269. Alien Dictionary

本质上就是topological sort.

1. 统计所有的点

　　对于每一个string,把所有的字母都加入indegree

2. 构建图，统计indegree

　　对于没连续的一组str，找到第一个不同的字母，把后一个加入前一个字母的neighbor，并且给后一个字母indegree+1. 

　　需要注意的是，如果要检查后一个字母是不是已经在第一个字母的neighbor，如果不是再做后续操作

3. 遍历图

　　注意可能没有邻节点

4. 检查是不是所有点都访问到了，返回结果

    public String alienOrder(String[] words) {
        if(words.length < 1) {
            return "";
        }   
        String res = "";
        Map<Character, Set<Character>> map = new HashMap<Character, Set<Character>>();
        Map<Character, Integer> indegree = new HashMap<Character, Integer>();
        for(String s: words) {
            for(char c: s.toCharArray()) {
                indegree.put(c, 0);
            }
        }
        for(int i = 0; i < words.length - 1; i++) {
            String cur = words[i];
            String next = words[i + 1];
            int len = Math.min(cur.length(), next.length());
            for(int j = 0; j < len; j++) {
                char c1 = cur.charAt(j);
                char c2 = next.charAt(j);
                if(c1 != c2) {
                    Set<Character> neighbors = map.get(c1);
                    if(neighbors == null) {
                        neighbors = new HashSet<Character>();
                    }
                    if(!neighbors.contains(c2)) {
                        neighbors.add(c2);
                        map.put(c1, neighbors);
                        indegree.put(c2, indegree.get(c2) + 1);
                    }
                    break;
                }
            }
        }
        Queue<Character> queue = new LinkedList<Character>();
        for(Character c: indegree.keySet()) {
            if(indegree.get(c) == 0) {
                queue.offer(c);
            }
        }
        while(!queue.isEmpty()) {
            Character c = queue.poll();
            res += c;
            Set<Character> neighbors = map.get(c);
            if(neighbors == null) {
                continue;
            }
            for(Character x: neighbors) {
                indegree.put(x, indegree.get(x) - 1);
                if(indegree.get(x) == 0) {
                    queue.offer(x);
                }
            }
        }
        return indegree.size() == res.length()? res: "";
    }