# Python - Advanced Linked list

We have already seen Linked List in earlier chapter in whihc it is possible only to travle forward. In this chapter we see another type of linked list in which it is possible to travle both forward and backward. Such a linked list is called Doubly Linked List. Following is the features of doubly linked list.

- Doubly Linked List contains a link element called first and last.
- Each link carries a data field(s) and two link fields called next and prev.
- Each link is linked with its next link using its next link.
- Each link is linked with its previous link using its previous link.
- The last link carries a link as null to mark the end of the list.

## Creating Doubly linked list

We create a Doubly Linked list by using the Node class. Now we use the same approach as used in the Singly Linked List but the head and next pointers will be used for proper assignation to create two links in each of the nodes in addition to the data present in the node.

class Node: def __init__(self, data): self.data = data self.next = None self.prev = None class doubly_linked_list: def __init__(self): self.head = None # Adding data elements def push(self, NewVal): NewNode = Node(NewVal) NewNode.next = self.head if self.head is not None: self.head.prev = NewNode self.head = NewNode # Print the Doubly Linked list def listprint(self, node): while (node is not None): print(node.data), last = node node = node.next dllist = doubly_linked_list() dllist.push(12) dllist.push(8) dllist.push(62) dllist.listprint(dllist.head)

When the above code is executed, it produces the following result −

62 8 12

## Inserting into Doubly Linked List

here we are going to see how to insert a node to the Doubly Link List using the following program. The program uses a menthod named insert which inserts the new node at the third position from the head of the doubly linked list.

# Create the Node class class Node: def __init__(self, data): self.data = data self.next = None self.prev = None # Create the doubly linked list class doubly_linked_list: def __init__(self): self.head = None # Define the push method to add elements def push(self, NewVal): NewNode = Node(NewVal) NewNode.next = self.head if self.head is not None: self.head.prev = NewNode self.head = NewNode # Define the insert method to insert the element def insert(self, prev_node, NewVal): if prev_node is None: return NewNode = Node(NewVal) NewNode.next = prev_node.next prev_node.next = NewNode NewNode.prev = prev_node if NewNode.next is not None: NewNode.next.prev = NewNode # Define the method to print the linked list def listprint(self, node): while (node is not None): print(node.data), last = node node = node.next dllist = doubly_linked_list() dllist.push(12) dllist.push(8) dllist.push(62) dllist.insert(dllist.head.next, 13) dllist.listprint(dllist.head)

When the above code is executed, it produces the following result −

62 8 13 12

## Appending to a Doubly linked list

Appending to a doubly linked list will add the element at the end.

# Create the node class class Node: def __init__(self, data): self.data = data self.next = None self.prev = None # Create the doubly linked list class class doubly_linked_list: def __init__(self): self.head = None # Define the push method to add elements at the begining def push(self, NewVal): NewNode = Node(NewVal) NewNode.next = self.head if self.head is not None: self.head.prev = NewNode self.head = NewNode # Define the append method to add elements at the end def append(self, NewVal): NewNode = Node(NewVal) NewNode.next = None if self.head is None: NewNode.prev = None self.head = NewNode return last = self.head while (last.next is not None): last = last.next last.next = NewNode NewNode.prev = last return # Define the method to print def listprint(self, node): while (node is not None): print(node.data), last = node node = node.next dllist = doubly_linked_list() dllist.push(12) dllist.append(9) dllist.push(8) dllist.push(62) dllist.append(45) dllist.listprint(dllist.head)

When the above code is executed, it produces the following result −

62 8 12 9 45

Please note the position of the elements 9 and 45 for the append operation.

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