133\. Clone Graph

Medium

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 value (`int`) and a list (`List[Node]`) of its neighbors.

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

**Test case format:**

For simplicity, 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.

**An 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:**

*   The number of nodes in the graph is in the range `[0, 100]`.
*   `1 <= Node.val <= 100`
*   `Node.val` is unique for each node.
*   There are 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.