Python - Delete the last node of the Circular Doubly Linked List

In this method, the last node of the circular doubly linked list is deleted. For example - if the given list is 10->20->30->40 and the last node is deleted, the list becomes 10->20->30.

Deleting the last node of the Circular Doubly Linked List involves checking the head for empty. If it is not empty and contains only one node then delete the head node. If the list contains more than one node then traverse to the second last node of the list and link it with the head. Finally, delete the last node.

Circular Doubly Linked List - Delete Last Node

The function pop_back is created for this purpose. It is a 3-step process.

def pop_back(self):
  if(self.head != None):
    
    #1. if head in not null and next of head
    #   is head, release the head
    if(self.head.next == self.head):
      self.head = None
    else:
      
      #2. Else, traverse to the second last 
      #   element of the list
      temp = self.head
      while(temp.next.next != self.head):
        temp = temp.next
      
      #3. Update links of head and second 
      #   last node, and delete the last node
      lastNode = temp.next
      temp.next = self.head
      self.head.prev = temp
      lastNode = None

The below is a complete program that uses above discussed concept of deleting the last node of the circular doubly linked list.

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

#class Linked List
class LinkedList:
  def __init__(self):
    self.head = None

  #Add new element at the end of the list
  def push_back(self, newElement):
    newNode = Node(newElement)
    if(self.head == None):
      self.head = newNode
      newNode.next = self.head
      newNode.prev = self.head
      return
    else:
      temp = self.head
      while(temp.next != self.head):
        temp = temp.next
      temp.next = newNode
      newNode.next = self.head
      newNode.prev = temp
      self.head.prev = newNode

  #Delete last node of the list
  def pop_back(self):
    if(self.head != None):
      if(self.head.next == self.head):
        self.head = None
      else:
        temp = self.head
        while(temp.next.next != self.head):
          temp = temp.next
        lastNode = temp.next
        temp.next = self.head
        self.head.prev = temp
        lastNode = None

  #display the content of the list
  def PrintList(self):
    temp = self.head
    if(temp != None):
      print("The list contains:", end=" ")
      while (True):
        print(temp.data, end=" ")
        temp = temp.next
        if(temp == self.head):
          break
      print()
    else:
      print("The list is empty.")

# test the code                  
MyList = LinkedList()

#Add four elements in the list.
MyList.push_back(10)
MyList.push_back(20)
MyList.push_back(30)
MyList.push_back(40)
MyList.PrintList()

#Delete the last node
MyList.pop_back()
MyList.PrintList()

The above code will give the following output:

The list contains: 10 20 30 40
The list contains: 10 20 30