133. Clone Graph

Given a reference of a node in a connected undirected graph, return a deep copy (clone) of the graph. Each node in the graph contains a val (int) and a list (List[Node]) of its neighbors.

Example:

Input:
{"$id":"1","neighbors":[{"$id":"2","neighbors":[{"$ref":"1"},{"$id":"3","neighbors":[{"$ref":"2"},{"$id":"4","neighbors":[{"$ref":"3"},{"$ref":"1"}],"val":4}],"val":3}],"val":2},{"$ref":"4"}],"val":1}

Explanation:
Node 1's value is 1, and it has two neighbors: Node 2 and 4.
Node 2's value is 2, and it has two neighbors: Node 1 and 3.
Node 3's value is 3, and it has two neighbors: Node 2 and 4.
Node 4's value is 4, and it has two neighbors: Node 1 and 3.

Note:

1. The number of nodes will be between 1 and 100.
2. The undirected graph is a simple graph, which means no repeated edges and no self-loops in the graph.
3. Since the graph is undirected, if node p has node q as neighbor, then node q must have node p as neighbor too.
4. You must return the copy of the given node as a reference to the cloned graph.

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> neighbors;

    public Node() {}

    public Node(int _val,List<Node> _neighbors) {
        val = _val;
        neighbors = _neighbors;
    }
};
*/
class Solution {
    public Node cloneGraph(Node node) {
        if (node == null) {
            return null;
        }
        
        return cloneHelper(node);        
    }
        
    public static Map<Node, Node> newNodesMap = new HashMap<>();
     
    public static Node cloneHelper(Node node) {
        
        if (!newNodesMap.containsKey(node)) {                   
            Node newNode = new Node(node.val, new ArrayList<>());
            newNodesMap.put(node, newNode);
        } else {
            return newNodesMap.get(node);
        }     
       
        for (Node temp : node.neighbors) {
             Node neighbour = cloneHelper(temp);              
             newNodesMap.get(node).neighbors.add(neighbour);
        }
        
        return newNodesMap.get(node);
    }
}

复杂DFS