【leetcode】Clone Graph

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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         /          \_/

題解：利用DFS遞迴的方法複製圖，用hashmap儲存原圖中的節點和新圖中的節點的對應關係。而且利用這個hashmap可以在O(1)的時間內判斷一個節點是否被複製過，並且知道複製它得到的新節點。

實現一個複製節點的方法，在這個方法中：

1. 判斷這個節點是否被clone過了，如果是，返回它對應的clone節點；
2. 如果沒有被clone過，那麼建立節點為該節點的clone節點，並且遞迴的clone它的鄰居節點，放入該節點的neighbors列表裡面；

代碼如下：

 1 /** 2  * Definition for undirected graph. 3  * class UndirectedGraphNode { 4  *     int label; 5  *     List<UndirectedGraphNode> neighbors; 6  *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); } 7  * }; 8  */ 9 public class Solution {10     private HashMap<UndirectedGraphNode,UndirectedGraphNode> mapClone = new HashMap<UndirectedGraphNode,UndirectedGraphNode>();11     12     private UndirectedGraphNode cloneNode(UndirectedGraphNode node){13         if(node == null)14             return null;15         //if this node already has a clone16         if(mapClone.containsKey(node))17             return mapClone.get(node);18         19         //if this node hasn‘t been cloned, we construct a new node copy20         UndirectedGraphNode copy = new UndirectedGraphNode(node.label);21         mapClone.put(node, copy);22         23         //clone all the neighbers of node24         for(UndirectedGraphNode n:node.neighbors){25             copy.neighbors.add(cloneNode(n));26         }27         return copy;28     }29     public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {30         return cloneNode(node);31     }32 }