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Greatest English Letter in Upper and Lower Case

Difficulty: Easy

Problem Description

Given a string of English letters s, return the greatest English letter which occurs as both a lowercase and uppercase letter in s. The returned letter should be in uppercase. If no such letter exists, return an empty string.

Key Insights

  • We need to identify letters that appear in both lowercase and uppercase forms.
  • The greatest letter is determined by its position in the English alphabet: 'Z' > 'Y' > ... > 'A'.
  • We can utilize a set to track the presence of lowercase and uppercase letters efficiently.

Space and Time Complexity

Time Complexity: O(n) - We traverse the string once. Space Complexity: O(1) - We use a fixed number of variables (sets for 26 letters).

Solution

To solve the problem, we will:

  1. Create two sets to track the lowercase and uppercase letters we encounter in the string.
  2. Iterate through each character in the string:
    • If the character is lowercase, add it to the lowercase set.
    • If it is uppercase, add it to the uppercase set.
  3. For each letter from 'Z' to 'A', check if both its uppercase and lowercase forms exist in the respective sets.
  4. Return the first uppercase letter found (the greatest one). If none is found, return an empty string.

Code Solutions

def greatestLetter(s: str) -> str:
    lower_set = set()
    upper_set = set()

    # Track lowercase and uppercase letters
    for char in s:
        if char.islower():
            lower_set.add(char)
        elif char.isupper():
            upper_set.add(char)

    # Check from 'Z' to 'A' for the greatest letter
    for char in range(ord('Z'), ord('A') - 1, -1):
        if chr(char) in upper_set and chr(char).lower() in lower_set:
            return chr(char)
    
    return ""
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