Skip to content
Related Articles

Related Articles

Improve Article

Reverse a Linked List in groups of given size | Set 2

  • Difficulty Level : Medium
  • Last Updated : 22 Oct, 2021

Given a linked list, write a function to reverse every k nodes (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 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. 
 

C++




// C++ program to reverse a linked list in groups
// of given size
#include <bits/stdc++.h>
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<Node*> 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 off 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<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);
}
}
 
// 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 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)
    {
        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




<script>
// javascript program to reverse a linked list in groups
// of given size class GfG {
 
    /* Link list node */
class Node {
    constructor() {
        this.data = 0;
        this.next = null;
    }
}
 
    var head = null;
 
    /*
     * Reverses the linked list in groups of size k and returns the pointer to the
     * new head node.
     */
    function Reverse(head , k) {
        // Create a stack of Node*
        var mystack = [];
var current = head;
var prev = null;
 
        while (current != null) {
 
            // Terminate the loop whichever comes first
            // either current == NULL or count >= k
            var 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.length > 0) {
 
                // If final list has not been started yet.
                if (prev == null) {
                    prev = mystack.pop();
                    head = prev;
                     
                } else {
                    prev.next = mystack.pop();
                    prev = prev.next;
                     
                }
            }
        }
 
        // Next of last element will point to NULL.
        prev.next = null;
 
        return head;
    }
 
    /* UTILITY FUNCTIONS */
    /* Function to push a node */
    function push(new_data) {
        /* allocate node */
var 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 */
    function printList(node) {
        while (node != null) {
            document.write(node.data + " ");
            node = node.next;
        }
    }
 
    /* Driver code */
     
        /* 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);
 
        document.write("Given linked list <br/>");
        printList(head);
        head = Reverse(head, 3);
        document.write("<br/>");
 
        document.write("Reversed Linked list <br/>");
        printList(head);
 
// This code contributed by aashish1995
</script>


Output: 
 

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

 



This article is contributed by Jatin Goyal. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

In case you wish to attend live classes with experts, please refer DSA Live Classes for Working Professionals and Competitive Programming Live for Students.




My Personal Notes arrow_drop_up
Recommended Articles
Page :