Python Data Structures - Doubly Linked List Other Related Topics

Python - Delete even nodes of the Doubly Linked List



Deleting even nodes of a doubly linked list requires traverse through the list and deleting even nodes one by one. It requires creating two nodes - oddNode and evenNode. If head is not null, make oddNode to first odd node of list and evenNode to first even node of the list. If both are not null, delete the evenNode and adjust links. Move both nodes to next set of odd-even nodes. Repeat the process till any or both nodes become null.

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

def deleteEvenNodes(self):  
  if (self.head != None):
    #1. if head is not null create nodes -
    #   evenNode and oddNode
    oddNode = self.head
    evenNode = self.head.next 

    while(oddNode != None and evenNode != None):
      
      #2. while oddNode and evenNode are not null
      #   make next of oddNode as next of evenNode
      #   and free evenNode   
      oddNode.next = evenNode.next
      evenNode = None

      #3. and make temp as oddNode and oddNode as 
      #   next of oddNode
      temp = oddNode
      oddNode = oddNode.next
      
      #4. Update prev link, oddNode and evenNode
      if(oddNode != None):
        oddNode.prev = temp
        evenNode = oddNode.next

The below is a complete program that uses above discussed concept of deleting even nodes of a 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
      return
    else:
      temp = self.head
      while(temp.next != None):
        temp = temp.next
      temp.next = newNode
      newNode.prev = temp

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

        temp = oddNode
        oddNode = oddNode.next
        if(oddNode != None):
          oddNode.prev = temp
          evenNode = oddNode.next

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

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