python || O(N)
Intuition\n Describe your first thoughts on how to solve this problem. \n\n# Approach\n Describe your approach to solving the problem. \n\n# Complexity\n- Time 
# Intuition\n<!-- Describe your first thoughts on how to solve this problem. -->\n\n# Approach\n<!-- Describe your approach to solving the problem. -->\n\n# Complexity\n- Time complexity:\n- O(N)\n<!-- Add your time complexity here, e.g. $$O(n)$$ -->\n\n- Space complexity:\n- O(1)\n<!-- Add your space complexity here, e.g. $$O(n)$$ -->\n\n# Code\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```
['Python3']