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.
'''

class Solution:
    def greatestLetter(self, s: str) -> str:
        res = ''
        for i in s:
            if i.isupper() and i.lower() in s:
                if i > res:
                    res = i.upper()
        return res