Program to Insert a New Node at the Beginning of the Doubly Linked List

Q. Program to insert a new node at the beginning of the doubly linked list.

Explanation

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 new node. Make new node as head of the list, and its previous will point to null.

Program to insert a new node at the beginning of the doubly linked list

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:
1. It first checks whether the head is null, then it will insert the node as the head.
2. Both head and tail will point to a newly added node.
3. Head's previous pointer will point to null and tail's next pointer will point to null.
4. If the head is not null, head's previous will point to new node.
5. Make new node as head of the list.
6. Since new node became head so its previous should point to null.
display() will show all the nodes present in the list.
1. Define a new node 'current' that will point to the head.
2. Print current.data till current points to null.
3. Current will point to the next node in the list in each iteration.

Solution

Python

#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

C

#include <stdio.h>
 
//Represent a node of the doubly linked list

struct node{
    int data;
    struct node *previous;
    struct node *next;
};    
 
//Represent the head and tail of the doubly linked list
struct node *head, *tail = NULL;
 
//addAtStart() will add a node to the starting of the list
void addAtStart(int data) {
    //Create a new node
    struct node *newNode = (struct node*)malloc(sizeof(struct node));
    newNode->data = data;
    
    //If list is empty
    if(head == NULL) {
        //Both head and tail will point to newNode
        head = tail = newNode;
        //head's previous will point to NULL
        head->previous = NULL;
        //tail's next will point to NULL, as it is the last node of the list
        tail->next = NULL;
    }
    //Add newNode as new head of the list
    else {
        //head's previous node will be newNode
        head->previous = newNode;
        //newNode's next node will be head
        newNode->next = head;
        //newNode's previous will point to NULL
        newNode->previous = NULL;
        //newNode will become new head
        head = newNode;
    }
}
 
//display() will print out the nodes of the list
void display() {
    //Node current will point to head
    struct node *current = head;
    if(head == NULL) {
        printf("List is empty\n");
        return;
    }
    printf("Adding a node to the start of the list: \n");
    while(current != NULL) {
        //Prints each node by incrementing pointer.
        printf("%d ", current->data);
        current = current->next;
    }
    printf("\n");
}
 
int main()
{
    //Adding 1 to the list
    addAtStart(1);
    display();
    //Adding 2 to the list
    addAtStart(2);
    display();
    //Adding 3 to the list
    addAtStart(3);
    display();
    //Adding 4 to the list
    addAtStart(4);
    display();
    //Adding 5 to the list
    addAtStart(5);
    display();
 
    return 0;
}

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

JAVA

public class InsertStart {
    
    //Represent a node of the doubly linked list

    class Node{
        int data;
        Node previous;
        Node next;
        
        public Node(int data) {
            this.data = data;
        }
    }
    
    //Represent the head and tail of the doubly linked list
    Node head, tail = null;
    
    //addAtStart() will add a node to the starting of the list
    public void addAtStart(int data) {
        //Create a new node
        Node newNode = new Node(data);
        
        //If list is empty
        if(head == null) {
            //Both head and tail will point to newNode
            head = tail = newNode;
            //head's previous will point to null
            head.previous = null;
            //tail's next will point to null, as it is the last node of the list
            tail.next = null;
        }
        //Add newNode as new head of the list
        else {
            //head's previous node will be newNode
            head.previous = newNode;
            //newNode's next node will be head
            newNode.next = head;
            //newNode's previous will point to null
            newNode.previous = null;
            //newNode will become new head
            head = newNode;
        }
    }
    
    //display() will print out the nodes of the list
    public void display() {
        //Node current will point to head
        Node current = head;
        if(head == null) {
            System.out.println("List is empty");
            return;
        }
        System.out.println("Adding a node to the start of the list: ");
        while(current != null) {
            //Prints each node by incrementing the pointer.

            System.out.print(current.data + " ");
            current = current.next;
        }
        System.out.println();
    }
    
    public static void main(String[] args) {
        
        InsertStart dList = new 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

C#

 using System; 
namespace DoublyLinkedList 
{                     
    public class Program
    {
        //Represent a node of the doubly linked list

        public class Node<T>{
            public T data;
            public Node<T> previous;
            public Node<T> next;
            
            public Node(T value) {
                data = value;
            }
        }
        
        public class InsertStart<T>{
            //Represent the head and tail of the doubly linked list
            protected Node<T> head = null;             
             protected Node<T> tail = null;
            
            //addAtStart() will add a node to the starting of the list
            public void addAtStart(T data) {
                //Create a new node
                Node<T> newNode = new Node<T>(data);
 
                //If list is empty
                if(head == null) {
                    //Both head and tail will point to newNode
                    head = tail = newNode;
                    //head's previous will point to null
                    head.previous = null;
                    //tail's next will point to null, as it is the last node of the list
                    tail.next = null;
                }
                //Add newNode as new head of the list
                else {
                    //head's previous node will be newNode
                    head.previous = newNode;
                    //newNode's next node will be head
                    newNode.next = head;
                    //newNode's previous will point to null
                    newNode.previous = null;
                    //newNode will become new head
                    head = newNode;
                }
            }
    
            //display() will print out the nodes of the list
            public void display() {
                //Node current will point to head
                Node<T> current = head;
                if(head == null) {
                    Console.WriteLine("List is empty");
                    return;
                }
                Console.WriteLine("Adding a node to the start of the list: ");
                while(current != null) {
                    //Prints each node by incrementing the pointer.

                    Console.Write(current.data + " ");
                    current = current.next;
                }
                Console.WriteLine();
            }
        }
        
        public static void Main()
        {
            InsertStart<int> dList = new InsertStart<int>();
 
            //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

PHP

<!DOCTYPE html>
<html>
<body>
<?php
//Represent a node of doubly linked list
class Node{
    public $data;
    public $previous;
    public $next;
    
    function __construct($data){
        $this->data = $data;
    }
}
class InsertStart{
    //Represent the head and tail of the doubly linked list
    public $head;
    public $tail;
    function __construct(){
        $this->head = NULL;
        $this->tail = NULL;
    }
    
    //addAtStart() will add a node to the starting of the list
    function addAtStart($data) {
        //Create a new node
        $newNode = new Node($data);
        
        //If list is empty
        if($this->head == NULL) {
            //Both head and tail will point to newNode
            $this->head = $this->tail = $newNode;
            //head's previous will point to NULL
            $this->head->previous = NULL;
            //tail's next will point to NULL, as it is the last node of the list
            $this->tail->next = NULL;
        }
        //Add newNode as new head of the list
        else {
            //head's previous node will be newNode
            $this->head->previous = $newNode;
            //newNode's next node will be head
            $newNode->next = $this->head;
            //newNode's previous will point to NULL
            $newNode->previous = NULL;
            //newNode will become new head
            $this->head = $newNode;
        }
    }
    
    //display() will print out the nodes of the list
    function display() {
        //Node current will point to head
        $current = $this->head;
        if($this->head == NULL) {
            print("List is empty <br>");
            return;
        }
        print("Adding a node to the start of the list: <br>");
        while($current != NULL) {
            //Prints each node by incrementing pointer.
            print($current->data . " ");
            $current = $current->next;
        }
        print("<br>");
    }
}
    
$dList = new 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();
?>
</body>
</html>

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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