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

// 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