Java Program For Reversing A Linked List In Groups Of Given Size- Set 2

Given a linked list, write a function to reverse every k nodes (where k is an input to the function). 
Examples:

Input: 1->2->3->4->5->6->7->8->NULL and k = 3 
Output: 3->2->1->6->5->4->8->7->NULL. 

Input: 1->2->3->4->5->6->7->8->NULL and k = 5
Output: 5->4->3->2->1->8->7->6->NULL.

We have already discussed its solution in below post 
Reverse a Linked List in groups of given size | Set 1
In this post, we have used a stack which will store the nodes of the given linked list. Firstly, push the k elements of the linked list in the stack. Now pop elements one by one and keep track of the previously popped node. Point the next pointer of prev node to top element of stack. Repeat this process, until NULL is reached.
This algorithm uses O(k) extra space.

Java

// Java program to reverse a linked list
// in groups of given size
import java.util.*;
class GfG
{
    // Link list node
    static class Node
    {
        int data;
        Node next;
    }
    static Node head = null;

    /* Reverses the linked list in groups
       of size k and returns the pointer
       to the new head node. */
    static Node Reverse(Node head, int k)
    {
        // Create a stack of Node*
        Stack<Node> mystack =
                    new Stack<Node> ();
        Node current = head;
        Node prev = null;

        while (current != null)
        {
            // Terminate the loop whichever
            // comes first either current == NULL
            // or count >= k
            int count = 0;
            while (current != null && count < k)
            {
                mystack.push(current);
                current = current.next;
                count++;
            }

            // Now pop the elements of stack
            // one by one
            while (mystack.size() > 0)
            {
                // If final list has not been
                // started yet.
                if (prev == null)
                {
                    prev = mystack.peek();
                    head = prev;
                    mystack.pop();
                }
                else
                {
                    prev.next = mystack.peek();
                    prev = prev.next;
                    mystack.pop();
                }
            }
        }

        // Next of last element will point
        // to NULL.
        prev.next = null;

        return head;
    }

    // UTILITY FUNCTIONS
    // Function to push a node
    static void push( int new_data)
    {
        // Allocate node
        Node new_node = new Node();

        // Put in the data
        new_node.data = new_data;

        // Link the old list of the
        // new node
        new_node.next = head;

        // Move the head to point to
        // the new node
        head = new_node;
    }

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

    // Driver code
    public static void main(String[] args)
    {
        // Start with the empty list
        //Node head = null;

        /* Created Linked list is
           1->2->3->4->5->6->7->8->9 */
        push( 9);
        push( 8);
        push( 7);
        push( 6);
        push( 5);
        push(4);
        push(3);
        push(2);
        push( 1);

        System.out.println(
               "Given linked list ");
        printList(head);
        head = Reverse(head, 3);
        System.out.println();

        System.out.println(
               "Reversed Linked list ");
        printList(head);
    }
}
// This code is contributed by Prerna Saini

Output: 

Given Linked List
1 2 3 4 5 6 7 8 9 
Reversed list
3 2 1 6 5 4 9 8 7

Please refer complete article on Reverse a Linked List in groups of given size | Set 2 for more details!