Insert a Node at Front/Beginning of a Linked List (original) (raw)

Last Updated : 26 Jul, 2024

Given a linked list, the task is to insert a new node at the beginning/start/front of the linked list.

**Example:

**Input: LinkedList = 2->3->4->5, NewNode = 1
**Output: 1 2 3 4 5

**Input: LinkedList = 2->10, NewNode = 1
**Output: 1 2 10

**Approach:

To insert a new node at the front, we create a **new node and point its **next reference to the **current head of the linked list. Then, we update the **head to be this new node. This operation is efficient because it only requires adjusting a few pointers.

Insertion-at-the-Beginning-of-Singly-Linked-List

Insertion at Front/Beginning of Linked List

**Algorithm:

Make the first node of Linked List linked to the new node
Remove the head from the original first node of Linked List
Make the new node as the Head of the Linked List.

Below is the implementation of the approach:

C++ `

// C++ Program to insert the node at the beginning of // Linked List

#include <bits/stdc++.h> using namespace std;

// Define a node in the linked list struct Node { int data; // Data stored in the node Node* next; // Pointer to the next node in the list

// Constructor to initialize the node
Node(int new_data)
{
    data = new_data;
    next = nullptr; // Set next to null
}

};

// Function to insert a new node at the beginning of the // list Node* insertAtFront(Node* head, int new_data) { // Create a new node with the given data Node* new_node = new Node(new_data);

// Make the next of the new node point to the current
// head
new_node->next = head;

// Return the new node as the new head of the list
return new_node;

}

// Function to print the contents of the linked list void printList(Node* head) { // Start from the head of the list Node* curr = head;

// Traverse the list
while (curr != nullptr) {
    // Print the current node's data
    cout << " " << curr->data;

    // Move to the next node
    curr = curr->next;
}

// Print a newline at the end
cout << endl;

}

// Driver code to test the functions int main() { // Create the linked list 2->3->4->5 Node* head = new Node(2); head->next = new Node(3); head->next->next = new Node(4); head->next->next->next = new Node(5);

// Print the original list
cout << "Original Linked List:";
printList(head);

// Insert a new node at the front of the list
cout << "After inserting Nodes at the front:";
int data = 1;
head = insertAtFront(head, data);

// Print the updated list
printList(head);

return 0;

}

C

// C Program to insert the node at the beginning of // Linked List

#include <stdio.h> #include <stdlib.h>

// Define a node in the linked list struct Node { int data; // Data stored in the node struct Node* next; // Pointer to the next node in the list };

// Function to create a new node with the given data struct Node* createNode(int new_data) { // Allocate memory for a new node struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));

// Initialize the node's data
new_node->data = new_data;

// Set the next pointer to NULL
new_node->next = NULL;

// Return the newly created node
return new_node;

}

// Function to insert a new node at the beginning of the // list struct Node* insertAtFront(struct Node* head, int new_data) { // Create a new node with the given data struct Node* new_node = createNode(new_data);

// Make the next of the new node point to the current
// head
new_node->next = head;

// Return the new node as the new head of the list
return new_node;

}

// Function to print the contents of the linked list void printList(struct Node* head) { // Start from the head of the list struct Node* curr = head;

// Traverse the list
while (curr != NULL) {
    // Print the current node's data
    printf(" %d", curr->data);

    // Move to the next node
    curr = curr->next;
}

// Print a newline at the end
printf("\n");

}

// Driver code to test the functions int main() { // Create the linked list 2->3->4->5 struct Node* head = createNode(2); head->next = createNode(3); head->next->next = createNode(4); head->next->next->next = createNode(5);

// Print the original list
printf("Original Linked List:");
printList(head);

// Insert a new node at the front of the list
printf("After inserting Nodes at the front:");
int data = 1;
head = insertAtFront(head, data);

// Print the updated list
printList(head);

return 0;

}

Java

// Java Program to insert the node at the beginning of // Linked List

class Node { int data; // Data stored in the node Node next; // Pointer to the next node in the list

// Constructor to initialize the node
Node(int new_data) {
    data = new_data;
    next = null;
}

}

public class GFG { // Function to insert a new node at the beginning of the list public static Node insertAtFront(Node head, int new_data) { // Create a new node with the given data Node new_node = new Node(new_data);

    // Make the next of the new node point to the current head
    new_node.next = head;
    
    // Return the new node as the new head of the list
    return new_node;
}

// Function to print the contents of the linked list
public static void printList(Node head) {
    // Start from the head of the list
    Node curr = head;
    
    // Traverse the list
    while (curr != null) {
        // Print the current node's data
        System.out.print(" " + curr.data);
        
        // Move to the next node
        curr = curr.next;
    }
    
    // Print a newline at the end
    System.out.println();
}

// Driver code to test the functions
public static void main(String[] args) {
    // Create the linked list 2->3->4->5
    Node head = new Node(2);
    head.next = new Node(3);
    head.next.next = new Node(4);
    head.next.next.next = new Node(5);

    // Print the original list
    System.out.println("Original Linked List:");
    printList(head);

    // Insert a new node at the front of the list
    System.out.println("After inserting Nodes at the front:");
    int data = 1;
    head = insertAtFront(head, data);
    
    // Print the updated list
    printList(head);
}

}

Python

Python Program to insert the node at the beginning of

Linked List

class Node: def init(self, new_data): # Initialize the node's data self.data = new_data

    # Set the next pointer to None
    self.next = None

def insert_at_front(head, new_data): # Create a new node with the given data new_node = Node(new_data)

# Make the next of the new node point to the current head
new_node.next = head

# Return the new node as the new head of the list
return new_node

def print_list(head): # Start from the head of the list curr = head

# Traverse the list
while curr is not None:
    # Print the current node's data
    print(f" {curr.data}", end="")

    # Move to the next node
    curr = curr.next

# Print a newline at the end
print()

Driver code to test the functions

if name == "main": # Create the linked list 2->3->4->5 head = Node(2) head.next = Node(3) head.next.next = Node(4) head.next.next.next = Node(5)

# Print the original list
print("Original Linked List:", end="")
print_list(head)

# Insert a new node at the front of the list
print("After inserting Nodes at the front:", end="")
data = 1
head = insert_at_front(head, data)

# Print the updated list
print_list(head)

JavaScript

// JavaScript Program to insert the node at the beginning of // Linked List

class Node { constructor(newData) { // Initialize the node's data this.data = newData;

    // Set the next pointer to null
    this.next = null;
}

}

// Function to insert a new node at the beginning of the // list function insertAtFront(head, newData) { // Create a new node with the given data const newNode = new Node(newData);

// Make the next of the new node point to the current
// head
newNode.next = head;

// Return the new node as the new head of the list
return newNode;

}

// Function to print the contents of the linked list function printList(head) { // Start from the head of the list let curr = head; let result = "";

// Traverse the list
while (curr !== null) {
    // Append the current node's data to result
    result += " " + curr.data;

    // Move to the next node
    curr = curr.next;
}

// Print the result
console.log(result);

}

// Driver code to test the functions function main() { // Create the linked list 2->3->4->5 let head = new Node(2); head.next = new Node(3); head.next.next = new Node(4); head.next.next.next = new Node(5);

// Print the original list
console.log("Original Linked List:");
printList(head);

// Insert a new node at the front of the list
console.log("After inserting Nodes at the front:");
const data = 1;
head = insertAtFront(head, data);

// Print the updated list
printList(head);

}

main(); // Execute the driver code

Output

Original Linked List: 2 3 4 5 After inserting Nodes at the front: 1 2 3 4 5

**Time Complexity: O(1), We have a pointer to the head and we can directly attach a node and update the head pointer. So, the Time complexity of inserting a node at the head position is O(1).
**Auxiliary Space: O(1)