public String alienOrder(String[] words) {

int[] indegree = new int[26];

Arrays.fill(indegree, -1);

/**

int count = 0;

for (String word : words) {

for (char c : word.toCharArray()) {

if (indegree[c - 'a'] != 0) {

indegree[c - 'a'] = 0;

count++;

}

}

}

/**

HashMap<Character, Set<Character>> map = new HashMap<>();

for (int i = 0; i < words.length - 1; i++) {

String first = words[i], second = words[i + 1];

int len = Math.min(first.length(), second.length());

for (int j = 0; j < len; j++) {

if (first.charAt(j) != second.charAt(j)) {

Set<Character> set = map.get(first.charAt(j));

if (set == null) {

set = new HashSet<Character>();

map.put(first.charAt(j), set);

}

/**

if (set.add(second.charAt(j))) {

indegree[second.charAt(j) - 'a']++;

}

/**

break;

} else {

/**

if (j + 1 >= second.length() && j + 1 < first.length()) {

return "";

}

}

}

}

/**

Queue<Character> queue = new LinkedList<Character>();

for (int i = 0; i < indegree.length; i++) {

if (indegree[i] == 0) {

queue.add((char) ('a' + i));

}

}

StringBuilder sb = new StringBuilder();

while (!queue.isEmpty()) {

Character from = queue.poll();

sb.append(from);

Set<Character> set = map.get(from);

if (set != null) {

for (Character to : map.get(from)) {

if (--indegree[to - 'a'] == 0) {

queue.add(to);

}

}

}

}

return sb.length() != count ? "" : sb.toString();

}