Simple Python 2-liner
\n\n# Code\n\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        a = [x for x in s if x.upper() in s and x.lower() in s] + [""]\n        ret
\n\n# Code\n```\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        a = [x for x in s if x.upper() in s and x.lower() in s] + [""]\n        return max(a).upper()\n```\n
['Python3']