Python - Circular Doubly Linked List Traversal

Traversing through a circular doubly linked list is very easy. It requires creating a temp node pointing to the head of the list. If the temp node is not null, display its content and move to the next node using temp next. Repeat the process till the temp node reaches the starting node (head of the list). If the temp node is empty at the start, then the list contains no item.

A circular doubly linked list can be traversed from any node as the starting point. Traverse the list till reaches the same node.

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

def PrintList(self):
  
  #1. create a temp node pointing to head
  temp = self.head
  
  #2. if the temp node is not null continue 
  #   displaying the content and move to the 
  #   next node till the temp becomes 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:
    
    #3. If the temp node is null at the start, 
    #   the list is empty
    print("The list is empty.")

The below is a complete program that uses above discussed concept to traverse through the circular doubly linked list and displaying its content.

# 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

  #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)

#traverse to display the content of the list.
MyList.PrintList()

The above code will give the following output:

The list contains: 10 20 30