Introduction

In this blog post, we will explore how to solve the LeetCode problem "2351" 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 consisting of lowercase English letters, return the first letter to appear twice. Note: A letter a appears twice before another letter b if the second occurrence of a is before the second occurrence of b. s will contain at least one letter that appears twice. Example 1: Input: s = "abccbaacz" Output: "c" Explanation: The letter 'a' appears on the indexes 0, 5 and 6. The letter 'b' appears on the indexes 1 and 4. The letter 'c' appears on the indexes 2, 3 and 7. The letter 'z' appears on the index 8. The letter 'c' is the first letter to appear twice, because out of all the letters the index of its second occurrence is the smallest. Example 2: Input: s = "abcdd" Output: "d" Explanation: The only letter that appears twice is 'd' so we return 'd'. Constraints: 2 <= s.length <= 100 s consists of lowercase English letters. s has at least one repeated letter.

Explanation

Here's an efficient solution to find the first letter to appear twice in a string:

• Use a set to track seen characters: Iterate through the string, adding each character to a set. If a character is already in the set, it means we've encountered it twice, and we can immediately return it.
• Leverage the properties of sets: Sets provide very fast lookups (typically O(1) on average), making the process of checking for duplicate characters highly efficient.

• Early exit: The moment a duplicate is found, the function returns, avoiding unnecessary iterations through the rest of the string.

• Runtime Complexity: O(n), where n is the length of the string. Storage Complexity: O(1), because the set can store at most 26 lowercase English letters.

Code

    def repeatedCharacter(s: str) -> str:
    """
    Finds the first letter to appear twice in a string.

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

    Returns:
        The first letter to appear twice.
    """
    seen = set()
    for char in s:
        if char in seen:
            return char
        seen.add(char)
    return ""  # This should never happen, as the problem statement guarantees a repeated character