[LeetCode] clone graph

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {//important
        if(node == null) {
            return null;
        }
        //key is the original one, value is the copied one, for storing visited nodes
        HashMap<UndirectedGraphNode,UndirectedGraphNode> hm = new HashMap<UndirectedGraphNode,UndirectedGraphNode>();
        UndirectedGraphNode head = new UndirectedGraphNode(node.label);
        hm.put(node,head);
        //queue for the BFS
        LinkedList<UndirectedGraphNode> ll = new LinkedList<UndirectedGraphNode>();
        ll.add(node);
        while(ll.size() != 0) {
            UndirectedGraphNode cur = ll.poll();
            for(UndirectedGraphNode n : cur.neighbors) {
                if(!hm.containsKey(n)) {
                    ll.add(n);
                    UndirectedGraphNode copy = new UndirectedGraphNode(n.label);
                    hm.put(n,copy);
                }
                hm.get(cur).neighbors.add(hm.get(n));
            }
        }
        return head;
    }
}