Introduction

In this blog post, we will explore how to solve the LeetCode problem "3084" 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 a character c. Return the total number of substrings of s that start and end with c. Example 1: Input: s = "abada", c = "a" Output: 6 Explanation: Substrings starting and ending with "a" are: "abada", "abada", "abada", "abada", "abada", "abada". Example 2: Input: s = "zzz", c = "z" Output: 6 Explanation: There are a total of 6 substrings in s and all start and end with "z". Constraints: 1 <= s.length <= 105 s and c consist only of lowercase English letters.

Explanation

Here's the solution to the problem:

• Count Occurrences: Iterate through the string and count the number of times the character c appears.

• Calculate Substrings: Use the formula n * (n + 1) // 2 where n is the count of character c. This formula calculates the number of substrings that can be formed using these occurrences as start and end points.

• Runtime Complexity: O(n), where n is the length of the string. Storage Complexity: O(1).

Code

    def count_substrings(s: str, c: str) -> int:
    """
    Given a string s and a character c. Return the total number of substrings of s that start and end with c.

    Example 1:
    Input: s = "abada", c = "a"
    Output: 6
    Explanation: Substrings starting and ending with "a" are: "a", "aba", "abada", "a", "ada", "a".

    Example 2:
    Input: s = "zzz", c = "z"
    Output: 6
    Explanation: There are a total of 6 substrings in s and all start and end with "z".

    Constraints:
    1 <= s.length <= 105
    s and c consist only of lowercase English letters.
    """
    count = 0
    for char in s:
        if char == c:
            count += 1
    return count * (count + 1) // 2