public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
if (node == null) {
            return null;
        }
        Queue<UndirectedGraphNode> q = new LinkedList<UndirectedGraphNode>();
        HashMap<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<>();

        q.add(node);
        map.put(node, new UndirectedGraphNode(node.label));

        while (!q.isEmpty()) {
            UndirectedGraphNode tmp = q.poll();

            for (UndirectedGraphNode neighbor : tmp.neighbors){
//poll 出来的neighbor
                if (!map.containsKey(neighbor)) {
// clone the neighbor
                map.put(neighbor, new UndirectedGraphNode(neighbor.label));
                q.add(neighbor);
            }
   // copy the neighbor  
       map.get(tmp).neighbors.add(map.get(neighbor));//由key找value； 
            }

        }
        return map.get(node);
}
}