clone graph

    private HashMap<UndirectedGraphNode, UndirectedGraphNode> hm = new HashMap<>();
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null){
            return null;
        }
        if (hm.containsKey(node)){
            return hm.get(node);
        }
        UndirectedGraphNode n = new UndirectedGraphNode(node.label);
        for (UndirectedGraphNode crt : node.neighbors){
            n.neighbors.add(cloneGraph(crt));
        }
        hm.put(node, n);
        return n;
    }
}