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Python program to create a doubly linked list of n nodes and display it in reverse order

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robaa
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2022-11-08
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In this program, we create a doubly linked list and then reverse the list by reversing the direction of the list and print out the nodes.

Traverse through the list by swapping the previous pointer with next pointer of each node. Then, swap the position of head and tail node that is, head of the original list will become tail of new list and tail of the original list will become head of the new list. So, the reversed list will be:

ALGORITHM:

  1. Define a Node class which represents a node in the list. It will have three properties: data, previous which will point to the previous node and next which will point to the next node.
  2. Define another class for creating a doubly linked list, and it has two nodes: head and tail. Initially, head and tail will point to null.
  3. addNode() will add node to the list:
  • It first checks whether the head is null, then it will insert the node as the head.
  • Both head and tail will point to a newly added node.
  • Head's previous pointer will point to null and tail's next pointer will point to null.
  • If the head is not null, the new node will be inserted at the end of the list such that new node's previous pointer will point to tail.
  • The new node will become the new tail. Tail's next pointer will point to null.

a. reverse() will reverse the given doubly linked list.

  • Define a node current which will initially point to head.
  • Traverse through the list by making current to point to current.next in each iteration till current points to null.
  • In each iteration, swap previous and next pointer of each node to reverse the direction of the list.
  • In the end, swap the position of head and tail.

a. display() will show all the nodes present in the list.

  • Define a new node 'current' that will point to the head.
  • Print current.data till current points to null.
  • Current will point to the next node in the list in each iteration.
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robaa
2022-11-08
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PROGRAM:

#Represent a node of doubly linked list    
class Node:    
    def __init__(self,data):    
        self.data = data;    
        self.previous = None;    
        self.next = None;    
            
class ReverseList:    
    #Represent the head and tail of the doubly linked list    
    def __init__(self):    
        self.head = None;    
        self.tail = None;    
            
    #addNode() will add a node to the list    
    def addNode(self, data):    
        #Create a new node    
        newNode = Node(data);    
            
        #If list is empty    
        if(self.head == None):    
            #Both head and tail will point to newNode    
            self.head = self.tail = newNode;    
            #head's previous will point to None    
            self.head.previous = None;    
            #tail's next will point to None, as it is the last node of the list    
            self.tail.next = None;    
        else:    
            #newNode will be added after tail such that tail's next will point to newNode    
            self.tail.next = newNode;    
            #newNode's previous will point to tail    
            newNode.previous = self.tail;    
            #newNode will become new tail    
            self.tail = newNode;    
            #As it is last node, tail's next will point to None    
            self.tail.next = None;    
                
    #reverse() will reverse the doubly linked list    
    def reverse(self):    
        #Node current will point to head    
        current = self.head;    
            
        #Swap the previous and next pointers of each node to reverse the direction of the list    
        while(current != None):    
            temp = current.next;    
            current.next = current.previous;    
            current.previous = temp;    
            current = current.previous;    
        #Swap the head and tail pointers.    
        temp = self.head;    
        self.head = self.tail;    
        self.tail = temp;    
            
    #display() will print out the elements of the list    
    def display(self):    
        #Node current will point to head    
        current = self.head;    
        if(self.head == None):    
            print("List is empty");    
            return;    
                
        while(current != None):    
            #Prints each node by incrementing pointer.    
            print(current.data),    
            current = current.next;    
                
dList = ReverseList();    
#Add nodes to the list    
dList.addNode(1);    
dList.addNode(2);    
dList.addNode(3);    
dList.addNode(4);    
dList.addNode(5);    
     
print("Original List: ");    
dList.display();    
     
#Reverse the given list    
dList.reverse();    
     
#Displays the reversed list    
print("\nReversed List: ");    
dList.display();

 

Output:

Original List: 
1 2 3 4 5 
Reversed List: 
5 4 3 2 1

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