# 0133. Clone Graph

https://leetcode.com/problems/clone-graph

## Description

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.

## ac1: BFS

```java
/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) return null;

        Map<Integer,UndirectedGraphNode> map = new HashMap<Integer,UndirectedGraphNode>();
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(node.label, newNode);

        Queue<UndirectedGraphNode> q = new LinkedList<UndirectedGraphNode>();
        q.offer(node);
        while (!q.isEmpty()) {
            UndirectedGraphNode head = q.poll();
            for (UndirectedGraphNode adj : head.neighbors) {
                if (!map.containsKey(adj.label)) {
                    UndirectedGraphNode clone = new UndirectedGraphNode(adj.label);
                    map.put(adj.label, clone);
                    q.offer(adj);
                }
                map.get(head.label).neighbors.add(map.get(adj.label));
            }
        }
        return newNode;
    }
}
```

## ac2: DFS

best answer, this one is easier to understand and more natural.

```java
public class Solution {
    Map<Integer,UndirectedGraphNode> map;
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        map = new HashMap<Integer,UndirectedGraphNode>();
        return clone(node);
    }

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

        if (map.containsKey(node.label)) return map.get(node.label);

        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(node.label, newNode);

        for (UndirectedGraphNode neighbor : node.neighbors) {
                map.get(newNode.label).neighbors.add(clone(neighbor));
            }
        return newNode;
    }
}
```

my original version

```java
public class Solution {
    Map<Integer,UndirectedGraphNode> map;
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) return null;
        map = new HashMap<Integer,UndirectedGraphNode>();
        clone(node);        
        return map.get(node.label);
    }

    private void clone(UndirectedGraphNode node) {
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(node.label, newNode);

        for (UndirectedGraphNode neighbor : node.neighbors) {
                if (!map.containsKey(neighbor.label)) {
                    clone(neighbor);
                }
                map.get(node.label).neighbors.add(map.get(neighbor.label));
            }
    }
}
```

```java
/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    Map<Integer, UndirectedGraphNode> map = new HashMap<>();

    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        // edge cases
        if (node == null) return node;

        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(node.label, newNode);

        for (UndirectedGraphNode n : node.neighbors) {
            if (map.containsKey(n.label)) {  // has been cloned before, get it
                newNode.neighbors.add(map.get(n.label));
            } else {
                UndirectedGraphNode tmp = cloneGraph(n);
                newNode.neighbors.add(tmp);
            }
        }

        return newNode;
    }
}
```
