Insert a Node at Front/Beginning of a Linked List

Like arrays, Linked List is a linear data structure. Unlike arrays, linked list elements are not stored at a contiguous location; the elements are linked using pointers. They include a series of connected nodes. Here, each node stores the data and the address of the next node.

To learn more about linked list refer to the article “Introduction to Linked LIst“.

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

Example:

List = 2->3->4->5, Insert a node with value 1 at beginning.
Output list will be: 1->2->3->4->5

Consider the following representations of the linked list. 

C++

// A linked list node
class Node {
public:
    int data;
    Node* next;
};

C

// A linked list node
struct Node {
    int data;
    struct Node* next;
};

Java

// Linked List Class
class LinkedList {
    Node head; // head of list

    /* Node Class */
    class Node {
        int data;
        Node next;

        // Constructor to create a new node
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
}

Python3

# Node class
class Node:

    # Function to initialize the node object
    def __init__(self, data):
        self.data = data  # Assign data
        self.next = None  # Initialize next as null

# Linked List class


class LinkedList:

    # Function to initialize the Linked List object
    def __init__(self):
        self.head = None

C#

/* Linked list Node*/
public class Node {
    public int data;
    public Node next;
    public Node(int d)
    {
        data = d;
        next = null;
    }
}

Javascript

<script>
// Linked List Class
    var head; // head of list

    /* Node Class */
    class Node {

        // Constructor to create a new node
        constructor(d) {
            this.data = d;
            this.next = null;
        }
    }
// This code is contributed by todaysgaurav 
</script>

Approach: The below steps should be followed to insert a new node at the front of the linked list

• Allocate a new node (say temp).
• Put the required data into temp.
• The ‘next’ pointer of the node should be pointed to the current head.
• Now make the head pointer point to temp.

See the below image for a better understanding:

Insert a new node at front of Linked List

Below is the implementation of the approach:

C++

// Given a reference (pointer to pointer)
// to the head of a list and an int,
// inserts a new node on the front of
// the list.
void push(Node** head_ref, int new_data)
{

    // 1. allocate node
    Node* new_node = new Node();

    // 2. put in the data
    new_node->data = new_data;

    // 3. Make next of new node as head
    new_node->next = (*head_ref);

    // 4. Move the head to point to
    // the new node
    (*head_ref) = new_node;
}

C

/* Given a reference (pointer to pointer) to the head of a
   list
   and an int,  inserts a new node on the front of the list.
 */
void push(struct Node** head_ref, int new_data)
{
    /* 1. allocate node */
    struct Node* new_node
        = (struct Node*)malloc(sizeof(struct Node));

    /* 2. put in the data  */
    new_node->data = new_data;

    /* 3. Make next of new node as head */
    new_node->next = (*head_ref);

    /* 4. move the head to point to the new node */
    (*head_ref) = new_node;
}

Java

/* This function is in LinkedList class. Inserts a
   new Node at front of the list. This method is
   defined inside LinkedList class shown above */
public void push(int new_data)
{
    /* 1 & 2: Allocate the Node &
              Put in the data*/
    Node new_node = new Node(new_data);

    /* 3. Make next of new Node as head */
    new_node.next = head;

    /* 4. Move the head to point to new Node */
    head = new_node;
}

Python3

# This function is in LinkedList class
# Function to insert a new node at the beginning
def push(self, new_data):

    # 1 & 2: Allocate the Node &
    # Put in the data
    new_node = Node(new_data)

    # 3. Make next of new Node as head
    new_node.next = self.head

    # 4. Move the head to point to new Node
    self.head = new_node

C#

/* Inserts a new Node at front of the list. */
public void push(int new_data)
{
    /* 1 & 2: Allocate the Node &
               Put in the data*/
    Node new_node = new Node(new_data);

    /* 3. Make next of new Node as head */
    new_node.next = head;

    /* 4. Move the head to point to new Node */
    head = new_node;
}

Javascript

<script>

/* This function is in LinkedList class. Inserts a
   new Node at front of the list. This method is 
   defined inside LinkedList class shown above */
   
 function push(new_data)
{
    /* 1 & 2: Allocate the Node &
              Put in the data*/
    var new_node = new Node(new_data);

    /* 3. Make next of new Node as head */
    new_node.next = head;

    /* 4. Move the head to point to new Node */
    head = new_node;
}

// This code contributed by Rajput-Ji 

</script>

Time Complexity: O(1)
Auxiliary Space: O(1)

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