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

Program to Insert a New Node at the Beginning of the Doubly Linked List on fibonacci, factorial, prime, armstrong, swap, reverse, search, sort, stack, queue, array, linkedlist, tree, graph etc.

## 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.

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

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. 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.
4. 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.tail = None;

#addAtStart() will add a node to the starting of the list
#Create a new node
newNode = Node(data);

#If list is empty
#Both head and tail will point to newNode
self.head = self.tail = newNode;
#head's previous will point to 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
#newNode's next node will be head
#newNode's previous will point to None
newNode.previous = None;
#newNode will become new head

#display() will print out the nodes of the list
def display(self):
#Node current will point to head
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.display();
#Adding 2 to the list
dList.display();
#Adding 3 to the list
dList.display();
#Adding 4 to the list
dList.display();
#Adding 5 to the list
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
//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
//newNode's next node will be head
//newNode's previous will point to NULL
newNode->previous = NULL;
//newNode will become new head
}
}

//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
display();
//Adding 2 to the list
display();
//Adding 3 to the list
display();
//Adding 4 to the list
display();
//Adding 5 to the list
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
//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
//newNode's next node will be head
//newNode's previous will point to null
newNode.previous = null;
//newNode will become new head
}
}

//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.display();
//Adding 2 to the list
dList.display();
//Adding 3 to the list
dList.display();
//Adding 4 to the list
dList.display();
//Adding 5 to the list
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;
{
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
//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
//newNode's next node will be head
//newNode's previous will point to null
newNode.previous = null;
//newNode will become new head
}
}

//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.display();
//Adding 2 to the list
dList.display();
//Adding 3 to the list
dList.display();
//Adding 4 to the list
dList.display();
//Adding 5 to the list
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 \$tail;
function __construct(){
\$this->tail = NULL;
}

//addAtStart() will add a node to the starting of the list
//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
//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
//newNode's next node will be head
//newNode's previous will point to NULL
\$newNode->previous = NULL;
//newNode will become new head
}
}

//display() will print out the nodes of the list
function display() {
//Node current will point to 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->display();
//Adding 2 to the list
\$dList->display();
//Adding 3 to the list
\$dList->display();
//Adding 4 to the list
\$dList->display();
//Adding 5 to the list
\$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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