Simple Python Solution using hashMap and set
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<!-- Add your time complexity here, e.g. $$O(n)$$ -->\n\n- Space complexity:\n<!-- Add your space complexity here, e.g. $$O(n)$$ -->\n\n# Code\n```\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        hMap = Counter()\n\n        for char in s:\n            temp = char.lower()\n\n            if temp in hMap:\n                hMap[temp].add(char)\n            else:\n                hMap[temp] = set(char)\n        \n        max_char = \'\'\n        for key, sets in hMap.items():\n            if len(sets) == 2:\n                max_char = max(max_char, key)\n        \n        return max_char.upper()\n```
['Python3']