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Javascript Program For Inserting A Node In A Linked List

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  • Last Updated : 27 Dec, 2021

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. 

Javascript




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

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)

linkedlist_insert_at_start

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

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);
  
    // 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 is contributed by Rajput-Ji 
</script>

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.

linkedlist_insert_middle

Javascript




<script>
/* This function is in LinkedList class. 
   Inserts a new node after the given prev_node. 
   This method is defined inside LinkedList 
   class shown above */
function insertAfter(prev_node , new_data) 
    // 1. Check if the given Node is null 
    if (prev_node == null
    
        document.write(
                 "The given previous node cannot be null"); 
        return
    
  
    /* 2. Allocate the Node & 
       3. Put in the data*/
    var 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; 
}
// This code is contributed by aashish1995
</script>

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.

linkedlist_insert_last

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

Javascript




<script>
/* Appends a new node at the end. This method is 
   defined inside LinkedList class shown above */
 function append(new_data)
{
    /* 1. Allocate the Node &
       2. Put in the data
       3. Set next as null */
    var 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 
    var last = head; 
    while (last.next != null)
        last = last.next;
  
    // 6. Change the next of last node 
    last.next = new_node;
    return;
}
// This code is contributed by aashish1995
</script>

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.

Javascript




<script>
// A complete working javascript program 
// to demonstrate all insertion methods
// on linked list
  
// Head of list
var head; 
  
// Linked list Node 
class Node 
{
    constructor(val) 
    {
        this.data = val;
        this.next = null;
    }
}
  
// Inserts a new Node at front 
// of the list. 
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;
}
  
// Inserts a new node after the given 
// prev_node. 
function insertAfter(prev_node, 
                     new_data) 
{
    // 1. Check if the given Node is null 
    if (prev_node == null
    {
        document.write(
                 "The given previous node cannot be null");
        return;
    }
  
    /* 2 & 3: Allocate the Node & 
              Put in the data */
    var 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 */
function append(new_data) 
{
    /* 1. Allocate the Node & 
       2. Put in the data 
       3. Set next as null */
    var 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 
    var 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 */
function printList() 
{
    var tnode = head;
    while (tnode != null
    {
        document.write(tnode.data + " ");
        tnode = tnode.next;
    }
}
  
// Driver code
  
// Start with the empty list 
  
// Insert 6. So linked list becomes 
// 6->NUllist
append(6);
  
// Insert 7 at the beginning. So 
// linked list becomes 7->6->NUllist
push(7);
  
// Insert 1 at the beginning. So 
// linked list becomes 1->7->6->NUllist
push(1);
  
// Insert 4 at the end. So linked list 
// becomes 1->7->6->4->NUllist
append(4);
  
// Insert 8, after 7. So linked list 
// becomes 1->7->8->6->4->NUllist
insertAfter(head.next, 8);
  
document.write(
         "Created Linked list is: ");
printList();
// This code is contributed by gauravrajput1
</script>

Output:

 Created Linked list is:  1  7  8  6  4

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


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