greatest_english_letter_in_upper_and_lower_case.py

# 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.

# An English letter b is greater than another letter a if b appears after a in the English alphabet.

 

# Example 1:

# Input: s = "lEeTcOdE"
# Output: "E"
# Explanation:
# The letter 'E' is the only letter to appear in both lower and upper case.
# Example 2:

# Input: s = "arRAzFif"
# Output: "R"
# Explanation:
# The letter 'R' is the greatest letter to appear in both lower and upper case.
# Note that 'A' and 'F' also appear in both lower and upper case, but 'R' is greater than 'F' or 'A'.
# Example 3:

# Input: s = "AbCdEfGhIjK"
# Output: ""
# Explanation:
# There is no letter that appears in both lower and upper case.

class Solution:
    def greatestLetter(self, s: str) -> str:
        greatest = 'a'
        changed = False
        for i in range(1, len(s)):
            if ord(greatest) <= ord(s[i]) and s[i].upper() in s:
                changed = True
                greatest = s[i]
                
        if changed:       
            return greatest.upper()
        else:
            return ''