# 133. Clone Graph

## Problem

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 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:**

```
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:**

```
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.
```

**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.

## Code

```
/*
// 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 node;
HashMap<Node, Node> map = new HashMap<>();
copyNode(node, map);
copyNei(node, map, new HashSet<Node>());
return map.get(node);
}
private void copyNode(Node node, HashMap<Node, Node> map) {
if (map.containsKey(node))
return;
map.put(node, new Node(node.val, new ArrayList<>()));
for (Node nei : node.neighbors) {
copyNode(nei, map);
}
}
private void copyNei(Node node, HashMap<Node, Node> map, HashSet<Node> visited) {
if (visited.contains(node))
return;
visited.add(node);
for (Node nei : node.neighbors) {
map.get(node).neighbors.add(map.get(nei));
copyNei(nei, map, visited);
}
}
}
```