_count, count)

    return max_count