133. Clone Graph

public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        return clone(node);
    }
    
    private HashMap<Integer, UndirectedGraphNode> map = new HashMap<>();
    
    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;
    }