Introduction

In this blog post, we will explore how to solve the LeetCode problem "2272" 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

The variance of a string is defined as the largest difference between the number of occurrences of any 2 characters present in the string. Note the two characters may or may not be the same. Given a string s consisting of lowercase English letters only, return the largest variance possible among all substrings of s. A substring is a contiguous sequence of characters within a string. Example 1: Input: s = "aababbb" Output: 3 Explanation: All possible variances along with their respective substrings are listed below: - Variance 0 for substrings "a", "aa", "ab", "abab", "aababb", "ba", "b", "bb", and "bbb". - Variance 1 for substrings "aab", "aba", "abb", "aabab", "ababb", "aababbb", and "bab". - Variance 2 for substrings "aaba", "ababbb", "abbb", and "babb". - Variance 3 for substring "babbb". Since the largest possible variance is 3, we return it. Example 2: Input: s = "abcde" Output: 0 Explanation: No letter occurs more than once in s, so the variance of every substring is 0. Constraints: 1 <= s.length <= 104 s consists of lowercase English letters.

Explanation

Here's the approach, analysis, and code:

• Pairwise Iteration: The core idea is to iterate through all possible pairs of distinct characters (major and minor) present in the string.
• Kadane's Algorithm Adaptation: For each character pair, we adapt Kadane's algorithm to find the maximum difference between the counts of the major and minor characters within substrings. We ensure that the minor character appears at least once in the substring.

• Maximization: The algorithm maintains a running maximum variance across all character pairs and substrings.

• Time Complexity: O(26 26 N), where N is the length of the string. This simplifies to O(N).

• Space Complexity: O(1).

Code

    def largestVariance(s: str) -> int:
    """
    Calculates the largest variance possible among all substrings of s.

    Args:
        s: The input string consisting of lowercase English letters.

    Returns:
        The largest variance possible among all substrings of s.
    """

    chars = set(s)
    max_variance = 0

    for major in chars:
        for minor in chars:
            if major == minor:
                continue

            major_count = 0
            minor_count = 0
            min_so_far = 0
            variance = 0
            minor_exists = False

            for char in s:
                if char == major:
                    major_count += 1
                elif char == minor:
                    minor_count += 1
                    minor_exists = True

                curr_diff = major_count - minor_count

                variance = max(variance, curr_diff - min_so_far)
                min_so_far = min(min_so_far, curr_diff)

                if minor_count > 0 and variance > max_variance:
                    max_variance = variance

                if major_count < minor_count:
                    major_count = 0
                    minor_count = 0
                    min_so_far = 0
                    minor_exists = False

    return max_variance