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

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

Deleting the first node of the circular doubly linked list is very easy. If the list contains one node, delete the node. If the list contains more than one node, then create two nodes - temp and firstNode both pointing to the head. Using temp node, traverse to the last node of the list. Make next of head as the head node and update all links. Finally, delete the first node.

Circular Singly Linked List - Delete First Node

The function pop_front is created for this purpose. It is a 4-step process.

def pop_front(self):
  if(self.head != None):
    
    #1. the list contains one node, delete
    #   make the head null
    if(self.head.next == self.head):
      self.head = None
    else:
      
      #2. if the list contains more than one node,
      #   create two nodes - temp and firstNode, both
      #   pointing to head node
      temp = self.head
      firstNode = self.head

      #3. using temp node, traverse to the last node
      while(temp.next != self.head):
        temp = temp.next

      #4. Make head as next of head, and 
      #   update links
      self.head = self.head.next
      self.head.prev = temp
      temp.next = self.head 
      firstNode = None

The below is a complete program that uses above discussed concept of deleting the first 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 LinkedList
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 first node of the list
  def pop_front(self):
    if(self.head != None):
      if(self.head.next == self.head):
        self.head = None
      else:
        temp = self.head
        firstNode = self.head
        while(temp.next != self.head):
          temp = temp.next
        self.head = self.head.next
        self.head.prev = temp
        temp.next = self.head 
        firstNode = 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 three elements at the end of the list.
MyList.push_back(10)
MyList.push_back(20)
MyList.push_back(30)
MyList.push_back(40)
MyList.PrintList()

#Delete the first node
MyList.pop_front()
MyList.PrintList()

The above code will give the following output:

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