Insert a node at the head of a linked list: Program and Algorithm

In this article, you’ll learn how to insert a node at the head of a linked list in C++, along with that you’ll also see the algorithm to insert a node at the head of a linked list.

So, Let’s get started!

Prerequisite: You should have the knowledge of How do you create a node in a linked list? and How to traverse a singly linked list?

How do you insert a node at the head of a linked list?

Let’s see how you can insert a node at the head of a linked list step-by-step:

Variables we are using here:

head: A pointer of Node type pointing to the first node of the linked list.
temp: The node to be inserted (It will also be of Node type).

The following algorithm inserts a node at the head of a linked list:

Algorithm to insert a node at the head of a linked list:

Step 01: Start
Step 02: Create a new node with the given data.
Step 03: If head == NULL
- Step 04: Set head = temp
- Step 05: Go to step 11
Step 06: [End of If ]
Step 07: Else
- Step 08: Set temp → next = head
- Step 09: Set head = temp
Step 10: [End of Else block. ]
Step 11: Stop

Visual Representation:

Insert a node at the head of a linked list

Implementation in C++:

Case 01: If the linked list is empty.

// writing a program in C++ to insert a node at the head of a linked list

#include<iostream>
using namespace std;

// creating a class "Node"
class Node
{
    public:
            int data;
            Node *next;

    //constructor
    Node(int data)
    {
        this -> data = data;
        this -> next = NULL;
    }
};


// Traversing a linked list
void TraverseList(Node* &head)
{
    if(head == NULL)
        cout << "List is empty" << endl;
    else
    {
        Node* temp = head;
        while(temp != NULL)
        {
            cout << temp -> data << " -> " ;
            temp = temp -> next;
        }
        cout << "Null" << endl;
    }
}


// function to insert a node at the head of a linked list
void insertAtHead(Node* &head, int data)
{
    //new node creation
    Node* temp = new Node(data);
    
    //if list is empty
    if(head == NULL)
        
        head = temp;
    
    else
    {
        temp -> next = head;
        head = temp;
    }

}


//Driver's code

int main()
{
    // if list is empty
    
    Node* head = NULL;
    insertAtHead(head, 786);
    TraverseList(head);

    return 0;


}

If you compile and run the above program, it will produce the following result:

Output:

786 -> Null

Case 02: If the linked list is not empty.

// writing a program in C++ to insert a node at the head of a linked list

#include<iostream>
using namespace std;

// creating a class "Node"
class Node
{
    public:
            int data;
            Node *next;

    //constructor
    Node(int data)
    {
        this -> data = data;
        this -> next = NULL;
    }
};


// Traversing a linked list
void TraverseList(Node* &head)
{
    if(head == NULL)
        cout << "List is empty" << endl;
    else
    {
        Node* temp = head;
        while(temp != NULL)
        {
            cout << temp -> data << " -> " ;
            temp = temp -> next;
        }
        cout << "Null" << endl;
    }
}


// function to insert a node at the head of a linked list
void insertAtHead(Node* &head, int data)
{
    //new node creation
    Node* temp = new Node(data);
    
    //if list is empty
    if(head == NULL)
        
        head = temp;
    
    else
    {
        temp -> next = head;
        head = temp;
    }

}


//Driver's code

int main()
{
    // if list is not empty (As you can see, initially this linked list contains a single node i.e., node1)
    
    Node* node1 = new Node(100);
    Node* head = node1;
    insertAtHead(head, 786);
    TraverseList(head);

    return 0;


}

If you compile and run the above program, it will produce the following result:

Output:

786 -> 100 -> Null

Conclusion:

In this article, you learned how to insert a node at the head of a linked list in C++, you also became familiar with the algorithm to insert a node at the head of a linked list.

So,

I really hope that you liked my article and got a lot of value from it.

Now, It’s your turn!

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