Python program to insert a new node at the beginning of the Doubly Linked list

10. Python program to insert a new node at the beginning of the Doubly Linked list.

In this program, we will create a doubly linked list and insert every new node at the beginning of the list. If the list is empty, then head and tail will point to the newly added node. If the list is not empty then, insert the new node at the beginning of the list such that the head's previous will point to the new node. Make new node as head of the list, and its previous will point to null.

Consider the above example. Initially, 1 was the head of the list. Now new will be inserted before node 1 such that node 1's previous will point to new. Make new as the head of the list, and its previous will point to null

ALGORITHM:

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.
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.
addAtStart() will add a node to the beginning of 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, head's previous will point to the new node.
Make new node as head of the list.
Since new node became head so its previous should point to null.

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.

PROGRAM:

#Represent a node of doubly linked list  
class Node:  
    def __init__(self,data):  
        self.data = data;  
        self.previous = None;  
        self.next = None;  
          
class InsertStart:  
    #Represent the head and tail of the doubly linked list  
    def __init__(self):  
        self.head = None;  
        self.tail = None;  
          
    #addAtStart() will add a node to the starting of the list  
    def addAtStart(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;  
        #Add newNode as new head of the list  
        else:  
            #head's previous node will be newNode  
            self.head.previous = newNode;  
            #newNode's next node will be head  
            newNode.next = self.head;  
            #newNode's previous will point to None  
            newNode.previous = None;  
            #newNode will become new head  
            self.head = newNode;  
              
    #display() will print out the nodes of the list  
    def display(self):  
        #Node current will point to head  
        current = self.head;  
        if(self.head == None):  
            print("List is empty");  
            return;  
        print("Adding a node to the start of the list: ");  
        while(current != None):  
            #Prints each node by incrementing pointer.  
            print(current.data),  
            current = current.next;  
              
        print();  
          
dList = InsertStart();  
   
#Adding 1 to the list  
dList.addAtStart(1);  
dList.display();  
#Adding 2 to the list  
dList.addAtStart(2);  
dList.display();  
#Adding 3 to the list  
dList.addAtStart(3);  
dList.display();  
#Adding 4 to the list  
dList.addAtStart(4);  
dList.display();  
#Adding 5 to the list  
dList.addAtStart(5);  
dList.display();  

Output:

Adding a node to the start of the list: 
1 
Adding a node to the start of the list: 
2 1 
Adding a node to the start of the list: 
3 2 1 
Adding a node to the start of the list: 
4 3 2 1 
Adding a node to the start of the list: 
5 4 3 2 1

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