Clone Graph

public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node==null) return null;
        HashMap<UndirectedGraphNode,UndirectedGraphNode> hm =  new HashMap<UndirectedGraphNode,UndirectedGraphNode>();
        UndirectedGraphNode head = new UndirectedGraphNode(node.label);
        hm.put(node, head);
        dfs(hm, node);
        return head;
    }
    private void dfs(HashMap<UndirectedGraphNode,UndirectedGraphNode> hm , UndirectedGraphNode node){
        if(node ==null) return;
        for(UndirectedGraphNode a_nb: node.neighbors){
            if(!hm.containsKey(a_nb)){
                UndirectedGraphNode nb = new UndirectedGraphNode(a_nb.label);
                hm.put(a_nb,nb);
                dfs(hm, a_nb);
            }
            //这里是关键!!
            hm.get(node).neighbors.add(hm.get(a_nb));
        }
    }
}