Clone graph

O(n) : where n is no. of nodes
Problem

/*
Definition for a Node.
class Node {
    public int val;
    public List<Node> neighbors;
    public Node() {
        val = 0;
        neighbors = new ArrayList<Node>();
    }
    public Node(int _val) {
        val = _val;
        neighbors = new ArrayList<Node>();
    }
    public Node(int _val, ArrayList<Node> _neighbors) {
        val = _val;
        neighbors = _neighbors;
    }
}
*/

class Solution {
    public Node cloneGraph(Node node) {
        if(node ==null) return null;
        Map<Node,Node> map = new HashMap<>();
        Node cloneNode = new Node(node.val);
        return dfs(node,cloneNode,map);
    }

    public Node dfs(Node a, Node b,Map<Node,Node> map){
        if(map.containsKey(a)) return map.get(a);
        map.put(a,b);
        for(Node adjNode : a.neighbors){
            if(!map.containsKey(adjNode)){
                Node newNode = new Node(adjNode.val);
                b.neighbors.add(dfs(adjNode,newNode,map));
            }
            else{
                b.neighbors.add(map.get(adjNode));
            } 
        }
        return b;
    }
}