Python - Insert a new node at a given position in the Circular Doubly Linked List

In this method, a new element is inserted at the specified position in the circular doubly linked list. For example - if the given list is 10->20->30 and a new element 100 is added at position 2, the list becomes 10->100->20->30.

First, create two nodes: 1. newNode with given element and 2. temp to traverse through the list. After that count the number of elements in the list to check whether the insertion position is valid or not (It must lie in the range of [1, n + 1], where n is number of elements in the list). If the insertion position is valid and equal to 1 then make the newNode as head and adjust links accordingly. If the insertion position is valid and greater than 1 then traverse to the specified position and insert the newNode and adjust links accordingly.

Circular Doubly Linked List - Add Node At Start

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

def push_at(self, newElement, position):     

  #1. allocate node to new element and 
  #   create a temp node to traverse the list
  newNode = Node(newElement)
  temp = self.head
  NoOfElements = 0

  #2. Find the number of elements in the list
  if(temp != None):
    NoOfElements += 1
    temp = temp.next
  while(temp != self.head):
    NoOfElements += 1
    temp = temp.next

  #3. check if the insertion position is valid
  if(position < 1 or position > (NoOfElements+1)):
    print("\nInavalid position.")
  elif (position == 1):
  
  #4. if the position is 1, make next of the
  #   new node as head and new node as head
    if(self.head == None):
      self.head = newNode
      self.head.next = self.head
      self.head.prev = self.head
    else:
      while(temp.next != self.head):
        temp = temp.next
      temp.next = newNode
      newNode.prev = temp
      newNode.next = self.head
      self.head.prev = newNode
      self.head = newNode

  else:

    #5. Else, traverse to the node previous to
    #   the given position, make newNode next 
    #   as temp next and temp next as newNode.
    temp = self.head
    for i in range(1, position-1):
      temp = temp.next 
    newNode.next = temp.next
    newNode.next.prev = newNode
    newNode.prev = temp
    temp.next = newNode

The below is a complete program that uses above discussed concept to insert a new node at given position in 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

  #Inserts a new element at the given position 
  def push_at(self, newElement, position):     
    newNode = Node(newElement)
    temp = self.head
    NoOfElements = 0
    if(temp != None):
      NoOfElements += 1
      temp = temp.next
    while(temp != self.head):
      NoOfElements += 1
      temp = temp.next

    if(position < 1 or position > (NoOfElements+1)):
      print("\nInavalid position.")
    elif (position == 1):
      if(self.head == None):
        self.head = newNode
        self.head.next = self.head
        self.head.prev = self.head
      else:
        while(temp.next != self.head):
          temp = temp.next
        temp.next = newNode
        newNode.prev = temp
        newNode.next = self.head
        self.head.prev = newNode
        self.head = newNode

    else:
      temp = self.head
      for i in range(1, position-1):
        temp = temp.next 
      newNode.next = temp.next
      newNode.next.prev = newNode
      newNode.prev = temp
      temp.next = newNode

  #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.PrintList()

#Insert an element at position 2
MyList.push_at(100, 2)
MyList.PrintList()

#Insert an element at position 1
MyList.push_at(200, 1)
MyList.PrintList()

The above code will give the following output:

The list contains: 10 20 30 
The list contains: 10 100 20 30 
The list contains: 200 10 100 20 30