Introduction

In this blog post, we will explore how to solve the LeetCode problem "2262" 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 appeal of a string is the number of distinct characters found in the string. For example, the appeal of "abbca" is 3 because it has 3 distinct characters: 'a', 'b', and 'c'. Given a string s, return the total appeal of all of its substrings. A substring is a contiguous sequence of characters within a string. Example 1: Input: s = "abbca" Output: 28 Explanation: The following are the substrings of "abbca": - Substrings of length 1: "a", "b", "b", "c", "a" have an appeal of 1, 1, 1, 1, and 1 respectively. The sum is 5. - Substrings of length 2: "ab", "bb", "bc", "ca" have an appeal of 2, 1, 2, and 2 respectively. The sum is 7. - Substrings of length 3: "abb", "bbc", "bca" have an appeal of 2, 2, and 3 respectively. The sum is 7. - Substrings of length 4: "abbc", "bbca" have an appeal of 3 and 3 respectively. The sum is 6. - Substrings of length 5: "abbca" has an appeal of 3. The sum is 3. The total sum is 5 + 7 + 7 + 6 + 3 = 28. Example 2: Input: s = "code" Output: 20 Explanation: The following are the substrings of "code": - Substrings of length 1: "c", "o", "d", "e" have an appeal of 1, 1, 1, and 1 respectively. The sum is 4. - Substrings of length 2: "co", "od", "de" have an appeal of 2, 2, and 2 respectively. The sum is 6. - Substrings of length 3: "cod", "ode" have an appeal of 3 and 3 respectively. The sum is 6. - Substrings of length 4: "code" has an appeal of 4. The sum is 4. The total sum is 4 + 6 + 6 + 4 = 20. Constraints: 1 <= s.length <= 105 s consists of lowercase English letters.

Explanation

Here's a breakdown of the problem and an efficient solution:

• Key Idea: The core idea is to iterate through the string and for each character, calculate its contribution to the total appeal. A character contributes to the appeal of a substring if it's the first occurrence of that character within that substring. Instead of generating all substrings, we can efficiently calculate this contribution by tracking the last occurrence of each character.

• Optimization: The contribution of character s[i] to the total appeal is the number of substrings ending at i where s[i] is a distinct character. This is i - last_occurrence[s[i]], where last_occurrence[s[i]] stores the index of the last time s[i] appeared. This allows us to avoid redundant calculations.

• Complexity:

  • Runtime: O(n), where n is the length of the string.
  • Storage: O(1), as we use a fixed-size array (26 elements) to store the last occurrence of each lowercase letter.

Code

    def appealSum(s: str) -> int:
    """
    Calculates the total appeal of all substrings of a given string.

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

    Returns:
        The total appeal of all substrings.
    """

    last_occurrence = [-1] * 26  # Store the last occurrence of each character
    total_appeal = 0
    n = len(s)

    for i in range(n):
        char_index = ord(s[i]) - ord('a')
        total_appeal += (i - last_occurrence[char_index]) * (n - i)
        last_occurrence[char_index] = i

    return total_appeal