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;
    }
}