Java Program For Inserting A Node In A Linked List

Last Updated : 01 Sep, 2022

We have introduced Linked Lists in the previous post. We also created a simple linked list with 3 nodes and discussed linked list traversal.
All programs discussed in this post consider the following representations of the linked list.

Java

// Linked List Class
class LinkedList
{
    // Head of list
    Node head;  
 
    // Node Class 
    class Node
    {
        int data;
        Node next;
          
        // Constructor to create
        // a new node
        Node(int d) 
        {
            data = d; 
            next = null; 
        }
    }
}

In this post, methods to insert a new node in linked list are discussed. A node can be added in three ways
1) At the front of the linked list
2) After a given node.
3) At the end of the linked list.

Add a node at the front: (4 steps process)
The new node is always added before the head of the given Linked List. And newly added node becomes the new head of the Linked List. For example, if the given Linked List is 10->15->20->25 and we add an item 5 at the front, then the Linked List becomes 5->10->15->20->25. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer, because push must change the head pointer to point to the new node (See this)

Following are the 4 steps to add a node at the front.

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;
}

Time complexity of push() is O(1) as it does a constant amount of work.
Add a node after a given node: (5 steps process)
We are given a pointer to a node, and the new node is inserted after the given node.

Java

/* This function is in LinkedList class. 
   Inserts a new node after the given 
   prev_node. This method is defined 
   inside LinkedList class shown above */
public void insertAfter(Node prev_node, 
                        int new_data) 
{ 
    // 1. Check if the given Node is null 
    if (prev_node == null) 
    { 
        System.out.println(
               "The given previous node cannot be null"); 
        return; 
    } 
 
    /* 2. Allocate the Node & 
       3. Put in the data*/
    Node new_node = new Node(new_data); 
 
    // 4. Make next of new Node as next
    // of prev_node 
    new_node.next = prev_node.next; 
 
    // 5. make next of prev_node as new_node 
    prev_node.next = new_node; 
}

Time complexity of insertAfter() is O(1) as it does a constant amount of work.

Add a node at the end: (6 steps process)
The new node is always added after the last node of the given Linked List. For example if the given Linked List is 5->10->15->20->25 and we add an item 30 at the end, then the Linked List becomes 5->10->15->20->25->30.
Since a Linked List is typically represented by the head of it, we have to traverse the list till the end and then change the next to last node to a new node.

Following are the 6 steps to add node at the end.

Java

/* Appends a new node at the end.  This method is 
   defined inside LinkedList class shown above */
public void append(int new_data)
{
    /* 1. Allocate the Node &
       2. Put in the data
       3. Set next as null */
    Node new_node = new Node(new_data);
 
    /* 4. If the Linked List is empty, then 
          make the new node as head */
    if (head == null)
    {
        head = new Node(new_data);
        return;
    }
 
    /* 4. This new node is going to be the 
          last node, so make next of it 
          as null */
    new_node.next = null;
 
    // 5. Else traverse till the last node 
    Node last = head; 
    while (last.next != null)
        last = last.next;
 
    // 6. Change the next of last node 
    last.next = new_node;
    return;
}

Time complexity of append is O(n) where n is the number of nodes in the linked list. Since there is a loop from head to end, the function does O(n) work.
This method can also be optimized to work in O(1) by keeping an extra pointer to the tail of the linked list/

Following is a complete program that uses all of the above methods to create a linked list.

Java

// A complete working Java program to 
// demonstrate all insertion methods
// on linked list
class LinkedList
{
    // head of list
    Node head;  
 
    // Linked list Node
    class Node
    {
        int data;
        Node next;
        Node(int d) 
        {
            data = d; 
            next = null; 
        }
    }
 
    // 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;
    }
 
    // Inserts a new node after the 
    // given prev_node. 
    public void insertAfter(Node prev_node, 
                            int new_data)
    {
        // 1. Check if the given Node is null 
        if (prev_node == null)
        {
            System.out.println(
                   "The given previous node cannot be null");
            return;
        }
 
        /* 2 & 3: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);
 
        // 4. Make next of new Node as next 
        // of prev_node 
        new_node.next = prev_node.next;
 
        // 5. make next of prev_node as 
        // new_node 
        prev_node.next = new_node;
    }
    
    /* Appends a new node at the end. 
       This method is defined inside 
       LinkedList class shown above */
    public void append(int new_data)
    {
        /* 1. Allocate the Node &
           2. Put in the data
           3. Set next as null */
        Node new_node = new Node(new_data);
 
        /* 4. If the Linked List is empty, 
              then make the new node as head */
        if (head == null)
        {
            head = new Node(new_data);
            return;
        }
 
        /* 4. This new node is going to be 
              the last node, so make next 
              of it as null */
        new_node.next = null;
 
        // 5. Else traverse till the last node 
        Node last = head; 
        while (last.next != null)
            last = last.next;
 
        // 6. Change the next of last node 
        last.next = new_node;
        return;
    }
 
    /* This function prints contents of 
       linked list starting from the 
       given node */
    public void printList()
    {
        Node tnode = head;
        while (tnode != null)
        {
            System.out.print(tnode.data + " ");
            tnode = tnode.next;
        }
    }
 
    // Driver code
    public static void main(String[] args)
    {
        // Start with the empty list 
        LinkedList llist = new LinkedList();
 
        // Insert 6. So linked list 
        // becomes 6->NUllist
        llist.append(6);
 
        // Insert 7 at the beginning. 
        // So linked list becomes 
        // 7->6->NUllist
        llist.push(7);
 
        // Insert 1 at the beginning. 
        // So linked list becomes
        // 1->7->6->NUllist
        llist.push(1);
 
        // Insert 4 at the end. 
        // So linked list becomes
        // 1->7->6->4->NUllist
        llist.append(4);
 
        // Insert 8, after 7. So linked 
        // list becomes
        // 1->7->8->6->4->NUllist
        llist.insertAfter(llist.head.next, 8);
 
        System.out.println(
               "Created Linked list is: ");
        llist.printList();
    }
}
// This code is contributed by Rajat Mishra

Output:

 Created Linked list is:  1  7  8  6  4

Time complexity: O(N) where N is size of given linked list

Auxiliary space: O(1) constant space is being used

Please refer complete article on Linked List | Set 2 (Inserting a node) for more details!

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