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

 1     public String alienOrder(String[] words) {
 2         if(words.length < 1) {
 3             return "";
 4         }
 5         String res = "";
 6         Map<Character, Set<Character>> map = new HashMap<Character, Set<Character>>();
 7         Map<Character, Integer> indegree = new HashMap<Character, Integer>();
 8         for(String s: words) {
 9             for(char c: s.toCharArray()) {
10                 indegree.put(c, 0);
11             }
12         }
13         for(int i = 0; i < words.length - 1; i++) {
14             String cur = words[i];
15             String next = words[i + 1];
16             int len = Math.min(cur.length(), next.length());
17             for(int j = 0; j < len; j++) {
18                 char c1 = cur.charAt(j);
19                 char c2 = next.charAt(j);
20                 if(c1 != c2) {
21                     Set<Character> neighbors = map.get(c1);
22                     if(neighbors == null) {
23                         neighbors = new HashSet<Character>();
24                     }
25                     if(!neighbors.contains(c2)) {
26                         neighbors.add(c2);
27                         map.put(c1, neighbors);
28                         indegree.put(c2, indegree.get(c2) + 1);
29                     }
30                     break;
31                 }
32             }
33         }
34         Queue<Character> queue = new LinkedList<Character>();
35         for(Character c: indegree.keySet()) {
36             if(indegree.get(c) == 0) {
37                 queue.offer(c);
38             }
39         }
40         while(!queue.isEmpty()) {
41             Character c = queue.poll();
42             res += c;
43             Set<Character> neighbors = map.get(c);
44             if(neighbors == null) {
45                 continue;
46             }
47             for(Character x: neighbors) {
48                 indegree.put(x, indegree.get(x) - 1);
49                 if(indegree.get(x) == 0) {
50                     queue.offer(x);
51                 }
52             }
53         }
54         return indegree.size() == res.length()? res: "";
55     }