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# Reverse a Linked List in groups of given size using Stack

• Difficulty Level : Medium
• Last Updated : 10 Jan, 2023

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

Examples:

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

Inputs:   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 the below post
Reverse a Linked List in groups of given size | Set 1
In this post, we have used a stack that 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 the prev node to the top element of the stack. Repeat this process, until NULL is reached.
This algorithm uses O(k) extra space.

Implementation:

## C++

 `// C++ program to reverse a linked list in groups``// of given size``#include ``using` `namespace` `std;`` ` `/* Link list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};`` ` `/* Reverses the linked list in groups of size k``   ``and returns the pointer to the new head node. */``struct` `Node* Reverse(``struct` `Node* head, ``int` `k)``{``    ``// Create a stack of Node*``    ``stack mystack;``    ``struct` `Node* current = head;``    ``struct` `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.top();``                ``head = prev;``                ``mystack.pop();``            ``} ``else` `{``                ``prev->next = mystack.top();``                ``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 */``void` `push(``struct` `Node** head_ref, ``int` `new_data)``{``    ``/* allocate node */``    ``struct` `Node* new_node = ``          ``(``struct` `Node*)``malloc``(``sizeof``(``struct` `Node));`` ` `    ``/* put in the data  */``    ``new_node->data = new_data;`` ` `    ``/* link the old list of the new node */``    ``new_node->next = (*head_ref);`` ` `    ``/* move the head to point to the new node */``    ``(*head_ref) = new_node;``}`` ` `/* Function to print linked list */``void` `printList(``struct` `Node* node)``{``    ``while` `(node != NULL) {``        ``printf``(``"%d  "``, node->data);``        ``node = node->next;``    ``}``}`` ` `/* Driver program to test above function*/``int` `main(``void``)``{``    ``/* Start with the empty list */``    ``struct` `Node* head = NULL;`` ` `    ``/* Created Linked list is 1->2->3->4->5->6->7->8->9 */``    ``push(&head, 9);``    ``push(&head, 8);``    ``push(&head, 7);``    ``push(&head, 6);``    ``push(&head, 5);``    ``push(&head, 4);``    ``push(&head, 3);``    ``push(&head, 2);``    ``push(&head, 1);`` ` `    ``printf``(``"\nGiven linked list \n"``);``    ``printList(head);``    ``head = Reverse(head, 3);`` ` `    ``printf``(``"\nReversed Linked list \n"``);``    ``printList(head);`` ` `    ``return` `0;``}`

## 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 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`

## Python3

 `# Python3 program to reverse a Linked List``# in groups of given size`` ` `# Node class``class` `Node(``object``):``    ``__slots__ ``=` `'data'``, ``'next'`` ` `    ``# Constructor to initialize the node object``    ``def` `__init__(``self``, data ``=` `None``, ``next` `=` `None``):``        ``self``.data ``=` `data``        ``self``.``next` `=` `next`` ` `    ``def` `__repr__(``self``):``        ``return` `repr``(``self``.data)`` ` `class` `LinkedList(``object``):`` ` `    ``# Function to initialize head``    ``def` `__init__(``self``):``        ``self``.head ``=` `None`` ` `    ``# Utility function to print nodes ``    ``# of LinkedList``    ``def` `__repr__(``self``):``        ``nodes ``=` `[]``        ``curr ``=` `self``.head``        ``while` `curr:``            ``nodes.append(``repr``(curr))``            ``curr ``=` `curr.``next``        ``return` `'['` `+` `', '``.join(nodes) ``+` `']'`` ` `    ``# Function to insert a new node at``    ``# the beginning``    ``def` `prepend(``self``, data):``        ``self``.head ``=` `Node(data ``=` `data,``                         ``next` `=` `self``.head)`` ` `    ``# Reverses the linked list in groups of size k``    ``# and returns the pointer to the new head node.``    ``def` `reverse(``self``, k ``=` `1``):``        ``if` `self``.head ``is` `None``:``            ``return`` ` `        ``curr ``=` `self``.head``        ``prev ``=` `None``        ``new_stack ``=` `[]``        ``while` `curr ``is` `not` `None``:``            ``val ``=` `0``             ` `            ``# Terminate the loop whichever comes first``            ``# either current == None or value >= k``            ``while` `curr ``is` `not` `None` `and` `val < k:``                ``new_stack.append(curr.data)``                ``curr ``=` `curr.``next``                ``val ``+``=` `1`` ` `            ``# Now pop the elements of stack one by one``            ``while` `new_stack:``                 ` `                ``# If final list has not been started yet.``                ``if` `prev ``is` `None``:``                    ``prev ``=` `Node(new_stack.pop())``                    ``self``.head ``=` `prev``                ``else``:``                    ``prev.``next` `=` `Node(new_stack.pop())``                    ``prev ``=` `prev.``next``                     ` `        ``# Next of last element will point to None.``        ``prev.``next` `=` `None``        ``return` `self``.head`` ` `# Driver Code``llist ``=` `LinkedList() ``llist.prepend(``9``)``llist.prepend(``8``)``llist.prepend(``7``)``llist.prepend(``6``)``llist.prepend(``5``)``llist.prepend(``4``)``llist.prepend(``3``)``llist.prepend(``2``)``llist.prepend(``1``)`` ` `print``(``"Given linked list"``)``print``(llist)``llist.head ``=` `llist.reverse(``3``)`` ` `print``(``"Reversed Linked list"``)``print``(llist)`` ` `# This code is contributed by ``# Sagar Kumar Sinha(sagarsinha7777)`

## C#

 `// C# program to reverse a linked list  ``// in groups of given size ``using` `System;``using` `System.Collections;`` ` `class` `GfG ``{ ``     ` `/* Link list node */``public` `class` `Node ``{ ``    ``public` `int` `data; ``    ``public` `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.Count > 0) ``        ``{ `` ` `            ``// If final list has not been started yet. ``            ``if` `(prev == ``null``) ``            ``{ ``                ``prev = (Node)mystack.Peek(); ``                ``head = prev; ``                ``mystack.Pop(); ``            ``} ``            ``else``            ``{ ``                ``prev.next = (Node)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``) ``    ``{ ``        ``Console.Write(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); `` ` `    ``Console.WriteLine(``"Given linked list "``); ``    ``printList(head); ``    ``head = Reverse(head, 3); ``    ``Console.WriteLine(); `` ` `    ``Console.WriteLine(``"Reversed Linked list "``); ``    ``printList(head); ``} ``} `` ` `// This code is contributed by Arnab Kundu`

## Javascript

 ``

Output

```Given linked list
1  2  3  4  5  6  7  8  9