【leetcode】Clone Graph

题目：

Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors.

OJ's undirected graph serialization:

Nodes are labeled uniquely.

We use  # as a separator for each node, and  , as a separator for node label and each neighbor of the node.

As an example, consider the serialized graph {0,1,2#1,2#2,2}.

The graph has a total of three nodes, and therefore contains three parts as separated by #.

1. First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
2. Second node is labeled as 1. Connect node 1 to node 2.
3. Third node is labeled as 2. Connect node 2 to node 2 (itself), thus forming a self-cycle.

Visually, the graph looks like the following:

       1
      / \
     /   \
    0 --- 2
         / \
         \_/

思路：

首先做一次BFS遍历，将所有结点创建出来。并且要保存在一个可以在短时间读取的结构中。map是个不错的结构并且每个结点label唯一，因此我们可以用label来作为索引。

第二次遍历的时候可以快速地找出结点并构造图的关系。

代码：

[java]  view plain copy

1. /** 
2.  * Definition for undirected graph. 
3.  * class UndirectedGraphNode { 
4.  *     int label; 
5.  *     ArrayList<UndirectedGraphNode> neighbors; 
6.  *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); } 
7.  * }; 
8.  */  
9. public class Solution {  
10.     Map<Integer, UndirectedGraphNode> nodeMap = new HashMap<Integer, UndirectedGraphNode>();  
11.     public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {  
12.         // Note: The Solution object is instantiated only once and is reused by each test case.  
13.         if (node == null) {  
14.             return null;  
15.         }  
16.           
17.         LinkedList<UndirectedGraphNode> stack = new LinkedList<UndirectedGraphNode>();  
18.         //Map<Integer, UndirectedGraphNode> nodeMap = new HashMap<Integer, UndirectedGraphNode>();  
19.         nodeMap.clear();  
20.         Set<Integer> isFound = new HashSet<Integer>();  
21.           
22.         stack.addLast(node);  
23.           
24.         UndirectedGraphNode head = null;  
25.           
26.         while (!stack.isEmpty()) {  
27.             UndirectedGraphNode first = stack.getFirst();  
28.             stack.removeFirst();  
29.               
30.             int label = first.label;  
31.               
32.             UndirectedGraphNode n = nodeMap.get(label);  
33.             if (n == null) {  
34.                 n = new UndirectedGraphNode(label);  
35.                 nodeMap.put(label, n);  
36.             }  
37.               
38.             if (head == null) {  
39.                 head = n;  
40.             }  
41.               
42.             if (!isFound.contains(label)) {  
43.                 isFound.add(label);  
44.                   
45.                 ArrayList<UndirectedGraphNode> neighbors = first.neighbors;  
46.                   
47.                 for (UndirectedGraphNode nb : neighbors) {  
48.                     UndirectedGraphNode nn = nodeMap.get(nb.label);  
49.                     if (nn == null) {  
50.                         nn = new UndirectedGraphNode(nb.label);  
51.                         nodeMap.put(nb.label, nn);  
52.                           
53.                         stack.addLast(nb);  
54.                     }  
55.                     n.neighbors.add(nn);                }  
56.             }  
57.               
58.               
59.         }  
60.           
61.         return head;  
62.     }  
63.   
64. }