clone-graph Java code

Clone an undirected graph. Each node in the graph contains alabeland a list of itsneighbors.
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 as0. Connect node0to both nodes1and2.
Second node is labeled as1. Connect node1to node2.
Third node is labeled as2. Connect node2to node2(itself), thus forming a self-cycle.
Visually, the graph looks like the following:
1
/ \
/ \
0 — 2
/ \
_/

import java.util.*;/** * Definition for undirected graph. * class UndirectedGraphNode { *     int label; *     ArrayList<UndirectedGraphNode> neighbors; *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); } * }; */public class Solution {    private HashMap<Integer,UndirectedGraphNode> map = new HashMap<>();    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {        return clone(node);    }    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){            clone.neighbors.add(clone(neighbor));        }        return clone;    }}