easy python solution
\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        letter_list, small_letter, capital_letter = [], [], []\n        for i in range(len(s)) 
```\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        letter_list, small_letter, capital_letter = [], [], []\n        for i in range(len(s)) :\n            if ord(s[i]) <= 90 : \n                if s[i] not in capital_letter : \n                    capital_letter.append(s[i])\n                    letter_list.append(s[i].lower())\n            else : \n                if s[i] not in small_letter : \n                    small_letter.append(s[i])\n                    letter_list.append(s[i])\n        \n        max_letter, max_ord = "", 0\n        for letter in letter_list : \n            if letter_list.count(letter) >= 2 : \n                if ord(letter) > max_ord : \n                    max_ord = ord(letter)\n                    max_letter = letter.upper()\n        return max_letter \n```
['String', 'Python', 'Python3']