CloneGraph.java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

// 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 CloneGraph {

    public static void main(String[] args) {


    }
    Map<Node, Node> map = new HashMap<>();
    public Node cloneGraph(Node node) {
        return dfs(node);
    }
    private Node dfs(Node node) {
        if (node == null) {
            return null;
        }
        if (map.containsKey(node)) {
            return map.get(node);
        }
        Node newNode = new Node(node.val);
        map.put(node, newNode);
        for (Node n : node.neighbors) {
            newNode.neighbors.add(dfs(n));
        }
        return newNode;
    }
}