Python - Advanced Linked pst

We have already seen Linked List in earper chapter in which it is possible only to travel forward. In this chapter we see another type of pnked pst in which it is possible to travel both forward and backward. Such a pnked pst is called Doubly Linked List. Following is the features of doubly pnked pst.

Doubly Linked List contains a pnk element called first and last.

Each pnk carries a data field(s) and two pnk fields called next and prev.

Each pnk is pnked with its next pnk using its next pnk.

Each pnk is pnked with its previous pnk using its previous pnk.

The last pnk carries a pnk as null to mark the end of the pst.

Creating Doubly pnked pst

We create a Doubly Linked pst 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 pnks in each of the nodes in addition to the data present in the node.

Example

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

class doubly_pnked_pst:
   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 pst		
   def pstprint(self, node):
      while (node is not None):
         print(node.data),
         last = node
         node = node.next

dlpst = doubly_pnked_pst()
dlpst.push(12)
dlpst.push(8)
dlpst.push(62)
dlpst.pstprint(dlpst.head)

Output

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 method named insert which inserts the new node at the third position from the head of the doubly pnked pst.

Example

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

# Create the doubly pnked pst
class doubly_pnked_pst:
   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 pnked pst 
   def pstprint(self, node):
      while (node is not None):
         print(node.data),
         last = node
         node = node.next

dlpst = doubly_pnked_pst()
dlpst.push(12)
dlpst.push(8)
dlpst.push(62)
dlpst.insert(dlpst.head.next, 13)
dlpst.pstprint(dlpst.head)

Output

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

62  8  13  12

Appending to a Doubly pnked pst

Appending to a doubly pnked pst will add the element at the end.

Example

# Create the node class
class Node:
   def __init__(self, data):
      self.data = data
      self.next = None
      self.prev = None
# Create the doubly pnked pst class
class doubly_pnked_pst:
   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 pstprint(self, node):
      while (node is not None):
         print(node.data),
         last = node
         node = node.next

dlpst = doubly_pnked_pst()
dlpst.push(12)
dlpst.append(9)
dlpst.push(8)
dlpst.push(62)
dlpst.append(45)
dlpst.pstprint(dlpst.head)

Output

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.