I

```
"""
LeetCode 133. Clone Graph
https://leetcode.com/problems/clone-graph/
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.
"""
from dataclasses import dataclass
@dataclass
class Node:
value: int = 0
neighbors: list["Node"] | None = None
def __post_init__(self) -> None:
"""
>>> Node(3).neighbors
[]
"""
self.neighbors = self.neighbors or []
def __hash__(self) -> int:
"""
>>> hash(Node(3)) != 0
True
"""
return id(self)
def clone_graph(node: Node | None) -> Node | None:
"""
This function returns a clone of a connected undirected graph.
>>> clone_graph(Node(1))
Node(value=1, neighbors=[])
>>> clone_graph(Node(1, [Node(2)]))
Node(value=1, neighbors=[Node(value=2, neighbors=[])])
>>> clone_graph(None) is None
True
"""
if not node:
return None
originals_to_clones = {} # map nodes to clones
stack = [node]
while stack:
original = stack.pop()
if original in originals_to_clones:
continue
originals_to_clones[original] = Node(original.value)
stack.extend(original.neighbors or [])
for original, clone in originals_to_clones.items():
for neighbor in original.neighbors or []:
cloned_neighbor = originals_to_clones[neighbor]
if not clone.neighbors:
clone.neighbors = []
clone.neighbors.append(cloned_neighbor)
return originals_to_clones[node]
if __name__ == "__main__":
import doctest
doctest.testmod()
```