LeetCode 269: Alien Dictionary

class Solution {
    private Map<Character, Set<Character>> dict;
    public String alienOrder(String[] words) {
        if (words.length == 0) {
            return "";
        }
        
        dict = new HashMap<>();
        for (String word : words) {
            for (char c : word.toCharArray()) {
                if (!dict.containsKey(c)) {
                    dict.put(c, new HashSet<>());
                }
            }
        }
        int total = dict.size();
        
        for (int i = 0; i < words.length - 1; i++) {
            generateDependencies(words[i], words[i + 1]);
        }

        Queue<Character> queue = new LinkedList<>();
        List<Character> toRemove = new ArrayList<>();
        for (char c : dict.keySet()) {
            if (dict.get(c).size() == 0) {
                queue.offer(c);
                toRemove.add(c);
            }
        }
        dict.keySet().removeAll(toRemove);

        StringBuilder result = new StringBuilder();
        while (!queue.isEmpty()) {
            int length = queue.size();
            for (int i = 0; i < length; i++) {
                char current = queue.poll();
                toRemove.clear();
                for (char c : dict.keySet()) {
                    if (dict.get(c).contains(current)) {
                        dict.get(c).remove(new Character(current));
                    }
                    if (dict.get(c).size() == 0) {
                        queue.offer(c);
                        toRemove.add(c);
                    }
                }
                dict.keySet().removeAll(toRemove);
                result.append(current);
            }
        }
        return total == result.length() ? result.toString() : "";
    }
    
    private void generateDependencies(String s1, String s2) {
        int index = 0;
        while (index < s1.length() && index < s2.length()) {
            if (s1.charAt(index) != s2.charAt(index)) {
                dict.get(s2.charAt(index)).add(s1.charAt(index));
                break;
            }
            index++;
        }
    }
}