public class Solution {
    Set<Character> visited = new HashSet<>();
    Set<Character> visiting = new HashSet<>();
    Set<Character> unmark = new HashSet<>();
     Set<Character> unvisit = new HashSet<>();
    String path = "";
     Map<Character, List<Character>> map = new HashMap<>();
     boolean shitvalue = false;
    public String alienOrder(String[] words) {
       if (words.length == 0) return "";
        if (words.length == 1) return words[0];
        int i = 0;
        char c = words[0].charAt(i);
        List<List<String>> buckets;


        do {
            buckets = new ArrayList<>();
            List<String> temp = new ArrayList<>();
            for (String word : words) {
                unmark.add(c);
                if (i >= word.length()) continue;
                if (word.charAt(i) != c) {
                    c = word.charAt(i);
                    buckets.add(temp);
                    temp = new ArrayList<>();
                }
                temp.add(word);
            }
            if (temp.size() > 0) buckets.add(temp);

            for (int j = 0; j < buckets.size(); j++) {
                if (j == 0) {
                    if (buckets.get(0).size() == 0) continue;
                    if (!map.containsKey(buckets.get(0))) map.put(buckets.get(0).get(0).charAt(i), new ArrayList<>());
                } else {
                    if (!map.containsKey(buckets.get(j - 1))) {
                        if (buckets.get(j - 1).size() == 0) continue;
                        map.put(buckets.get(j - 1).get(0).charAt(i), new ArrayList<>());
                    }
                    map.get(buckets.get(j - 1).get(0).charAt(i)).add(buckets.get(j).get(0).charAt(i));
                }
            }
            i++;
        } while (buckets.size() > 0);


        while (unmark.size() > 0) {

            char character = 's';
            for (char asdf : unmark) {
                character = asdf;
                break;
            }
            unmark.remove(character);
            visit(character);

        }
        return shitvalue ? "" : path;

    }
    public void visit(char c) {
        if (visiting.contains(c)) {
            shitvalue =true;
            return;
        }
        if (!visited.contains(c)) {
            if (unmark.contains(c)) unmark.remove(c);
            visiting.add(c);
            if (map.get(c) != null) {
                for (char adj : map.get(c)) {
                    visit(adj);
                }
            }
            visited.add(c);
            visiting.remove(c);
            path = c + path;
        }
    }
}