2309. Greatest English Letter in Upper and Lower Case

Given a string of English letters s, return the greatest English letter which occurs as both a lowercase and uppercase letter in s. The returned letter should be in uppercase. If no such letter exists, return an empty string.

An English letter b is greater than another letter a if b appears after a in the English alphabet.

class Solution {
public:
    string greatestLetter(string s) {
        unordered_set<char> us(begin(s), end(s));
        for(char ch = 'Z'; ch >= 'A'; ch--) {
            if(us.count(ch) and us.count(ch-'A'+'a'))
                return string(1,ch);
        }
        
        return "";
    }
};