Python Program For Rotating A Linked List

Given a singly linked list, rotate the linked list counter-clockwise by k nodes. Where k is a given positive integer. For example, if the given linked list is 10->20->30->40->50->60 and k is 4, the list should be modified to 50->60->10->20->30->40. Assume that k is smaller than the count of nodes in a linked list.

Method 1:
To rotate the linked list, we need to change the next of kth node to NULL, the next of the last node to the previous head node, and finally, change the head to (k+1)th node. So we need to get hold of three nodes: kth node, (k+1)th node, and last node. 
Traverse the list from the beginning and stop at kth node. Store pointer to kth node. We can get (k+1)th node using kthNode->next. Keep traversing till the end and store a pointer to the last node also. Finally, change pointers as stated above.

Below image shows how to rotate function works in the code :

Python

# Python program to rotate
# a linked list
# Node class
class Node:

    # Constructor to initialize
    # the node object
    def __init__(self, data):
        self.data = data
        self.next = None

class LinkedList:

    # Function to initialize head
    def __init__(self):
        self.head = None

    # Function to insert a new node
    # at the beginning
    def push(self, new_data):
        # allocate node and put the data
        new_node = Node(new_data)

        # Make next of new node as head
        new_node.next = self.head

        # Move the head to point to the
        # new Node
        self.head = new_node

    # Utility function to print it the
    # linked LinkedList
    def printList(self):
        temp = self.head
        while(temp):
            print temp.data,
            temp = temp.next

    # This function rotates a linked list
    # counter-clockwise and updates the
    # head. The function assumes that k
    # is smaller than size of linked list.
    # It doesn't modify the list if k is
    # greater than of equal to size
    def rotate(self, k):
        if k == 0:
            return

        # Let us understand the below code
        # for example k = 4 and list =
        # 10->20->30->40->50->60
        current = self.head

        # current will either point to kth
        # or NULL after this loop current
        # will point to node 40 in the above
        # example
        count = 1
        while(count <k and
              current is not None):
            current = current.next
            count += 1

        # If current is None, k is greater
        # than or equal to count of nodes
        # in linked list. Don't change
        # the list in this case
        if current is None:
            return

        # current points to kth node. Store
        # it in a variable kth node points
        # to node 40 in the above example
        kthNode = current

        # current will point to last node
        # after this loop current will point
        # to node 60 in above example
        while(current.next is not None):
            current = current.next

        # Change next of last node to previous
        # head Next of 60 is now changed to
        # node 10
        current.next = self.head

        # Change head to (k + 1)th node
        # head is not changed to node 50
        self.head = kthNode.next

        # change next of kth node to NULL
        # next of 40 is not NULL
        kthNode.next = None

# Driver code
llist = LinkedList()

# Create a list
# 10->20->30->40->50->60
for i in range(60, 0, -10):
    llist.push(i)

print "Given linked list"
llist.printList()
llist.rotate(4)

print "Rotated Linked list"
llist.printList()
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

p>Output:

Given linked list
10  20  30  40  50  60
Rotated Linked list
50  60  10  20  30  40

Time Complexity: O(n) where n is the number of nodes in Linked List. The code traverses the linked list only once.

Method 2:
To rotate a linked list by k, we can first make the linked list circular and then moving k-1 steps forward from head node, making (k-1)th node’s next to null and make kth node as head.

Python3

# Python3 program to rotate a
# linked list counter clock wise

# Link list node
class Node:

    def __init__(self):

        self.data = 0
        self.next = None

# This function rotates a linked list
# counter-clockwise and updates the
# head. The function assumes that k is
# smaller than size of linked list.
def rotate(head_ref, k):

    if (k == 0):
        return

    # Let us understand the below
    # code for example k = 4 and
    # list = 10.20.30.40.50.60.
    current = head_ref

    # Traverse till the end.
    while (current.next != None):
        current = current.next

    current.next = head_ref
    current = head_ref

    # Traverse the linked list to k-1
    # position which will be last
    # element for rotated array.
    for i in range(k - 1):
        current = current.next

    # Update the head_ref and last
    # element pointer to None
    head_ref = current.next
    current.next = None
    return head_ref

# UTILITY FUNCTIONS
# Function to push a node
def push(head_ref, new_data):

    # Allocate node
    new_node = Node()

    # Put in the data
    new_node.data = new_data

    # Link the old list off
    # the new node
    new_node.next = (head_ref)

    # Move the head to point
    # to the new node
    (head_ref) = new_node
    return head_ref

# Function to print linked list
def printList(node):

    while (node != None):
        print(node.data, end = ' ')
        node = node.next

# Driver code
if __name__=='__main__':

    # Start with the empty list
    head = None

    # Create a list
    # 10.20.30.40.50.60
    for i in range(60, 0, -10):
        head = push(head, i)

    print("Given linked list ")
    printList(head)
    head = rotate(head, 4)

    print("\nRotated Linked list ")
    printList(head)
# This code is contributed by rutvik_56

Output:

Given linked list 
10 20 30 40 50 60 
Rotated Linked list 
50 60 10 20 30 40

Please refer complete article on Rotate a Linked List for more details!