Python - Delete even nodes of the Circular Singly Linked List

Deleting even nodes of a circular singly linked list requires traverse through the list and deleting even nodes one by one. It requires creating two nodes - oddNode and evenNode. If the list contains more than one node, make oddNode to first odd node of list and evenNode to first even node of the list. Store the current odd node into temp node and delete the even node. Move both nodes to next set of odd-even nodes. Repeat the process till any of nodes reaches head. Finally, link the last node with the head.

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

def deleteEvenNodes(self):  
  if (self.head != None and self.head.next != self.head):
    #1. if the list has more than one element
    #   create evenNode node - pointing to head
    #   oddNode node - pointing to next of head
    #   temp node - to store last odd node
    oddNode = self.head
    evenNode = self.head.next 

    while(True):
      
      #2. delete even node and update evenNode and 
      #   oddNode to next set of odd-even nodes
      #   update temp node to latest oddNode node
      #   continue the process till any of the node 
      #   reaches head  
      temp = oddNode
      oddNode.next = evenNode.next
      evenNode = None
      oddNode = oddNode.next
      evenNode = oddNode.next
      if(oddNode == self.head or evenNode == self.head):
        break

    #3. if oddNode reaches head, make next of 
    #   temp as head else make next of oddNode
    #   as head
    if(oddNode == self.head):
      temp.next = self.head
    else:
      oddNode.next = self.head

The below is a complete program that uses above discussed concept of deleting even nodes of a circular singly linked list.

# node structure
class Node:
  def __init__(self, data):
    self.data = data
    self.next = 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
      return
    else:
      temp = self.head
      while(temp.next != self.head):
        temp = temp.next
      temp.next = newNode
      newNode.next = self.head

  #delete even nodes of the list
  def deleteEvenNodes(self):  
    if (self.head != None and self.head.next != self.head):
      oddNode = self.head
      evenNode = self.head.next 
      while(True):
        temp = oddNode
        oddNode.next = evenNode.next
        evenNode = None
        oddNode = oddNode.next
        evenNode = oddNode.next
        if(oddNode == self.head or evenNode == self.head):
          break
      if(oddNode == self.head):
        temp.next = self.head
      else:
        oddNode.next = self.head

  #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 even nodes of the list
MyList.deleteEvenNodes()

print("After deleting even 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 even nodes.
The list contains: 10 30 50