Find the number Weak Connected Component in the directed graph. Each node in the graph contains a label and a list of its neighbors. (a connected set of a directed graph is a subgraph in which any two vertices are connected by direct edge path.)

 1 /**
 2  * Definition for Directed graph.
 3  * class DirectedGraphNode {
 4  *     int label;
 5  *     ArrayList<DirectedGraphNode> neighbors;
 6  *     DirectedGraphNode(int x) { label = x; neighbors = new ArrayList<DirectedGraphNode>(); }
 7  * };
 8  */
 9 public class Solution {
10     public List<List<Integer>> connectedSet2(ArrayList<DirectedGraphNode> nodes) {
11
12         List<List<Integer>> res = new ArrayList<List<Integer>>();
13         if(nodes == null || nodes.size() == 0){
14             return res;
15         }
16
17         HashSet<Integer> hs = new HashSet<Integer>();
18         for(DirectedGraphNode node : nodes){
19             hs.add(node.label);
20             for(DirectedGraphNode neigh : node.neighbors){
21                 hs.add(neigh.label);
22             }
23         }
24
25         UnionFind forest = new UnionFind(hs);
26         for(DirectedGraphNode node : nodes){
27             for(DirectedGraphNode neigh : node.neighbors){
28                 forest.union(node.label, neigh.label);
29             }
30         }
31
32         HashMap<Integer, List<Integer>> resHashMap = new HashMap<Integer, List<Integer>>();
33         for(int i : hs){
34             //找到root
35             int rootParent = forest.root(i);
36             if(!resHashMap.containsKey(rootParent)){
37                 resHashMap.put(rootParent, new ArrayList<Integer>());
38             }
39             //每个root下面的值都放在一个list里，包括root本身
40             resHashMap.get(rootParent).add(i);
41         }
42
43         for(List<Integer> item : resHashMap.values()){
44             Collections.sort(item);
45             res.add(item);
46         }
47         return res;
48     }
49 }
50
51 class UnionFind{
52
53     //HashMap maintaining key - > value (child -> parent) relationship
54     HashMap<Integer, Integer> parent;
55     public UnionFind(HashSet<Integer> hs){
56         parent = new HashMap<Integer, Integer>();
57         for(int i : hs){
58             parent.put(i, i);
59         }
60     }
61
62     public int root(int i){
63         while(i != parent.get(i)){
64             parent.put(i, parent.get(parent.get(i)));
65             i = parent.get(i);
66         }
67         return i;
68     }
69
70     public void union(int i, int j){
71         int p = root(i);
72         int q = root(j);
73         if(p != q){
74             parent.put(p, q);
75         }
76     }
77 }