Clone Graph (LeetCode Graph)

Question: 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 #.
First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
Second node is labeled as 1. Connect node 1 to node 2.
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
         / \
         \_/

Idea: Recursion with caching the former results.

Time: O(n^2) Space: O(1)

Code:

 public class Solution {  
   HashMap<UndirectedGraphNode,UndirectedGraphNode> copied=new HashMap<UndirectedGraphNode,UndirectedGraphNode>();  
   public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {  
     if(node==null)  
       return null;  
     if(copied.containsKey(node))  
       return copied.get(node);  
     UndirectedGraphNode newNode=new UndirectedGraphNode(node.label);  
     copied.put(node, newNode);  
     List<UndirectedGraphNode> nbs=new ArrayList<UndirectedGraphNode>();  
     for(UndirectedGraphNode oriNeighbor:node.neighbors)  
     {  
       nbs.add(cloneGraph(oriNeighbor));  
     }  
     newNode.neighbors=nbs;  
     return newNode;  
   }  
 }