Python Solution | Complexity O(nlogn)
\ndef greatestLetter(self, s: str) -> str:\n        leng = len(s)\n        if leng == 1:\n            return \'\'\n        greatest_char = []\n        char_hash
```\ndef greatestLetter(self, s: str) -> str:\n        leng = len(s)\n        if leng == 1:\n            return \'\'\n        greatest_char = []\n        char_hash = {}\n        \n        for i in range(1, leng):\n            if (ord(s[i]) < 91 and ord(s[i])+32 in char_hash):\n                greatest_char.append(ord(s[i]))\n            elif(ord(s[i]) > 96 and ord(s[i]) - 32 in char_hash):\n                greatest_char.append(ord(s[i])-32)\n            char_hash[ord(s[i])] = 1\n            \n        greatest_char.sort()\n        return chr(greatest_char[-1]) if len(greatest_char) else ""\n                        \n\n```
['Sorting', 'Python']