/*
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) {
        // Create a map to store original nodes -> cloned nodes.
        Map<Node, Node> clonedMap = new HashMap<>();

        return dfs(node, clonedMap);
    }

    public Node dfs(Node node, Map<Node, Node> clonedMap)
    {
        if(node == null)
            return null;
        
        if(clonedMap.containsKey(node))
            return clonedMap.get(node);
        
        // Create a new node with the same value.
        Node newNode = new Node(node.val);
        // Store it in the map.
        clonedMap.put(node, newNode);

        for(Node u : node.neighbors)
        {
            // Recursively clone all neighbors and add them to the clone’s neighbor list.
            Node nextNode = dfs(u, clonedMap);
            newNode.neighbors.add(nextNode);
        }
        return newNode;
    }
}