Reverse a Linked List in groups of given size Java

PROGRAM TO REVERSE A LINKED LIST IN GROUPS OF GIVEN SIZE

import java.util.*;

class MAIN

{

      

/* 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 off 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); 

} 

} 




OUTPUT

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

CREDITS: https://www.geeksforgeeks.org/reverse-linked-list-groups-given-size-set-2/