Reverse a Linked List in groups of given size Python

PROGRAM TO REVERSE A LINKED LIST IN GROUPS OF GIVEN SIZE

# Node class

class Node(object):

    __slots__ = 'data', 'next'

  

    # Constructor to initialize the node object

    def __init__(self, data = None, next = None):

        self.data = data

        self.next = next

  

    def __repr__(self):

        return repr(self.data)

  

class LinkedList(object):

  

    # Function to initialize head

    def __init__(self):

        self.head = None

  

    # Utility function to print nodes 

    # of LinkedList

    def __repr__(self):

        nodes = []

        curr = self.head

        while curr:

            nodes.append(repr(curr))

            curr = curr.next

        return '[' + ', '.join(nodes) + ']'

  

    # Function to insert a new node at

    # the beginning

    def prepend(self, data):

        self.head = Node(data = data,

                         next = self.head)

  

    # Reverses the linked list in groups of size k

    # and returns the pointer to the new head node.

    def reverse(self, k = 1):

        if self.head is None:

            return

  

        curr = self.head

        prev = None

        new_stack = []

        while curr is not None:

            val = 0

              

            # Terminate the loop whichever comes first

            # either current == None or value >= k

            while curr is not None and val < k:

                new_stack.append(curr.data)

                curr = curr.next

                val += 1

  

            # Now pop the elements of stack one by one

            while new_stack:

                  

                # If final list has not been started yet.

                if prev is None:

                    prev = Node(new_stack.pop())

                    self.head = prev

                else:

                    prev.next = Node(new_stack.pop())

                    prev = prev.next

                      

        # Next of last element will point to None.

        prev.next = None

        return self.head

  

# Driver Code

llist = LinkedList() 

llist.prepend(9)

llist.prepend(8)

llist.prepend(7)

llist.prepend(6)

llist.prepend(5)

llist.prepend(4)

llist.prepend(3)

llist.prepend(2)

llist.prepend(1)

  

print("Given linked list")

print(llist)

llist.head = llist.reverse(3)

  

print("Reversed Linked list")

print(llist)




OUTPUT

Given Linked List
1 2 3 4 5 6 7 8 9 
Reversed list
3 2 1 6 5 4 9 8 7

CREDITS: https://www.geeksforgeeks.org/reverse-linked-list-groups-given-size-set-2/