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[NEW ALGORITHM] Rotate linked list by K. #9278

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Oct 1, 2023
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156 changes: 156 additions & 0 deletions data_structures/linked_list/rotate_to_the_right.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,156 @@
from __future__ import annotations

from dataclasses import dataclass


@dataclass
class Node:
data: int
next_node: Node | None = None


def print_linked_list(head: Node | None) -> None:
"""
Print the entire linked list iteratively.

This function prints the elements of a linked list separated by '->'.

Parameters:
head (Node | None): The head of the linked list to be printed,
or None if the linked list is empty.

>>> head = insert_node(None, 0)
>>> head = insert_node(head, 2)
>>> head = insert_node(head, 1)
>>> print_linked_list(head)
0->2->1
>>> head = insert_node(head, 4)
>>> head = insert_node(head, 5)
>>> print_linked_list(head)
0->2->1->4->5
"""
if head is None:
return
while head.next_node is not None:
print(head.data, end="->")
head = head.next_node
print(head.data)


def insert_node(head: Node | None, data: int) -> Node:
"""
Insert a new node at the end of a linked list and return the new head.

Parameters:
head (Node | None): The head of the linked list.
data (int): The data to be inserted into the new node.

Returns:
Node: The new head of the linked list.

>>> head = insert_node(None, 10)
>>> head = insert_node(head, 9)
>>> head = insert_node(head, 8)
>>> print_linked_list(head)
10->9->8
"""
new_node = Node(data)
# If the linked list is empty, the new_node becomes the head
if head is None:
return new_node

temp_node = head
while temp_node.next_node:
temp_node = temp_node.next_node

temp_node.next_node = new_node # type: ignore
return head


def rotate_to_the_right(head: Node, places: int) -> Node:
"""
Rotate a linked list to the right by places times.

Parameters:
head: The head of the linked list.
places: The number of places to rotate.

Returns:
Node: The head of the rotated linked list.

>>> rotate_to_the_right(None, places=1)
Traceback (most recent call last):
...
ValueError: The linked list is empty.
>>> head = insert_node(None, 1)
>>> rotate_to_the_right(head, places=1) == head
True
>>> head = insert_node(None, 1)
>>> head = insert_node(head, 2)
>>> head = insert_node(head, 3)
>>> head = insert_node(head, 4)
>>> head = insert_node(head, 5)
>>> new_head = rotate_to_the_right(head, places=2)
>>> print_linked_list(new_head)
4->5->1->2->3
"""
# Check if the list is empty or has only one element
if not head:
raise ValueError("The linked list is empty.")

if head.next_node is None:
return head

# Calculate the length of the linked list
length = 1
temp_node = head
while temp_node.next_node is not None:
length += 1
temp_node = temp_node.next_node

# Adjust the value of places to avoid places longer than the list.
places %= length

if places == 0:
return head # As no rotation is needed.

# Find the new head position after rotation.
new_head_index = length - places

# Traverse to the new head position
temp_node = head
for _ in range(new_head_index - 1):
assert temp_node.next_node
temp_node = temp_node.next_node

# Update pointers to perform rotation
assert temp_node.next_node
new_head = temp_node.next_node
temp_node.next_node = None
temp_node = new_head
while temp_node.next_node:
temp_node = temp_node.next_node
temp_node.next_node = head

assert new_head
return new_head


if __name__ == "__main__":
import doctest

doctest.testmod()
head = insert_node(None, 5)
head = insert_node(head, 1)
head = insert_node(head, 2)
head = insert_node(head, 4)
head = insert_node(head, 3)

print("Original list: ", end="")
print_linked_list(head)

places = 3
new_head = rotate_to_the_right(head, places)

print(f"After {places} iterations: ", end="")
print_linked_list(new_head)