2309_GreatestEnglishLetterinUpperandLowerCase - a920604a/leetcode GitHub Wiki

class Solution {
public:
    string greatestLetter(string s) {
        vector<int> lower(26,0), upper(26,0);
        for(char c:s){
            if(c>=65 && c<=90) upper[c-'A']++;
            else lower[c-'a']++;
        }
        for(int i=25;i>-1;--i){
            if(lower[i]>0 && upper[i]>0){
                
                return string(1, 'A'+i);
            }
        }
        return "";
    }
};