doubly_linked_list.py

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

class DoublyLinkedList:
    def __init__(self):
        self.head = None

    def is_empty(self):
        return self.head is None

    def insert_at_head(self, data):
        new_node = Node(data)
        if self.is_empty():
            self.head = new_node
        else:
            new_node.next = self.head
            self.head.prev = new_node
            self.head = new_node

    def insert_at_tail(self, data):
        new_node = Node(data)
        if self.is_empty():
            self.head = new_node
        else:
            current = self.head
            while current.next:
                current = current.next
            current.next = new_node
            new_node.prev = current

    def delete_at_head(self):
        if self.is_empty():
            print("Linked list is empty. No node to delete.")
        else:
            if self.head.next is None:
                self.head = None
            else:
                self.head = self.head.next
                self.head.prev = None

    def delete_at_tail(self):
        if self.is_empty():
            print("Linked list is empty. No node to delete.")
        else:
            if self.head.next is None:
                self.head = None
            else:
                current = self.head
                while current.next:
                    current = current.next
                current.prev.next = None

    def display_forward(self):
        if self.is_empty():
            print("Linked list is empty.")
        else:
            current = self.head
            while current:
                print(current.data, end=" ")
                current = current.next
            print()

    def display_backward(self):
        if self.is_empty():
            print("Linked list is empty.")
        else:
            current = self.head
            while current.next:
                current = current.next
            while current:
                print(current.data, end=" ")
                current = current.prev
            print()

# Usage example:
linked_list = DoublyLinkedList()
linked_list.insert_at_head(5)
linked_list.insert_at_head(3)
linked_list.insert_at_tail(7)
linked_list.display_forward()  # Output: 3 5 7
linked_list.display_backward()  # Output: 7 5 3
linked_list.delete_at_head()
linked_list.delete_at_tail()
linked_list.display_forward()  # Output: 5