**Problem Statement:** Given the **pointer to a node** belonging to a doubly linked list, and a value **val**, your task is to insert a new node with the given val before the referenced node. It is guaranteed that the node **will not be the head** of the doubly linked list.

##
**
Examples
**

Example 1:

Input Format:DLL: 1 <-> 2 <-> 3 <-> 4

Give Node: 3

Value to be Inserted: 6

Result: DLL: 1 <-> 2 <-> 6 <-> 3 <-> 4

Explanation: We have to insert a new node with value 6 before a node with value 3.

Example 2:

Input Format:DLL: 10 <-> 20 <-> 30

Given Node: 30

Value to be Inserted: 5

Result: DLL: 10 <-> 20 <-> 5 <->30

Explanation: In this case, a new node of value 5 has to be inserted before the 1st node ie. head of the doubly linked list. 5 will be the new head of the doubly linked list.

**Solution**

** Disclaimer**:

*Don’t jump directly to the solution, try it out yourself first.*

**Approach**:

To insert before a given node, begin by identifying its **previous** **node**. This step is assured since the provided node is never the head. Create a **new** **node** with the specified value and set its **back** and **next** pointers to the **previous** **node** and the **given** **node**, respectively. To seamlessly integrate the new node into the doubly linked list, set the previous node’s **next** pointer and the given node’s **back** pointer to the **new** **node**.

### Algorithm:

**Step 1: **Track the node to which the **back pointer **of the** referenced node** is pointing and the name is `**prev**`.

**Step 2: **Create a** new node** with its **data as val **and back **pointer as prev** and **next pointer **as the given node**.**

**Step 3:** Update the **next** **pointer** of the **prev** **node** to point to the newly created node, and also, update the **back** **pointer** of the **referenced** **node** to point to the new node.

**Code:**

## C++ Code

```
#include <iostream>
#include <bits/stdc++.h>
using namespace std;
// Define a Node class for doubly linked list
class Node {
public:
// Data stored in the node
int data;
// Pointer to the next node in the list (forward direction)
Node* next;
// Pointer to the previous node in the list (backward direction)
Node* back;
// Constructor for a Node with both data, a reference to the next node, and a reference to the previous node
Node(int data1, Node* next1, Node* back1) {
data = data1;
next = next1;
back = back1;
}
// Constructor for a Node with data, and no references to the next and previous nodes (end of the list)
Node(int data1) {
data = data1;
next = nullptr;
back = nullptr;
}
};
// Function to convert an array to a doubly linked list
Node* convertArr2DLL(vector<int> arr) {
// Create the head node with the first element of the array
Node* head = new Node(arr[0]);
// Initialize 'prev' to the head node
Node* prev = head;
for (int i = 1; i < arr.size(); i++) {
// Create a new node with data from the array and set its 'back' pointer to the previous node
Node* temp = new Node(arr[i], nullptr, prev);
// Update the 'next' pointer of the previous node to point to the new node
prev->next = temp;
// Move 'prev' to the newly created node for the next iteration
prev = temp;
}
// Return the head of the doubly linked list
return head;
}
// Function to print the elements of the doubly linked list
void print(Node* head) {
while (head != nullptr) {
// Print the data in the current node
cout << head->data << " ";
// Move to the next node
head = head->next;
}
}
// Function to insert before the Kth element
Node* insertBeforeKthElement(Node* head, int k, int val){
if(k==1){
// K = 1 means node has to be insert before the head
return insertBeforeHead(head, val);
}
// temp will keep track of the node
Node* temp = head;
// count so that the Kth element can be reached
int count = 0;
while(temp!=NULL){
count ++;
// On reaching the Kth position, break out of the loop
if(count == k) break;
// keep moving temp forward till count != K
temp = temp->next;
}
// track the node before the Kth node
Node* prev = temp->back;
// create new node with data as val
Node* newNode = new Node(val, temp, prev);
//join the new node in between prev and temp
prev->next = newNode;
temp->back = newNode;
// Set newNode's pointers to prev and temp
newNode->next = temp;
newNode->back = prev;
// Return the head of the updated doubly linked list
return head;
}
// Function to insert a new node before a given node
void insertBeforeNode(Node*node, int val){
// Get the node before the given node
Node* prev = node->back;
// Create a new node with the given val
Node* newNode = new Node(val, node, prev);
// Connect the newNode to the doubly linked List
prev->next = newNode;
node->back = newNode;
// void function to just update
return;
}
int main() {
vector<int> arr = {12, 5, 8, 7, 4};
Node* head = convertArr2DLL(arr);
print(head);
cout << endl << "Doubly Linked List After Inserting before the node with value 8: " << endl;
insertBeforeNode(head->next->next, 100);
print(head);
return 0;
}
```

**Output:**

12 5 8 7 4

Doubly Linked List After Inserting before the node with value 8:

12 5 100 8 7 4

**Time Complexity: O(1): **The time complexity of this insertion operation is O(1) because only a constant number of pointer updates are being performed regardless of the size of the Doubly Linked List.

**Space Complexity: O(1): **The space complexity is also O(1) because a constant amount of extra space is used to create the new node

## Java Code

```
public class DLinkedList {
public static class Node {
// Data stored in the node
public int data;
// Reference to the next node in the list (forward direction)
public Node next;
// Reference to the previous node in the list (backward direction)
public Node back;
// Constructor for a Node with both data, a reference to the next node, and a reference to the previous node
public Node(int data1, Node next1, Node back1) {
data = data1;
next = next1;
back = back1;
}
// Constructor for a Node with data, and no references to the next and previous nodes (end of the list)
public Node(int data1) {
data = data1;
next = null;
back = null;
}
}
private static Node convertArr2DLL(int[] arr) {
// Create the head node with the first element of the array
Node head = new Node(arr[0]);
// Initialize 'prev' to the head node
Node prev = head;
for (int i = 1; i < arr.length; i++) {
// Create a new node with data from the array and set its 'back' pointer to the previous node
Node temp = new Node(arr[i], null, prev);
// Update the 'next' pointer of the previous node to point to the new node
prev.next = temp;
// Move 'prev' to the newly created node for the next iteration
prev = temp;
}
// Return the head of the doubly linked list
return head;
}
private static void print(Node head) {
while (head != null) {
// Print the data in the current node
System.out.print(head.data + " ");
// Move to the next node
head = head.next; // Move to the next node
}
System.out.println();
}
// Function to insert a new node before a given node
private static void insertBeforeNode(Node node, int val) {
// Get the node before the given node
Node prev = node.back;
// Create a new node with the given val
Node newNode = new Node(val, node, prev);
// Connect the newNode to the doubly linked list
prev.next = newNode;
node.back = newNode;
}
public static void main(String[] args) {
int[] arr = {12, 5, 6, 8, 4};
// Convert the array to a doubly linked list
Node head = convertArr2DLL(arr);
// Print the doubly linked list
print(head);
System.out.println("Doubly Linked List After Inserting before the node with value 8:");
insertBeforeNode(head.next.next, 100);
print(head);
}
}
```

**Output:**

12 5 8 7 4

Doubly Linked List After Inserting before the node with value 8:

12 5 100 8 7 4

**Time Complexity: O(1): **The time complexity of this insertion operation is O(1) because only a constant number of pointer updates are being performed regardless of the size of the Doubly Linked List.

**Space Complexity: O(1): **The space complexity is also O(1) because a constant amount of extra space is used to create the new node.

## Python Code

```
class DLinkedList:
class Node:
def __init__(self, data, next_node=None, back_node=None):
# Data stored in the node
self.data = data
# Reference to the next node in the list (forward direction)
self.next = next_node
# Reference to the previous node in the list (backward direction)
self.back = back_node
@staticmethod
def convertArr2DLL(arr):
# Create the head node with the first element of the array
head = DLinkedList.Node(arr[0])
# Initialize 'prev' to the head node
prev = head
for i in range(1, len(arr)):
# Create a new node with data from the array and set its 'back' pointer to the previous node
temp = DLinkedList.Node(arr[i], None, prev)
# Update the 'next' pointer of the previous node to point to the new node
prev.next = temp
# Move 'prev' to the newly created node for the next iteration
prev = temp
# Return the head of the doubly linked list
return head
@staticmethod
def print_linked_list(head):
while head is not None:
# Print the data in the current node
print(head.data, end=" ")
# Move to the next node
head = head.next
print()
@staticmethod
def insert_before_node(node, val):
# Get the node before the given node
prev = node.back
# Create a new node with the given val
new_node = DLinkedList.Node(val, node, prev)
# Connect the new_node to the doubly linked list
prev.next = new_node
node.back = new_node
if __name__ == "__main__":
arr = [12, 5, 6, 8, 4]
# Convert the array to a doubly linked list
head = DLinkedList.convertArr2DLL(arr)
# Print the doubly linked list
DLinkedList.print_linked_list(head)
print("Doubly Linked List After Inserting before the node with value 8:")
# Insert a new node before the node with value 8
DLinkedList.insert_before_node(head.next.next, 100)
# Print the updated doubly linked list
DLinkedList.print_linked_list(head)
```

**Output:**

12 5 8 7 4

Doubly Linked List After Inserting before the node with value 8:

12 5 100 8 7 4

**Time Complexity: O(1): **The time complexity of this insertion operation is O(1) because only a constant number of pointer updates are being performed regardless of the size of the Doubly Linked List.

**Space Complexity: O(1): **The space complexity is also O(1) because a constant amount of extra space is used to create the new node.

## JavaScript Code

```
class Node {
// Data stored in the node
constructor(data1, next1, back1) {
this.data = data1;
this.next = next1;
this.back = back1;
}
// Constructor for a Node with data, and no references to the next and previous nodes (end of the list)
constructor(data1) {
this.data = data1;
this.next = null;
this.back = null;
}
}
function convertArr2DLL(arr) {
// Create the head node with the first element of the array
let head = new Node(arr[0]);
// Initialize 'prev' to the head node
let prev = head;
for (let i = 1; i < arr.length; i++) {
// Create a new node with data from the array and set its 'back' pointer to the previous node
let temp = new Node(arr[i], null, prev);
// Update the 'next' pointer of the previous node to point to the new node
prev.next = temp;
// Move 'prev' to the newly created node for the next iteration
prev = temp;
}
// Return the head of the doubly linked list
return head;
}
function print(head) {
while (head !== null) {
// Print the data in the current node
console.log(head.data + " ");
// Move to the next node
head = head.next; // Move to the next node
}
console.log();
}
// Function to insert a new node before a given node
function insertBeforeNode(node, val) {
// Get the node before the given node
let prev = node.back;
// Create a new node with the given val
let newNode = new Node(val, node, prev);
// Connect the newNode to the doubly linked list
prev.next = newNode;
node.back = newNode;
}
// Main function
let arr = [12, 5, 6, 8, 4];
// Convert the array to a doubly linked list
let head = convertArr2DLL(arr);
// Print the doubly linked list
print(head);
console.log("Doubly Linked List After Inserting before the node with value 8:");
insertBeforeNode(head.next.next, 100);
print(head);
```

**Output:**

12 5 8 7 4

Doubly Linked List After Inserting before the node with value 8:

12 5 100 8 7 4

**Time Complexity: O(1): **The time complexity of this insertion operation is O(1) because only a constant number of pointer updates are being performed regardless of the size of the Doubly Linked List.

**Space Complexity: O(1): **The space complexity is also O(1) because a constant amount of extra space is used to create the new node

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Special thanks toGauri Tomarfor contributing to this article on takeUforward. If you also wish to share your knowledge with the takeUforward fam,please check out this article