Javascript Program For Inserting A Node After The N-th Node From The End

Insert a node x after the nth node from the end in the given singly linked list. It is guaranteed that the list contains the nth node from the end. Also 1 <= n.

Examples: 

Input : list: 1->3->4->5
        n = 4, x = 2
Output : 1->2->3->4->5
4th node from the end is 1 and
insertion has been done after this node.

Input : list: 10->8->3->12->5->18
        n = 2, x = 11
Output : 10->8->3->12->5->11->18

Method 1 (Using length of the list):
Find the length of the linked list, i.e, the number of nodes in the list. Let it be len. Now traverse the list from the 1st node upto the (len-n+1)th node from the beginning and insert the new node after this node. This method requires two traversals of the list. 

Javascript

<script>

// JavaScript implementation to
// insert a node after
// the n-th node from the end

    // structure of a node
     class Node {
            constructor() {
                this.data = 0;
                this.next = null;
            }
        }

    // function to get a new node
    function getNode(data) {
        // allocate memory for the node
var newNode = new Node();

        // put in the data
        newNode.data = data;
        newNode.next = null;
        return newNode;
    }

    // function to insert a node after the
    // nth node from the end
    function insertAfterNthNode(head , n , x) {
        // if list is empty
        if (head == null)
            return;

        // get a new node for the value 'x'
var newNode = getNode(x);
var ptr = head;
        var len = 0, i;

        // find length of the list, i.e, the
        // number of nodes in the list
        while (ptr != null) {
            len++;
            ptr = ptr.next;
        }

        // traverse up to the nth node from the end
        ptr = head;
        for (i = 1; i <= (len - n); i++)
            ptr = ptr.next;

        // insert the 'newNode' by making the
        // necessary adjustment in the links
        newNode.next = ptr.next;
        ptr.next = newNode;
    }

    // function to print the list
    function printList(head) {
        while (head != null) {
            document.write(head.data + " ");
            head = head.next;
        }
    }

    // Driver code

        // Creating list 1->3->4->5
var head = getNode(1);
        head.next = getNode(3);
        head.next.next = getNode(4);
        head.next.next.next = getNode(5);

        var n = 4, x = 2;

        document.write("Original Linked List: ");
        printList(head);

        insertAfterNthNode(head, n, x);
        document.write();
        document.write("<br/>Linked List After Insertion: ");
        printList(head);

// This code contributed by gauravrajput1

</script>

Output:

Original Linked List: 1 3 4 5
Linked List After Insertion: 1 2 3 4 5

Time Complexity: O(n), where n is the number of nodes in the list.

Auxiliary space: O(1) because using constant variables

Method 2 (Single traversal):
This method uses two pointers, one is slow_ptr and the other is fast_ptr. First move the fast_ptr up to the nth node from the beginning. Make the slow_ptr point to the 1st node of the list. Now, simultaneously move both the pointers until fast_ptr points to the last node. At this point the slow_ptr will be pointing to the nth node from the end. Insert the new node after this node. This method requires single traversal of the list.

Javascript

<script>

      // JavaScript implementation to
      // insert a node after the
      // nth node from the end
      // structure of a node
      class Node {
        constructor() {
          this.data = 0;
          this.next = null;
        }
      }

      // function to get a new node
      function getNode(data) {
        // allocate memory for the node
        var newNode = new Node();

        // put in the data
        newNode.data = data;
        newNode.next = null;
        return newNode;
      }

      // function to insert a node after
      // the nth node from the end
      function insertAfterNthNode(head, n, x) {
        // if list is empty
        if (head == null) return;

        // get a new node for the value 'x'
        var newNode = getNode(x);

        // Initializing the slow
        // and fast pointers
        var slow_ptr = head;
        var fast_ptr = head;

        // move 'fast_ptr' to point to the
        // nth node from the beginning
        for (var i = 1; i <= n - 1; i++)
        fast_ptr = fast_ptr.next;

        // iterate until 'fast_ptr' points
        // to the last node
        while (fast_ptr.next != null) {
          // move both the pointers to the
          // respective next nodes
          slow_ptr = slow_ptr.next;
          fast_ptr = fast_ptr.next;
        }

        // insert the 'newNode' by making the
        // necessary adjustment in the links
        newNode.next = slow_ptr.next;
        slow_ptr.next = newNode;
      }

      // function to print the list
      function printList(head) {
        while (head != null) {
          document.write(head.data + " ");
          head = head.next;
        }
      }

      // Driver code
      // Creating list 1->3->4->5
      var head = getNode(1);
      head.next = getNode(3);
      head.next.next = getNode(4);
      head.next.next.next = getNode(5);

      var n = 4,
        x = 2;
      document.write("Original Linked List: ");
      printList(head);

      insertAfterNthNode(head, n, x);
      document.write("<br>");
      document.write("Linked List After Insertion:");
      printList(head);

</script>

Output:

Original Linked List: 1 3 4 5
Linked List After Insertion: 1 2 3 4 5

Time Complexity: O(n), where n is the number of nodes in the list.

Space Complexity: O(1) using constant space 

Please refer complete article on Insert a node after the n-th node from the end for more details!