> For the complete documentation index, see [llms.txt](https://mayanktyagi3111.gitbook.io/interview-prep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mayanktyagi3111.gitbook.io/interview-prep/graphs-bfs-and-dfs/clone-graph.md).

# Clone Graph

Given a reference of a node in a [**connected**](https://en.wikipedia.org/wiki/Connectivity_\(graph_theory\)#Connected_graph) undirected graph.

Return a [**deep copy**](https://en.wikipedia.org/wiki/Object_copying#Deep_copy) (clone) of the graph.

Each node in the graph contains a val (`int`) and a list (`List[Node]`) of its neighbors.

```
class Node {
    public int val;
    public List<Node> neighbors;
}
```

**Test case format:**

For simplicity sake, each node's value is the same as the node's index (1-indexed). For example, the first node with `val = 1`, the second node with `val = 2`, and so on. The graph is represented in the test case using an adjacency list.

**Adjacency list** is a collection of unordered **lists** used to represent a finite graph. Each list describes the set of neighbors of a node in the graph.

The given node will always be the first node with `val = 1`. You must return the **copy of the given node** as a reference to the cloned graph.

**Example 1:**

![](https://assets.leetcode.com/uploads/2019/11/04/133_clone_graph_question.png)

```
Input: adjList = [[2,4],[1,3],[2,4],[1,3]]
Output: [[2,4],[1,3],[2,4],[1,3]]
Explanation: There are 4 nodes in the graph.
1st node (val = 1)'s neighbors are 2nd node (val = 2) and 4th node (val = 4).
2nd node (val = 2)'s neighbors are 1st node (val = 1) and 3rd node (val = 3).
3rd node (val = 3)'s neighbors are 2nd node (val = 2) and 4th node (val = 4).
4th node (val = 4)'s neighbors are 1st node (val = 1) and 3rd node (val = 3).
```

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/01/07/graph.png)

```
Input: adjList = [[]]
Output: [[]]
Explanation: Note that the input contains one empty list. The graph consists of only one node with val = 1 and it does not have any neighbors.
```

**Example 3:**

```
Input: adjList = []
Output: []
Explanation: This an empty graph, it does not have any nodes.
```

**Example 4:**

![](https://assets.leetcode.com/uploads/2020/01/07/graph-1.png)

```
Input: adjList = [[2],[1]]
Output: [[2],[1]]
```

**Constraints:**

* `1 <= Node.val <= 100`
* `Node.val` is unique for each node.
* Number of Nodes will not exceed 100.
* There is no repeated edges and no self-loops in the graph.
* The Graph is connected and all nodes can be visited starting from the given node.

```java
/*
// 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;
    }
}
*/

public class Solution {
    private HashMap<Integer, Node> map = new HashMap<>();

    public Node cloneGraph(Node node) {
        return clone(node);
    }

    private Node clone(Node node) {
        if (node == null)
            return null;

        if (map.containsKey(node.val)) {
            return map.get(node.val);
        }
        Node clone = new Node(node.val);
        map.put(clone.val, clone);
        for (Node neighbor : node.neighbors) {
            clone.neighbors.add(clone(neighbor));
        }
        return clone;
    }
}
```