Python - Delete odd nodes of the Circular Doubly Linked List

Deleting odd nodes of a circular doubly linked list requires traverse through the list and deleting odd nodes one by one. It involves the following process. If the list contains only head then make the head null. If the list contains more than one element then delete the head and adjust the link of last element with the new head. If next of head is not head then, create three nodes - evenNode pointing to head, oddNode pointing to next of head and temp to store last even node. Then delete oddNode and update evenNode and oddNode to next set of odd-even nodes and continue the process till any of the node reaches head. After that if evenNode reaches head, make next of temp as head else make next of evenNode as head and adjust other links accordingly.

The function deleteOddNodes is created for this purpose. It is a 5-step process.

def deleteOddNodes(self):  
  #1. if head is the only element element in 
  #   list make the head as null  
  if(self.head != None and self.head.next == self.head):
    self.head = None
  elif (self.head != None):

    #2. if the list contains more than one element
    #   delete the head and adjust the link of
    #   last element with the new head       
    temp = self.head
    while(temp.next != self.head):
      temp = temp.next
    temp.next = self.head.next
    self.head.next.prev = temp
    self.head = None
    self.head = temp.next

    #3. create evenNode node - pointing to head
    #   oddNode node - pointing to next of head
    #   temp node - to store last even node
    if(self.head != None and self.head.next != self.head):

      evenNode = self.head
      oddNode = self.head.next
      while(True):
        
        #4. delete odd node and update evenNode and 
        #   oddNode to next set of odd-even nodes update
        #   temp node to latest evenNode node continue
        #   the process till any of the node reaches head
        temp = evenNode
        evenNode.next = oddNode.next
        evenNode.next.prev = evenNode
        oddNode = None
        evenNode = evenNode.next
        oddNode = evenNode.next
        if(evenNode == self.head or oddNode == self.head):
          break
      
      #5. if evenNode reaches head, make next of temp
      #   as head else make next of evenNode as head
      #   and adjust other links accordingly
      if(evenNode == self.head):
        temp.next = self.head
        self.head.prev = temp
      else:
        evenNode.next = self.head
        self.head.prev = evenNode

The below is a complete program that uses above discussed concept of deleting odd nodes of a 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 odd nodes of the list
  def deleteOddNodes(self):  
    if(self.head != None and self.head.next == self.head):
      self.head = None
    elif (self.head != None):
     
      temp = self.head
      while(temp.next != self.head):
        temp = temp.next
      temp.next = self.head.next
      self.head.next.prev = temp
      self.head = None
      self.head = temp.next

      if(self.head != None and self.head.next != self.head):

        evenNode = self.head
        oddNode = self.head.next
        while(True):
          temp = evenNode
          evenNode.next = oddNode.next
          evenNode.next.prev = evenNode
          oddNode = None
          evenNode = evenNode.next
          oddNode = evenNode.next
          if(evenNode == self.head or oddNode == self.head):
            break

        if(evenNode == self.head):
          temp.next = self.head
          self.head.prev = temp
        else:
          evenNode.next = self.head
          self.head.prev = evenNode

  #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 five elements in the list.
MyList.push_back(10)
MyList.push_back(20)
MyList.push_back(30)
MyList.push_back(40)
MyList.push_back(50)

#Display the content of the list.
MyList.PrintList()

#delete odd nodes of the list
MyList.deleteOddNodes()

print("After deleting odd nodes:")
#Display the content of the list.
MyList.PrintList()

The above code will give the following output:

The list contains: 10 20 30 40 50 
After deleting odd nodes:
The list contains: 20 40