Introduction

In this blog post, we will explore how to solve the LeetCode problem "3325" 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 and an integer k, return the total number of substrings of s where at least one character appears at least k times. Example 1: Input: s = "abacb", k = 2 Output: 4 Explanation: The valid substrings are: "aba" (character 'a' appears 2 times). "abac" (character 'a' appears 2 times). "abacb" (character 'a' appears 2 times). "bacb" (character 'b' appears 2 times). Example 2: Input: s = "abcde", k = 1 Output: 15 Explanation: All substrings are valid because every character appears at least once. Constraints: 1 <= s.length <= 3000 1 <= k <= s.length s consists only of lowercase English letters.

Explanation

Here's an efficient solution to the problem, along with explanations and code.

• Iterate through all possible substrings: The core idea is to generate all possible substrings of the input string s.
• Check frequency condition: For each substring, we count the frequency of each character and check if at least one character's frequency is greater than or equal to k.

• Count valid substrings: If the frequency condition is met, we increment a counter.

• Runtime Complexity: O(n^2), where n is the length of the string s.

• Storage Complexity: O(1), excluding the input string. The character counts use a fixed amount of space (26 integers).

Code

    def count_substrings(s: str, k: int) -> int:
    """
    Given a string s and an integer k, return the total number of substrings of s
    where at least one character appears at least k times.

    Example 1:
    Input: s = "abacb", k = 2
    Output: 4
    Explanation: The valid substrings are:
    "aba" (character 'a' appears 2 times).
    "abac" (character 'a' appears 2 times).
    "abacb" (character 'a' appears 2 times).
    "bacb" (character 'b' appears 2 times).

    Example 2:
    Input: s = "abcde", k = 1
    Output: 15
    Explanation: All substrings are valid because every character appears at least once.

    Constraints:
    1 <= s.length <= 3000
    1 <= k <= s.length
    s consists only of lowercase English letters.
    """
    n = len(s)
    count = 0

    for i in range(n):
        for j in range(i, n):
            substring = s[i:j+1]
            char_counts = {}
            for char in substring:
                char_counts[char] = char_counts.get(char, 0) + 1

            valid = False
            for char in char_counts:
                if char_counts[char] >= k:
                    valid = True
                    break

            if valid:
                count += 1

    return count