Fast Python O(n) using set
The main idea is to enumerate a char (in both its upper and lower cases) from \'z\' to \'a\', and "probe"  if they exist in the input string. Also implement an 
The main idea is to enumerate a char (in both its upper and lower cases) from \'z\' to \'a\', and "probe"  if they exist in the input string. Also implement an early return. \n```\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        s = set(s)\n        upper, lower = ord(\'Z\'), ord(\'z\')\n        for i in range(26):\n            if chr(upper - i) in s and chr(lower - i) in s:\n                return chr(upper - i)\n        return \'\'\n```
['Ordered Set', 'Python']