133.Clone Graph
===

###### tags: `Medium`,`Hash Table`,`DFS`,`BFS`,`Graph`

[133. Clone Graph](https://leetcode.com/problems/clone-graph/)

### 題目描述

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.

### 範例

**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.

### 解答

class Solution:
    def cloneGraph(self, node: 'Node') -> 'Node':
        visited = {}

        def clone(node):
            if node in visited:
                return visited[node]
            copy = Node(node.val)
            visited[node] = copy
            for nei in node.neighbors:
                copy.neighbors.append(clone(nei))
            return copy
        
        return clone(node) if node else None

> [name=Yen-Chi Chen][time=Sat, Apr 8, 2023]
### Reference

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