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Reverse alternate K nodes in a Singly Linked List
• Difficulty Level : Medium
• Last Updated : 19 May, 2021

Given a linked list, write a function to reverse every alternate k nodes (where k is an input to the function) in an efficient way. Give the complexity of your algorithm.
Example:

```Inputs:   1->2->3->4->5->6->7->8->9->NULL and k = 3
Output:   3->2->1->4->5->6->9->8->7->NULL. ```

Method 1 (Process 2k nodes and recursively call for rest of the list)
This method is basically an extension of the method discussed in this post.

```kAltReverse(struct node *head, int k)
1)  Reverse first k nodes.
2)  In the modified list head points to the kth node.  So change next
of head to (k+1)th node
3)  Move the current pointer to skip next k nodes.
4)  Call the kAltReverse() recursively for rest of the n - 2k nodes.
5)  Return new head of the list.```

## C++

 `// C++ program to reverse alternate``// k nodes in a linked list``#include ``using` `namespace` `std;` `/* Link list node */``class` `Node``{``    ``public``:``    ``int` `data;``    ``Node* next;``};` `/* Reverses alternate k nodes and``returns the pointer to the new head node */``Node *kAltReverse(Node *head, ``int` `k)``{``    ``Node* current = head;``    ``Node* next;``    ``Node* prev = NULL;``    ``int` `count = 0;` `    ``/*1) reverse first k nodes of the linked list */``    ``while` `(current != NULL && count < k)``    ``{``    ``next = current->next;``    ``current->next = prev;``    ``prev = current;``    ``current = next;``    ``count++;``    ``}``    ` `    ``/* 2) Now head points to the kth node.``    ``So change next  of head to (k+1)th node*/``    ``if``(head != NULL)``    ``head->next = current;` `    ``/* 3) We do not want to reverse next k``       ``nodes. So move the current``        ``pointer to skip next k nodes */``    ``count = 0;``    ``while``(count < k-1 && current != NULL )``    ``{``    ``current = current->next;``    ``count++;``    ``}` `    ``/* 4) Recursively call for the list``    ``starting from current->next. And make``    ``rest of the list as next of first node */``    ``if``(current != NULL)``    ``current->next = kAltReverse(current->next, k);` `    ``/* 5) prev is new head of the input list */``    ``return` `prev;``}` `/* UTILITY FUNCTIONS */``/* Function to push a node */``void` `push(Node** head_ref, ``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_ref);    ` `    ``/* move the head to point to the new node */``    ``(*head_ref) = new_node;``}` `/* Function to print linked list */``void` `printList(Node *node)``{``    ``int` `count = 0;``    ``while``(node != NULL)``    ``{``        ``cout<data<<``" "``;``        ``node = node->next;``        ``count++;``    ``}``}` `/* Driver code*/``int` `main(``void``)``{``    ``/* Start with the empty list */``    ``Node* head = NULL;``    ``int` `i;``    ` `    ``// create a list 1->2->3->4->5...... ->20``    ``for``(i = 20; i > 0; i--)``    ``push(&head, i);` `    ``cout<<``"Given linked list \n"``;``    ``printList(head);``    ``head = kAltReverse(head, 3);` `    ``cout<<``"\n Modified Linked list \n"``;``    ``printList(head);``    ``return``(0);``}`  `// This code is contributed by rathbhupendra`

C]

``````
// Java program to reverse alternate k nodes in a linked list

class LinkedList {

static Node head;

class Node {

int data;
Node next;

Node(int d) {
data = d;
next = null;
}
}

/* Reverses alternate k nodes and
returns the pointer to the new head node */
Node kAltReverse(Node node, int k) {
Node current = node;
Node next = null, prev = null;
int count = 0;

/*1) reverse first k nodes of the linked list */
while (current != null && count < k) {
next = current.next;
current.next = prev;
prev = current;
current = next;
count++;
}

/* 2) Now head points to the kth node.  So change next
of head to (k+1)th node*/
if (node != null) {
node.next = current;
}

/* 3) We do not want to reverse next k nodes. So move the current
pointer to skip next k nodes */
count = 0;
while (count < k - 1 && current != null) {
current = current.next;
count++;
}

/* 4) Recursively call for the list starting from current->next.
And make rest of the list as next of first node */
if (current != null) {
current.next = kAltReverse(current.next, k);
}

/* 5) prev is new head of the input list */
return prev;
}

void printList(Node node) {
while (node != null) {
System.out.print(node.data + " ");
node = node.next;
}
}

void push(int newdata) {
Node mynode = new Node(newdata);
mynode.next = head;
head = mynode;
}

public static void main(String[] args) {
LinkedList list = new LinkedList();

// Creating the linkedlist
for (int i = 20; i > 0; i--) {
list.push(i);
}
System.out.println("Given Linked List :");
list.printList(head);
head = list.kAltReverse(head, 3);
System.out.println("");
System.out.println("Modified Linked List :");
list.printList(head);

}
}

// This code has been contributed by Mayank Jaiswal

``````

## Python3

 `# Python3 program to reverse alternate``# k nodes in a linked list``import` `math` `# Link list node``class` `Node:``    ``def` `__init__(``self``, data):``        ``self``.data ``=` `data``        ``self``.``next` `=` `None` `# Reverses alternate k nodes and``#returns the poer to the new head node``def` `kAltReverse(head, k) :``    ``current ``=` `head``    ``next` `=` `None``    ``prev ``=` `None``    ``count ``=` `0` `    ``#1) reverse first k nodes of the linked list``    ``while` `(current !``=` `None` `and` `count < k) :``        ``next` `=` `current.``next``        ``current.``next` `=` `prev``        ``prev ``=` `current``        ``current ``=` `next``        ``count ``=` `count ``+` `1``;``    ` `    ``# 2) Now head pos to the kth node.``    ``# So change next of head to (k+1)th node``    ``if``(head !``=` `None``):``        ``head.``next` `=` `current` `    ``# 3) We do not want to reverse next k``    ``# nodes. So move the current``    ``# poer to skip next k nodes``    ``count ``=` `0``    ``while``(count < k ``-` `1` `and` `current !``=` `None` `):``        ``current ``=` `current.``next``        ``count ``=` `count ``+` `1``    ` `    ``# 4) Recursively call for the list``    ``# starting from current.next. And make``    ``# rest of the list as next of first node``    ``if``(current !``=` `None``):``        ``current.``next` `=` `kAltReverse(current.``next``, k)` `    ``# 5) prev is new head of the input list``    ``return` `prev` `# UTILITY FUNCTIONS``# Function to push a node``def` `push(head_ref, new_data):``    ` `    ``# allocate node``    ``new_node ``=` `Node(new_data)` `    ``# 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 po to the new node``    ``head_ref ``=` `new_node``    ` `    ``return` `head_ref` `# Function to print linked list``def` `prList(node):``    ``count ``=` `0``    ``while``(node !``=` `None``):``        ``print``(node.data, end ``=` `" "``)``        ``node ``=` `node.``next``        ``count ``=` `count ``+` `1``    ` `# Driver code``if` `__name__``=``=``'__main__'``:``    ` `    ``# Start with the empty list``    ``head ``=` `None` `    ``# create a list 1.2.3.4.5...... .20``    ``for` `i ``in` `range``(``20``, ``0``, ``-``1``):``        ``head ``=` `push(head, i)``        ` `    ``print``(``"Given linked list "``)``    ``prList(head)``    ``head ``=` `kAltReverse(head, ``3``)` `    ``print``(``"\nModified Linked list"``)``    ``prList(head)``    ` `# This code is contributed by Srathore`

## C#

 `// C# program to reverse alternate``// k nodes in a linked list``using` `System;``class` `LinkedList``{` `    ``static` `Node head;` `    ``public` `class` `Node``    ``{` `        ``public` `int` `data;``        ``public` `Node next;` `        ``public` `Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Reverses alternate k nodes and``    ``returns the pointer to the new head node */``    ``Node kAltReverse(Node node, ``int` `k)``    ``{``        ``Node current = node;``        ``Node next = ``null``, prev = ``null``;``        ``int` `count = 0;` `        ``/*1) reverse first k nodes of the linked list */``        ``while` `(current != ``null` `&& count < k)``        ``{``            ``next = current.next;``            ``current.next = prev;``            ``prev = current;``            ``current = next;``            ``count++;``        ``}` `        ``/* 2) Now head points to the kth``        ``node. So change next``        ``of head to (k+1)th node*/``        ``if` `(node != ``null``)``        ``{``            ``node.next = current;``        ``}` `        ``/* 3) We do not want to reverse``        ``next k nodes. So move the current``        ``pointer to skip next k nodes */``        ``count = 0;``        ``while` `(count < k - 1 && current != ``null``)``        ``{``            ``current = current.next;``            ``count++;``        ``}` `        ``/* 4) Recursively call for the``        ``list starting from current->next.``        ``And make rest of the list as``        ``next of first node */``        ``if` `(current != ``null``)``        ``{``            ``current.next = kAltReverse(current.next, k);``        ``}` `        ``/* 5) prev is new head of the input list */``        ``return` `prev;``    ``}` `    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``)``        ``{``            ``Console.Write(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``void` `push(``int` `newdata)``    ``{``        ``Node mynode = ``new` `Node(newdata);``        ``mynode.next = head;``        ``head = mynode;``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main(String []args)``    ``{``        ``LinkedList list = ``new` `LinkedList();` `        ``// Creating the linkedlist``        ``for` `(``int` `i = 20; i > 0; i--)``        ``{``            ``list.push(i);``        ``}``        ``Console.WriteLine(``"Given Linked List :"``);``        ``list.printList(head);``        ``head = list.kAltReverse(head, 3);``        ``Console.WriteLine(``""``);``        ``Console.WriteLine(``"Modified Linked List :"``);``        ``list.printList(head);``    ``}``}` `// This code has been contributed by Arnab Kundu`

Output:

```Given linked list
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Modified Linked list
3 2 1 4 5 6 9 8 7 10 11 12 15 14 13 16 17 18 20 19```

Time Complexity: O(n)
Method 2 (Process k nodes and recursively call for rest of the list)
The method 1 reverses the first k node and then moves the pointer to k nodes ahead. So method 1 uses two while loops and processes 2k nodes in one recursive call.
This method processes only k nodes in a recursive call. It uses a third bool parameter b which decides whether to reverse the k elements or simply move the pointer.

```_kAltReverse(struct node *head, int k, bool b)
1)  If b is true, then reverse first k nodes.
2)  If b is false, then move the pointer k nodes ahead.
3)  Call the kAltReverse() recursively for rest of the n - k nodes and link
rest of the modified list with end of first k nodes.
4)  Return new head of the list.```

## C++

 `#include ``using` `namespace` `std;` `/* Link list node */``class` `node``{``    ``public``:``    ``int` `data;``    ``node* next;``};` `/* Helper function for kAltReverse() */``node * _kAltReverse(node *node, ``int` `k, ``bool` `b);` `/* Alternatively reverses the given linked list``in groups of given size k. */``node *kAltReverse(node *head, ``int` `k)``{``    ``return` `_kAltReverse(head, k, ``true``);``}` `/* Helper function for kAltReverse(). ``It reverses k nodes of the list only if``the third parameter b is passed as true,``otherwise moves the pointer k nodes ahead``and recursively calls iteself */``node * _kAltReverse(node *Node, ``int` `k, ``bool` `b)``{``    ``if``(Node == NULL)``        ``return` `NULL;``    ` `    ``int` `count = 1;``    ``node *prev = NULL;``    ``node *current = Node;``    ``node *next;``    ` `    ``/* The loop serves two purposes``        ``1) If b is true,``           ``then it reverses the k nodes``        ``2) If b is false,``           ``then it moves the current pointer */``    ``while``(current != NULL && count <= k)``    ``{``        ``next = current->next;``    ` `        ``/* Reverse the nodes only if b is true*/``        ``if``(b == ``true``)``            ``current->next = prev;``                ` `        ``prev = current;``        ``current = next;``        ``count++;``    ``}``        ` `    ``/* 3) If b is true, then node is the kth node.``        ``So attach rest of the list after node.``        ``4) After attaching, return the new head */``    ``if``(b == ``true``)``    ``{``        ``Node->next = _kAltReverse(current, k, !b);``        ``return` `prev;        ``    ``}``        ` `    ``/* If b is not true, then attach``    ``rest of the list after prev.``    ``So attach rest of the list after prev */``    ``else``    ``{``        ``prev->next = _kAltReverse(current, k, !b);``        ``return` `Node;    ``    ``}``}`  `/* UTILITY FUNCTIONS */``/* Function to push a node */``void` `push(node** head_ref, ``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_ref);` `    ``/* move the head to point to the new node */``    ``(*head_ref) = new_node;``}` `/* Function to print linked list */``void` `printList(node *node)``{``    ``int` `count = 0;``    ``while``(node != NULL)``    ``{``        ``cout << node->data << ``" "``;``        ``node = node->next;``        ``count++;``    ``}``}` `// Driver Code``int` `main(``void``)``{``    ``/* Start with the empty list */``    ``node* head = NULL;``    ``int` `i;` `    ``// create a list 1->2->3->4->5...... ->20``    ``for``(i = 20; i > 0; i--)``    ``push(&head, i);` `    ``cout << ``"Given linked list \n"``;``    ``printList(head);``    ``head = kAltReverse(head, 3);` `    ``cout << ``"\nModified Linked list \n"``;``    ``printList(head);``    ``return``(0);``}` `// This code is contributed by rathbhupendra`

## C

 `#include``#include` `/* Link list node */``struct` `node``{``    ``int` `data;``    ``struct` `node* next;``};` `/* Helper function for kAltReverse() */``struct` `node * _kAltReverse(``struct` `node *node, ``int` `k, ``bool` `b);` `/* Alternatively reverses the given linked list in groups of``   ``given size k. */``struct` `node *kAltReverse(``struct` `node *head, ``int` `k)``{``  ``return` `_kAltReverse(head, k, ``true``);``}`` ` `/*  Helper function for kAltReverse().  It reverses k nodes of the list only if``    ``the third parameter b is passed as true, otherwise moves the pointer k``    ``nodes ahead and recursively calls iteself  */``struct` `node * _kAltReverse(``struct` `node *node, ``int` `k, ``bool` `b)``{``   ``if``(node == NULL)``       ``return` `NULL;` `   ``int` `count = 1;``   ``struct` `node *prev = NULL;``   ``struct` `node  *current = node;``   ``struct` `node *next;`` ` `   ``/* The loop serves two purposes``      ``1) If b is true, then it reverses the k nodes``      ``2) If b is false, then it moves the current pointer */``   ``while``(current != NULL && count <= k)``   ``{``       ``next = current->next;` `       ``/* Reverse the nodes only if b is true*/``       ``if``(b == ``true``)``          ``current->next = prev;``            ` `       ``prev = current;``       ``current = next;``       ``count++;``   ``}``   ` `   ``/* 3) If b is true, then node is the kth node.``       ``So attach rest of the list after node.``     ``4) After attaching, return the new head */``   ``if``(b == ``true``)``   ``{``        ``node->next = _kAltReverse(current,k,!b);``        ``return` `prev;       ``   ``}``   ` `   ``/* If b is not true, then attach rest of the list after prev.``     ``So attach rest of the list after prev */`   `   ``else``   ``{``        ``prev->next = _kAltReverse(current, k, !b);``        ``return` `node;      ``   ``}``}`` `  `/* 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)``{``    ``int` `count = 0;``    ``while``(node != NULL)``    ``{``        ``printf``(``"%d  "``, node->data);``        ``node = node->next;``        ``count++;``    ``}``}`` ` `/* Driver program to test above function*/``int` `main(``void``)``{``    ``/* Start with the empty list */``    ``struct` `node* head = NULL;``    ``int` `i;`` ` `    ``// create a list 1->2->3->4->5...... ->20``    ``for``(i = 20; i > 0; i--)``      ``push(&head, i);`` ` `    ``printf``(``"\n Given linked list \n"``);``    ``printList(head);``    ``head = kAltReverse(head, 3);`` ` `    ``printf``(``"\n Modified Linked list \n"``);``    ``printList(head);`` ` `    ``getchar``();``    ``return``(0);``}`

## Java

 `// Java program to reverse alternate k nodes in a linked list` `class` `LinkedList {` `    ``static` `Node head;` `    ``class` `Node {` `        ``int` `data;``        ``Node next;` `        ``Node(``int` `d) {``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Alternatively reverses the given linked list in groups of``     ``given size k. */``    ``Node kAltReverse(Node head, ``int` `k) {``        ``return` `_kAltReverse(head, k, ``true``);``    ``}` `    ``/*  Helper function for kAltReverse().  It reverses k nodes of the list only if``     ``the third parameter b is passed as true, otherwise moves the pointer k``     ``nodes ahead and recursively calls iteself  */``    ``Node _kAltReverse(Node node, ``int` `k, ``boolean` `b) {``        ``if` `(node == ``null``) {``            ``return` `null``;``        ``}` `        ``int` `count = ``1``;``        ``Node prev = ``null``;``        ``Node current = node;``        ``Node next = ``null``;` `        ``/* The loop serves two purposes``         ``1) If b is true, then it reverses the k nodes``         ``2) If b is false, then it moves the current pointer */``        ``while` `(current != ``null` `&& count <= k) {``            ``next = current.next;` `            ``/* Reverse the nodes only if b is true*/``            ``if` `(b == ``true``) {``                ``current.next = prev;``            ``}` `            ``prev = current;``            ``current = next;``            ``count++;``        ``}` `        ``/* 3) If b is true, then node is the kth node.``         ``So attach rest of the list after node.``         ``4) After attaching, return the new head */``        ``if` `(b == ``true``) {``            ``node.next = _kAltReverse(current, k, !b);``            ``return` `prev;``        ``} ``/* If b is not true, then attach rest of the list after prev.``         ``So attach rest of the list after prev */` `else` `{``            ``prev.next = _kAltReverse(current, k, !b);``            ``return` `node;``        ``}``    ``}` `    ``void` `printList(Node node) {``        ``while` `(node != ``null``) {``            ``System.out.print(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``void` `push(``int` `newdata) {``        ``Node mynode = ``new` `Node(newdata);``        ``mynode.next = head;``        ``head = mynode;``    ``}` `    ``public` `static` `void` `main(String[] args) {``        ``LinkedList list = ``new` `LinkedList();` `        ``// Creating the linkedlist``        ``for` `(``int` `i = ``20``; i > ``0``; i--) {``            ``list.push(i);``        ``}``        ``System.out.println(``"Given Linked List :"``);``        ``list.printList(head);``        ``head = list.kAltReverse(head, ``3``);``        ``System.out.println(``""``);``        ``System.out.println(``"Modified Linked List :"``);``        ``list.printList(head);` `    ``}``}` `// This code has been contributed by Mayank Jaiswal`

## Python

 `# Python code for above algorithm` `# Link list node``class` `node:``    ` `    ``def` `__init__(``self``, data):``        ``self``.data ``=` `data``        ``self``.``next` `=` `next``        ` `# function to insert a node at the``# beginning of the linked list``def` `push(head_ref, new_data):` `    ``# allocate node``    ``new_node ``=` `node(``0``)` `    ``# put in the data``    ``new_node.data ``=` `new_data` `    ``# link the old list to the new node``    ``new_node.``next` `=` `(head_ref)` `    ``# move the head to point to the new node``    ``(head_ref) ``=` `new_node``    ` `    ``return` `head_ref``    ` `""" Alternatively reverses the given linked list``in groups of given size k. """``def` `kAltReverse(head, k) :` `    ``return` `_kAltReverse(head, k, ``True``)` `""" Helper function for kAltReverse().``It reverses k nodes of the list only if``the third parameter b is passed as True,``otherwise moves the pointer k nodes ahead``and recursively calls iteself """``def` `_kAltReverse(Node, k, b) :` `    ``if``(Node ``=``=` `None``) :``        ``return` `None``    ` `    ``count ``=` `1``    ``prev ``=` `None``    ``current ``=` `Node``    ``next` `=` `None``    ` `    ``""" The loop serves two purposes``        ``1) If b is True,``        ``then it reverses the k nodes``        ``2) If b is false,``        ``then it moves the current pointer """``    ``while``(current !``=` `None` `and` `count <``=` `k) :``    ` `        ``next` `=` `current.``next``    ` `        ``""" Reverse the nodes only if b is True"""``        ``if``(b ``=``=` `True``) :``            ``current.``next` `=` `prev``                ` `        ``prev ``=` `current``        ``current ``=` `next``        ``count ``=` `count ``+` `1``    ` `        ` `    ``""" 3) If b is True, then node is the kth node.``        ``So attach rest of the list after node.``        ``4) After attaching, return the new head """``    ``if``(b ``=``=` `True``) :``    ` `        ``Node.``next` `=` `_kAltReverse(current, k, ``not` `b)``        ``return` `prev        ``    ` `    ``else` `:``        ``""" If b is not True, then attach``        ``rest of the list after prev.``        ``So attach rest of the list after prev """``        ``prev.``next` `=` `_kAltReverse(current, k, ``not` `b)``        ``return` `Node    ``    ` `""" Function to print linked list """``def` `printList(node) :` `    ``count ``=` `0``    ``while``(node !``=` `None``) :``    ` `        ``print``( node.data, end ``=` `" "``)``        ``node ``=` `node.``next``        ``count ``=` `count ``+` `1` `# Driver Code` `""" Start with the empty list """``head ``=` `None``i ``=` `20` `# create a list 1->2->3->4->5...... ->20``while``(i > ``0` `):``    ``head ``=` `push(head, i)``    ``i ``=` `i ``-` `1` `print``( ``"Given linked list "``)``printList(head)``head ``=` `kAltReverse(head, ``3``)` `print``( ``"\nModified Linked list "``)``printList(head)` `# This code is contributed by Arnab Kundu`

## C#

 `// C# Program for converting``// singly linked list into``// circular linked list.``using` `System;` `public` `class` `LinkedList``{` `    ``static` `Node head;` `    ``public` `class` `Node``    ``{` `        ``public` `int` `data;``        ``public` `Node next;` `        ``public` `Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Reverses alternate k nodes and``    ``returns the pointer to the new head node */``    ``Node kAltReverse(Node node, ``int` `k)``    ``{``        ``Node current = node;``        ``Node next = ``null``, prev = ``null``;``        ``int` `count = 0;` `        ``/*1) reverse first k nodes of the linked list */``        ``while` `(current != ``null` `&& count < k)``        ``{``            ``next = current.next;``            ``current.next = prev;``            ``prev = current;``            ``current = next;``            ``count++;``        ``}` `        ``/* 2) Now head points to the kth node.``        ``So change next of head to (k+1)th node*/``        ``if` `(node != ``null``)``        ``{``            ``node.next = current;``        ``}` `        ``/* 3) We do not want to reverse next``        ``k nodes. So move the current``        ``pointer to skip next k nodes */``        ``count = 0;``        ``while` `(count < k - 1 && current != ``null``)``        ``{``            ``current = current.next;``            ``count++;``        ``}` `        ``/* 4) Recursively call for the list``        ``starting from current->next. And make``         ``rest of the list as next of first node */``        ``if` `(current != ``null``)``        ``{``            ``current.next = kAltReverse(current.next, k);``        ``}` `        ``/* 5) prev is new head of the input list */``        ``return` `prev;``    ``}` `    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``)``        ``{``            ``Console.Write(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``void` `push(``int` `newdata)``    ``{``        ``Node mynode = ``new` `Node(newdata);``        ``mynode.next = head;``        ``head = mynode;``    ``}` `    ``public` `static` `void` `Main(String[] args)``    ``{``        ``LinkedList list = ``new` `LinkedList();` `        ``// Creating the linkedlist``        ``for` `(``int` `i = 20; i > 0; i--)``        ``{``            ``list.push(i);``        ``}``        ``Console.WriteLine(``"Given Linked List :"``);``        ``list.printList(head);``        ``head = list.kAltReverse(head, 3);``        ``Console.WriteLine(``""``);``        ``Console.WriteLine(``"Modified Linked List :"``);``        ``list.printList(head);``    ``}``}` `// This code is contributed 29AjayKumar`

## Javascript

 ``

Output:

```Given linked list
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Modified Linked list
3 2 1 4 5 6 9 8 7 10 11 12 15 14 13 16 17 18 20 19```

Time Complexity: O(n)
Please write comments if you find the above code/algorithm incorrect, or find other ways to solve the same problem.

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