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++
#include <bits/stdc++.h>
using namespace std;
class Node
{
public :
int data;
Node* next;
};
Node *kAltReverse(Node *head, int k)
{
Node* current = head;
Node* next;
Node* prev = NULL;
int count = 0;
while (current != NULL && count < k)
{
next = current->next;
current->next = prev;
prev = current;
current = next;
count++;
}
if (head != NULL)
head->next = current;
count = 0;
while (count < k-1 && current != NULL )
{
current = current->next;
count++;
}
if (current != NULL)
current->next = kAltReverse(current->next, k);
return prev;
}
void push(Node** head_ref, int new_data)
{
Node* new_node = new Node();
new_node->data = new_data;
new_node->next = (*head_ref);
(*head_ref) = new_node;
}
void printList(Node *node)
{
int count = 0;
while (node != NULL)
{
cout<<node->data<< " " ;
node = node->next;
count++;
}
}
int main( void )
{
Node* head = NULL;
int i;
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);
}
|
Java
class LinkedList {
static Node head;
class Node {
int data;
Node next;
Node( int d) {
data = d;
next = null ;
}
}
Node kAltReverse(Node node, int k) {
Node current = node;
Node next = null , prev = null ;
int count = 0 ;
while (current != null && count < k) {
next = current.next;
current.next = prev;
prev = current;
current = next;
count++;
}
if (node != null ) {
node.next = current;
}
count = 0 ;
while (count < k - 1 && current != null ) {
current = current.next;
count++;
}
if (current != null ) {
current.next = kAltReverse(current.next, k);
}
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();
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);
}
}
|
Python3
import math
class Node:
def __init__( self , data):
self .data = data
self . next = None
def kAltReverse(head, k) :
current = head
next = None
prev = None
count = 0
while (current ! = None and count < k) :
next = current. next
current. next = prev
prev = current
current = next
count = count + 1 ;
if (head ! = None ):
head. next = current
count = 0
while (count < k - 1 and current ! = None ):
current = current. next
count = count + 1
if (current ! = None ):
current. next = kAltReverse(current. next , k)
return prev
def push(head_ref, new_data):
new_node = Node(new_data)
new_node. next = head_ref
head_ref = new_node
return head_ref
def printList(node):
count = 0
while (node ! = None ):
print (node.data, end = " " )
node = node. next
count = count + 1
if __name__ = = '__main__' :
head = None
for i in range ( 20 , 0 , - 1 ):
head = push(head, i)
print ( "Given linked list " )
printList(head)
head = kAltReverse(head, 3 )
print ( "\nModified Linked list" )
printList(head)
|
C#
using System;
class LinkedList
{
static Node head;
public class Node
{
public int data;
public Node next;
public Node( int d)
{
data = d;
next = null ;
}
}
Node kAltReverse(Node node, int k)
{
Node current = node;
Node next = null , prev = null ;
int count = 0;
while (current != null && count < k)
{
next = current.next;
current.next = prev;
prev = current;
current = next;
count++;
}
if (node != null )
{
node.next = current;
}
count = 0;
while (count < k - 1 && current != null )
{
current = current.next;
count++;
}
if (current != null )
{
current.next = kAltReverse(current.next, k);
}
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();
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);
}
}
|
Javascript
<script>
class Node
{
constructor(d)
{
this .data = d;
this .next = null ;
}
}
let head;
function kAltReverse(node, k)
{
let current = node;
let next = null , prev = null ;
let count = 0;
while (current != null && count < k)
{
next = current.next;
current.next = prev;
prev = current;
current = next;
count++;
}
if (node != null )
{
node.next = current;
}
count = 0;
while (count < k - 1 && current != null )
{
current = current.next;
count++;
}
if (current != null )
{
current.next = kAltReverse(current.next, k);
}
return prev;
}
function printList(node)
{
while (node != null )
{
document.write(node.data + " " );
node = node.next;
}
}
function push(newdata)
{
let mynode = new Node(newdata);
mynode.next = head;
head = mynode;
}
for (let i = 20; i > 0; i--)
{
push(i);
}
document.write( "Given Linked List :<br>" );
printList(head);
head = kAltReverse(head, 3);
document.write( "<br>" );
document.write( "Modified Linked List :<br>" );
printList(head);
</script>
|
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)
Space 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 <bits/stdc++.h>
using namespace std;
class node
{
public :
int data;
node* next;
};
node * _kAltReverse(node *node, int k, bool b);
node *kAltReverse(node *head, int k)
{
return _kAltReverse(head, k, true );
}
node * _kAltReverse(node *Node, int k, bool b)
{
if (Node == NULL)
return NULL;
int count = 1;
node *prev = NULL;
node *current = Node;
node *next;
while (current != NULL && count <= k)
{
next = current->next;
if (b == true )
current->next = prev;
prev = current;
current = next;
count++;
}
if (b == true )
{
Node->next = _kAltReverse(current, k, !b);
return prev;
}
else
{
prev->next = _kAltReverse(current, k, !b);
return Node;
}
}
void push(node** head_ref, int new_data)
{
node* new_node = new node();
new_node->data = new_data;
new_node->next = (*head_ref);
(*head_ref) = new_node;
}
void printList(node *node)
{
int count = 0;
while (node != NULL)
{
cout << node->data << " " ;
node = node->next;
count++;
}
}
int main( void )
{
node* head = NULL;
int i;
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);
}
|
C
#include<stdio.h>
#include<stdlib.h>
struct node
{
int data;
struct node* next;
};
struct node * _kAltReverse( struct node *node, int k, bool b);
struct node *kAltReverse( struct node *head, int k)
{
return _kAltReverse(head, k, true );
}
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;
while (current != NULL && count <= k)
{
next = current->next;
if (b == true )
current->next = prev;
prev = current;
current = next;
count++;
}
if (b == true )
{
node->next = _kAltReverse(current,k,!b);
return prev;
}
else
{
prev->next = _kAltReverse(current, k, !b);
return node;
}
}
void push( struct node** head_ref, int new_data)
{
struct node* new_node =
( struct node*) malloc ( sizeof ( struct node));
new_node->data = new_data;
new_node->next = (*head_ref);
(*head_ref) = new_node;
}
void printList( struct node *node)
{
int count = 0;
while (node != NULL)
{
printf ( "%d " , node->data);
node = node->next;
count++;
}
}
int main( void )
{
struct node* head = NULL;
int i;
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
class LinkedList {
static Node head;
class Node {
int data;
Node next;
Node( int d) {
data = d;
next = null ;
}
}
Node kAltReverse(Node head, int k) {
return _kAltReverse(head, k, true );
}
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 ;
while (current != null && count <= k) {
next = current.next;
if (b == true ) {
current.next = prev;
}
prev = current;
current = next;
count++;
}
if (b == true ) {
node.next = _kAltReverse(current, k, !b);
return 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();
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);
}
}
|
Python3
class node:
def __init__( self , data):
self .data = data
self . next = next
def push(head_ref, new_data):
new_node = node( 0 )
new_node.data = new_data
new_node. next = (head_ref)
(head_ref) = new_node
return head_ref
def kAltReverse(head, k) :
return _kAltReverse(head, k, True )
def _kAltReverse(Node, k, b) :
if (Node = = None ) :
return None
count = 1
prev = None
current = Node
next = None
while (current ! = None and count < = k) :
next = current. next
if (b = = True ) :
current. next = prev
prev = current
current = next
count = count + 1
if (b = = True ) :
Node. next = _kAltReverse(current, k, not b)
return prev
else :
prev. next = _kAltReverse(current, k, not b)
return Node
def printList(node) :
count = 0
while (node ! = None ) :
print ( node.data, end = " " )
node = node. next
count = count + 1
head = None
i = 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)
|
C#
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 ;
}
}
Node kAltReverse(Node node, int k)
{
Node current = node;
Node next = null , prev = null ;
int count = 0;
while (current != null && count < k)
{
next = current.next;
current.next = prev;
prev = current;
current = next;
count++;
}
if (node != null )
{
node.next = current;
}
count = 0;
while (count < k - 1 && current != null )
{
current = current.next;
count++;
}
if (current != null )
{
current.next = kAltReverse(current.next, k);
}
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();
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);
}
}
|
Javascript
<script>
var head;
class Node {
constructor(val) {
this .data = val;
this .next = null ;
}
}
function kAltReverse(head , k) {
return _kAltReverse(head, k, true );
}
function _kAltReverse(node , k, b) {
if (node == null ) {
return null ;
}
var count = 1;
var prev = null ;
var current = node;
var next = null ;
while (current != null && count <= k) {
next = current.next;
if (b == true ) {
current.next = prev;
}
prev = current;
current = next;
count++;
}
if (b == true ) {
node.next = _kAltReverse(current, k, !b);
return prev;
}
else {
prev.next = _kAltReverse(current, k, !b);
return node;
}
}
function printList(node) {
while (node != null ) {
document.write(node.data + " " );
node = node.next;
}
}
function push(newdata) {
var mynode = new Node(newdata);
mynode.next = head;
head = mynode;
}
for (i = 20; i > 0; i--) {
push(i);
}
document.write( "Given Linked List :<br/>" );
printList(head);
head = kAltReverse(head, 3);
document.write( "<br/>" );
document.write( "Modified Linked List :<br/>" );
printList(head);
</script>
|
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)
Space 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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