Python || Hash Map || O(N) || Easy Solution
Code\n\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        dic={}\n        res=""\n        for i in s:\n            if i>=\'A\' and i<=\'Z\'
# Code\n```\nclass Solution:\n    def greatestLetter(self, s: str) -> str:\n        dic={}\n        res=""\n        for i in s:\n            if i>=\'A\' and i<=\'Z\':\n                if i in dic and dic[i]%2==1:\n                    dic[i]+=2\n                elif i not in dic:\n                    dic[i]=2\n            if i>=\'a\' and i<=\'z\':\n                i=i.upper()\n                if i in dic and dic[i]%2==0:\n                    dic[i]+=1\n                elif i not in dic:\n                    dic[i]=1\n        for i in dic:\n            if dic[i]>=3:\n                res=max(i,res)\n        return res   \n```\n\n***Please Upvote***
['Python3']