133. Clone Graph (Medium)


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.




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.



  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.


class Solution {
    // record the visited node to avoid multi times access
    Map<Node, Node> visited = new HashMap<>();

    public Node cloneGraph(Node node) {
        if (node == null) {
            return null;

        if (visited.containsKey(node)) {
            return visited.get(node);

        Node newNode = new Node(node.val, new ArrayList<>());

        visited.put(node, newNode);

        if (node.neighbors == null) {
            return newNode;

        for (Node n : node.neighbors) {
            // DFS technique to go through and clone all children nodes
            Node xn = cloneGraph(n);


        return newNode;

Incorrect Solutions


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