Python program to rotate doubly linked list by N nodes

When working with doubly linked lists, rotation involves moving the first N nodes from the beginning to the end of the list. This requires a Node class with three attributes: data, next pointer, and previous pointer for bidirectional traversal.

Node Class Structure

Each node contains data and pointers to both next and previous nodes ?

class Node:
    def __init__(self, my_data):
        self.previous = None
        self.data = my_data
        self.next = None

class DoublyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None
        self.size = 0
    
    def add_data(self, my_data):
        new_node = Node(my_data)
        if self.head is None:
            self.head = self.tail = new_node
        else:
            self.tail.next = new_node
            new_node.previous = self.tail
            self.tail = new_node
        self.size += 1
    
    def print_list(self):
        curr = self.head
        if curr is None:
            print("The list is empty")
            return
        print("The nodes in the doubly linked list are:")
        while curr is not None:
            print(curr.data)
            curr = curr.next
    
    def rotate_list(self, n):
        if n == 0 or n >= self.size or self.head is None:
            return
        
        # Find the nth node
        curr = self.head
        for i in range(1, n):
            curr = curr.next
        
        # Store new head
        new_head = curr.next
        
        # Break the connection and rotate
        self.tail.next = self.head
        self.head.previous = self.tail
        self.head = new_head
        self.head.previous = None
        self.tail = curr
        self.tail.next = None

# Example usage
dll = DoublyLinkedList()
print("Elements are being added to the doubly linked list")
dll.add_data(10)
dll.add_data(24)
dll.add_data(54)
dll.add_data(77)
dll.add_data(24)
dll.add_data(0)
dll.print_list()

print("\nThe elements in the list after rotating by 4 nodes:")
dll.rotate_list(4)
dll.print_list()

Elements are being added to the doubly linked list
The nodes in the doubly linked list are:
10
24
54
77
24
0

The elements in the list after rotating by 4 nodes:
The nodes in the doubly linked list are:
24
0
10
24
54
77

How the Rotation Works

The rotation algorithm follows these steps ?

1. Find the nth node from the beginning
2. The (n+1)th node becomes the new head
3. Connect the original tail to the original head
4. Update head and tail pointers accordingly

Visualization

Key Points

• Edge Cases: Handle empty lists and rotation values greater than list size
• Bidirectional Links: Update both next and previous pointers during rotation
• Circular Connection: Temporarily connect tail to head during rotation
• Pointer Management: Carefully update head and tail references

Conclusion

Rotating a doubly linked list by N nodes involves finding the split point, reconnecting pointers, and updating head/tail references. The algorithm maintains the bidirectional nature of the list while efficiently repositioning elements.