Java Program To Delete Alternate Nodes Of A Linked List

Given a Singly Linked List, starting from the second node delete all alternate nodes of it. For example, if the given linked list is 1->2->3->4->5 then your function should convert it to 1->3->5, and if the given linked list is 1->2->3->4 then convert it to 1->3.

Method 1 (Iterative):
Keep track of previous of the node to be deleted. First, change the next link of the previous node and iteratively move to the next node.

Java

// Java program to delete alternate
// nodes of a 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;
        }
    }

    void deleteAlt()
    {
       if (head == null)
          return;

       Node prev = head;
       Node now = head.next;

       while (prev != null &&
              now != null)
       {
           // Change next link of previous node
           prev.next = now.next;

           // Free node
           now = null;

           // Update prev and now
           prev = prev.next;
           if (prev != null)
              now = prev.next;
       }
    }


    // Utility functions
    // 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;
    }

    // Function to print linked list
    void printList()
    {
        Node temp = head;
        while(temp != null)
        {
           System.out.print(temp.data + " ");
           temp = temp.next;
        }
        System.out.println();
    }

     // Driver code
    public static void main(String args[])
    {
        LinkedList llist = new LinkedList();

        /* Constructed Linked List is
           1->2->3->4->5->null */
        llist.push(5);
        llist.push(4);
        llist.push(3);
        llist.push(2);
        llist.push(1);

        System.out.println(
               "Linked List before calling deleteAlt() ");
        llist.printList();

        llist.deleteAlt();

        System.out.println(
               "Linked List after calling deleteAlt() ");
        llist.printList();
    }
}
// This code is contributed by Rajat Mishra

Output: 

List before calling deleteAlt() 
1 2 3 4 5 
List after calling deleteAlt() 
1 3 5 

Time Complexity: O(n) where n is the number of nodes in the given Linked List.

Auxiliary Space: O(1) because it is using constant space

Method 2 (Recursive):
Recursive code uses the same approach as method 1. The recursive code is simple and short but causes O(n) recursive function calls for a linked list of size n.

Java

/* Deletes alternate nodes of
   a list starting with head */
static Node deleteAlt(Node head)
{
    if (head == null)
        return;

    Node node = head.next;

    if (node == null)
        return;

    // Change the next link of head
    head.next = node.next;


    /* Recursively call for the new
       next of head */
    head.next = deleteAlt(head.next);
}
// This code is contributed by Arnab Kundu

Time Complexity: O(n)

Auxiliary space: O(n) for call stack because using recursion

Please refer complete article on Delete alternate nodes of a Linked List for more details!