"""
Floyd's cycle detection algorithm is a popular algorithm used to detect cycles
in a linked list. It uses two pointers, a slow pointer and a fast pointer,
to traverse the linked list. The slow pointer moves one node at a time while the fast
pointer moves two nodes at a time. If there is a cycle in the linked list,
the fast pointer will eventually catch up to the slow pointer and they will
meet at the same node. If there is no cycle, the fast pointer will reach the end of
the linked list and the algorithm will terminate.
For more information: https://en.wikipedia.org/wiki/Cycle_detection#Floyd's_tortoise_and_hare
"""
from collections.abc import Iterator
from dataclasses import dataclass
from typing import Any, Self
@dataclass
class Node:
"""
A class representing a node in a singly linked list.
"""
data: Any
next_node: Self | None = None
@dataclass
class LinkedList:
"""
A class representing a singly linked list.
"""
head: Node | None = None
def __iter__(self) -> Iterator:
"""
Iterates through the linked list.
Returns:
Iterator: An iterator over the linked list.
Examples:
>>> linked_list = LinkedList()
>>> list(linked_list)
[]
>>> linked_list.add_node(1)
>>> tuple(linked_list)
(1,)
"""
visited = []
node = self.head
while node:
if node in visited:
return
visited.append(node)
yield node.data
node = node.next_node
def add_node(self, data: Any) -> None:
"""
Adds a new node to the end of the linked list.
Args:
data (Any): The data to be stored in the new node.
Examples:
>>> linked_list = LinkedList()
>>> linked_list.add_node(1)
>>> linked_list.add_node(2)
>>> linked_list.add_node(3)
>>> linked_list.add_node(4)
>>> tuple(linked_list)
(1, 2, 3, 4)
"""
new_node = Node(data)
if self.head is None:
self.head = new_node
return
current_node = self.head
while current_node.next_node is not None:
current_node = current_node.next_node
current_node.next_node = new_node
def detect_cycle(self) -> bool:
"""
Detects if there is a cycle in the linked list using
Floyd's cycle detection algorithm.
Returns:
bool: True if there is a cycle, False otherwise.
Examples:
>>> linked_list = LinkedList()
>>> linked_list.add_node(1)
>>> linked_list.add_node(2)
>>> linked_list.add_node(3)
>>> linked_list.add_node(4)
>>> linked_list.detect_cycle()
False
# Create a cycle in the linked list
>>> linked_list.head.next_node.next_node.next_node = linked_list.head.next_node
>>> linked_list.detect_cycle()
True
"""
if self.head is None:
return False
slow_pointer: Node | None = self.head
fast_pointer: Node | None = self.head
while fast_pointer is not None and fast_pointer.next_node is not None:
slow_pointer = slow_pointer.next_node if slow_pointer else None
fast_pointer = fast_pointer.next_node.next_node
if slow_pointer == fast_pointer:
return True
return False
if __name__ == "__main__":
import doctest
doctest.testmod()
linked_list = LinkedList()
linked_list.add_node(1)
linked_list.add_node(2)
linked_list.add_node(3)
linked_list.add_node(4)
if (
linked_list.head
and linked_list.head.next_node
and linked_list.head.next_node.next_node
):
linked_list.head.next_node.next_node.next_node = linked_list.head.next_node
has_cycle = linked_list.detect_cycle()
print(has_cycle)