public static final char END = '_';
public static final String BAD_SET_FORMAT = "BAD SET\n%s";

public static void noPrefix(List<String> words) {
// Write your code here
    Map<Character, Object> trie = new HashMap<>();
    Map<Character, Object> current = null;
    // incremental insertion of words
    for (String word: words) {
        current = trie;
        // for each word, insert and change current map/dict per letter
        for (char c: word.toCharArray()) {
            // FAIL: a processed word is the prefix of the current word
            if (current.containsKey(END)) {
                System.out.printf(BAD_SET_FORMAT, word);
                return;
            }
            // create map/dict if missing
            if (!current.containsKey(c)) {
                current.put(c, new HashMap<Character, Object>());
            }
            current = (HashMap<Character, Object>) current.get(c);
        }
        
        // FAIL: current word map/dict is not empty
        // meaning either:
        // - current word was already inserted in the trie
        // - current word was a prefix of previously inserted words
        if (!current.isEmpty()) {
            System.out.printf(BAD_SET_FORMAT, word);
            return;
        }
        
        // mark the current node where the current word ends here
        current.put(END, null);
    }
    System.out.println("GOOD SET");
}