克隆一个无向图

import java.util.ArrayList;
import java.util.HashMap;
class UndirectedGraphNode {
      int label;
      ArrayList<UndirectedGraphNode> neighbors;
      UndirectedGraphNode(int x) { 
        label = x; 
        neighbors = new ArrayList<UndirectedGraphNode>(); 
      }
  };

public class Solution {
    HashMap<Integer,UndirectedGraphNode>map=new HashMap<>();
    //递归实现无向图的克隆
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {

           return clone(node);

    }
    //递归函数
    private UndirectedGraphNode clone(UndirectedGraphNode node){
           if(node==null) 
               return null;
           if(map.containsKey(node.label)){
            return map.get(node.label);
           } 
           UndirectedGraphNode clone=new UndirectedGraphNode(node.label);
           map.put(clone.label,clone);
           for(UndirectedGraphNode neighbor:node.neighbors){
              //递归调用
              clone.neighbors.add(clone(neighbor));
           }
           return clone; 
    }
    public static void main(String[]args){
      System.out.println("Hello World!");

    }
}