Introduction

In this blog post, we will explore how to solve the LeetCode problem "424" 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 and an integer k. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most k times. Return the length of the longest substring containing the same letter you can get after performing the above operations. Example 1: Input: s = "ABAB", k = 2 Output: 4 Explanation: Replace the two 'A's with two 'B's or vice versa. Example 2: Input: s = "AABABBA", k = 1 Output: 4 Explanation: Replace the one 'A' in the middle with 'B' and form "AABBBBA". The substring "BBBB" has the longest repeating letters, which is 4. There may exists other ways to achieve this answer too. Constraints: 1 <= s.length <= 105 s consists of only uppercase English letters. 0 <= k <= s.length

Explanation

Here's the solution to the problem, with a focus on efficiency and clarity:

• Sliding Window: Use a sliding window to explore all possible substrings.
• Character Count: Maintain a count of each character within the current window.

• Window Adjustment: Expand the window while the number of replacements needed (window size minus the count of the most frequent character) is within the allowed k. If it exceeds k, shrink the window from the left.

• Runtime Complexity: O(n), where n is the length of the string.

• Storage Complexity: O(26) which simplifies to O(1), since the size of the character count array is fixed.

Code

    def characterReplacement(s: str, k: int) -> int:
    """
    Finds the length of the longest substring containing the same letter after performing at most k replacements.

    Args:
        s: The input string consisting of uppercase English letters.
        k: The maximum number of replacements allowed.

    Returns:
        The length of the longest substring with repeating characters.
    """

    left = 0
    max_length = 0
    char_counts = {}

    for right in range(len(s)):
        char_counts[s[right]] = char_counts.get(s[right], 0) + 1

        # Calculate the number of replacements needed
        max_char_count = max(char_counts.values())
        replacements_needed = (right - left + 1) - max_char_count

        # If the number of replacements needed exceeds k, shrink the window
        if replacements_needed > k:
            char_counts[s[left]] -= 1
            left += 1

        # Update the maximum length of the substring
        max_length = max(max_length, right - left + 1)

    return max_length