Greatest English Letter in Upper and Lower Case

Below is a step-by-step approach and code to find the greatest English letter that occurs as both a lowercase and uppercase letter in the given string s. If no such letter exists, we’ll return an empty string.

Understanding the Problem

You are given a string s containing English letters. You need to find the letter that appears in both lowercase and uppercase forms in s and is the greatest according to the English alphabet. The returned letter should be in uppercase.

Solution Approach

  1. Create a Set of Candidates: Create an empty set to store letters that are found in both lowercase and uppercase forms.

  2. Iterate Through the String: Iterate through the string, checking for letters that appear in both forms. Add such letters to the set.

  3. Find the Greatest Letter: Find the greatest letter from the set.

  4. Return the Result: Return the greatest letter in uppercase or an empty string if no such letter exists.

Here’s the code:

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class Solution:
    def greatestLetter(self, s: str) -> str:
        # Create a set to store letters that are found in both forms
        found_letters = set()

        # Iterate through the string to find letters that appear in both forms
        for letter in s:
            if letter.lower() in s and letter.upper() in s:
                found_letters.add(letter.lower())

        # If no such letters are found, return an empty string
        if not found_letters:
            return ""

        # Find and return the greatest letter from the set, in uppercase
        greatest_letter = max(found_letters)
        return greatest_letter.upper()

Key Takeaways

  • We have used a set to keep track of letters found in both forms, ensuring uniqueness.
  • We have iterated through the string to find such letters, adding them to the set.
  • We have used the max function to find the greatest letter and returned it in uppercase.
  • The code handles the case where no such letter is found, returning an empty string.

This code meets the problem’s constraints and provides a clear solution to find the desired letter from the given string.