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

• Last Updated : 11 Jan, 2022

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 mystack = ``                    ``new` `Stack (); ``        ``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); ``    ``} ``} ``// 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!

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