2023.1.27-----leetcode.2309

public String greatestLetter(String s) {
        Set<Character> ht = new HashSet<Character>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            ht.add(c);
        }
        for (int i = 25; i >= 0; i--) {
            if (ht.contains((char) ('a' + i)) && ht.contains((char) ('A' + i))) {
                return String.valueOf((char) ('A' + i));
            }
        }
        return "";
    }