Simple Python3 Solution
\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        chars = set(s)\n        l = []\n        for i in chars:\n            if i.upper() in ch
```\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        chars = set(s)\n        l = []\n        for i in chars:\n            if i.upper() in chars and i.lower() in chars:\n                l.append(i)\n        if l:\n            l.sort()\n            return l[-1].upper()\n        else:\n            return ""\n```
['Python', 'Python3']