Pairwise swap adjacent nodes of a linked list by changing pointers | Set 2

Given a singly linked list, write a function to swap elements pairwise. 

Input : 1->2->3->4->5->6->7
Output : 2->1->4->3->6->5->7,

Input : 1->2->3->4->5->6 
Output : 2->1->4->3->6->5

A solution has been discussed set 1. Here a simpler solution is discussed. We explicitly change pointers of first two nodes, then fix remaining nodes.  

Implementation:

C++

/* This program swaps the nodes of linked list
   rather than swapping the field from the nodes.
   Imagine a case where a node contains many
   fields, there will be plenty of unnecessary
   swap calls. */
#include<bits/stdc++.h>
using namespace std;

/* A linked list node */
struct Node
{
    int data;
    struct Node *next;
};

/* Function to pairwise swap elements of a
   linked list */
Node *pairWiseSwap(Node *head)
{
    // If linked list is empty or there is only
    // one node in list
    if (head == NULL || head->next == NULL)
        return head;

    // Fix the head and its next explicitly to
    // avoid many if else in while loop
    Node *curr = head->next->next;
    Node *prev = head;
    head = head->next;
    head->next = prev;

    // Fix remaining nodes
    while (curr != NULL && curr->next != NULL)
    {
        prev->next = curr->next;
        prev = curr;
        Node *next = curr->next->next;
        curr->next->next = curr;
        curr = next;
    }

    prev->next = curr;

    return head;
}

/* Function to add a node at the beginning of
   Linked List */
void push(struct Node** head_ref, int new_data)
{
    struct Node* new_node = new Node;
    new_node->data = new_data;
    new_node->next = (*head_ref);
    (*head_ref) = new_node;
}

/* Function to print nodes in a given 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()
{
    struct Node *start = NULL;

    /* The constructed linked list is:
    1->2->3->4->5->6->7 */
    push(&start, 7);
    push(&start, 6);
    push(&start, 5);
    push(&start, 4);
    push(&start, 3);
    push(&start, 2);
    push(&start, 1);

    printf("\n Linked list before calling pairWiseSwap() ");
    printList(start);

    start = pairWiseSwap(start);

    printf("\n Linked list after calling pairWiseSwap() ");
    printList(start);

    return 0;
}

Java

/* This program swaps the nodes of linked list
rather than swapping the field from the nodes.
Imagine a case where a node contains many
fields, there will be plenty of unnecessary
swap calls. */
class GfG
{

/* A linked list node */
static class Node
{
    int data;
    Node next;
}
static Node head = null;

/* Function to pairwise swap elements of a
linked list */
static Node pairWiseSwap(Node head)
{

    // If linked list is empty or there is only
    // one node in list
    if (head == null || head.next == null)
        return head;

    // Fix the head and its next explicitly to
    // avoid many if else in while loop
    Node curr = head.next.next;
    Node prev = head;
    head = head.next;
    head.next = prev;

    // Fix remaining nodes
    while (curr != null && curr.next != null)
    {
        prev.next = curr.next;
        prev = curr;
        Node next = curr.next.next;
        curr.next.next = curr;
        curr = next;
    }

    prev.next = curr;

    return head;
}

/* Function to add a node at the
beginning of Linked List */
static void push(int new_data)
{
    Node new_node = new Node();
    new_node.data = new_data;
    new_node.next = (head);
    (head) = new_node;
}

/* Function to print nodes in a given 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)
{
    //Node head = null;

    /* The constructed linked list is:
    1->2->3->4->5->6->7 */
    push( 7);
    push( 6);
    push( 5);
    push( 4);
    push(3);
    push( 2);
    push( 1);

    System.out.print("\n Linked list before calling pairWiseSwap() ");
    printList(head);

    Node start = pairWiseSwap(head);

    System.out.print("\n Linked list after calling pairWiseSwap() ");
    printList(start);
}
}

// This code is contributed by Prerna Saini.

Python3

# This program swaps the nodes of linked list
# rather than swapping the field from the nodes.
# Imagine a case where a node contains many
# fields, there will be plenty of unnecessary
# swap calls.

# A linked list node
class Node:

    def __init__(self, data):

        self.data = data
        self.next = None

# Function to pairwise swap elements of a
# linked list
def pairWiseSwap(head):

    # If linked list is empty or there is only
    # one node in list
    if (head == None or head.next == None):
        return head

    # Fix the head and its next explicitly to
    # avoid many if else in while loop
    curr = head.next.next
    prev = head
    head = head.next
    head.next = prev

    # Fix remaining nodes
    while (curr != None and curr.next != None):
        prev.next = curr.next
        prev = curr
        next = curr.next.next
        curr.next.next = curr
        curr = next

    prev.next = curr

    return head

# Function to add a node at the beginning
# of Linked List
def push(head_ref, new_data):

    new_node = Node(new_data)
    new_node.next = (head_ref)
    (head_ref) = new_node
    return head_ref

# Function to print nodes in a
# given linked list
def printList(node):

    while (node != None):
        print(node.data, end = ' ')
        node = node.next

# Driver Code
if __name__=='__main__':

    start = None

    # The constructed linked list is:
    # 1.2.3.4.5.6.7
    start = push(start, 7)
    start = push(start, 6)
    start = push(start, 5)
    start = push(start, 4)
    start = push(start, 3)
    start = push(start, 2)
    start = push(start, 1)

    print("\nLinked list before "
          "calling pairWiseSwap() ", end = '')
    printList(start)

    start = pairWiseSwap(start)

    print("\nLinked list after calling "
          "pairWiseSwap() ", end = '')
    printList(start)

# This code is contributed by rutvik_56

C#

/* This program swaps the nodes of linked list
rather than swapping the field from the nodes.
Imagine a case where a node contains many
fields, there will be plenty of unnecessary
swap calls. */
using System;

class GfG
{

/* A linked list node */
class Node
{
    public int data;
    public Node next;
}
static Node head = null;

/* Function to pairwise swap
elements of a linked list */
static Node pairWiseSwap(Node head)
{

    // If linked list is empty or there
    //  is only one node in list
    if (head == null || head.next == null)
        return head;

    // Fix the head and its next explicitly to
    // avoid many if else in while loop
    Node curr = head.next.next;
    Node prev = head;
    head = head.next;
    head.next = prev;

    // Fix remaining nodes
    while (curr != null && curr.next != null)
    {
        prev.next = curr.next;
        prev = curr;
        Node next = curr.next.next;
        curr.next.next = curr;
        curr = next;
    }

    prev.next = curr;
    return head;
}

/* Function to add a node at the
beginning of Linked List */
static void push(int new_data)
{
    Node new_node = new Node();
    new_node.data = new_data;
    new_node.next = (head);
    (head) = new_node;
}

/* Function to print nodes
in a given linked list */
static void printList(Node node)
{
    while (node != null)
    {
        Console.Write(node.data + " ");
        node = node.next;
    }
}

/* Driver code */
public static void Main()
{
    //Node head = null;

    /* The constructed linked list is:
    1->2->3->4->5->6->7 */
    push( 7);
    push( 6);
    push( 5);
    push( 4);
    push(3);
    push( 2);
    push( 1);

    Console.Write("\n Linked list before" +
                "calling pairWiseSwap() ");
    printList(head);

    Node start = pairWiseSwap(head);

    Console.Write("\n Linked list after" +
                "calling pairWiseSwap() ");
    printList(start);
}
}

// This code is contributed by PrinciRaj1992

Javascript

<script>

/* This program swaps the nodes of linked list
rather than swapping the field from the nodes.
Imagine a case where a node contains many
fields, there will be plenty of unnecessary
swap calls. */

/* A linked list node */
class Node
{
    constructor()
    {
        this.data = 0;
        this.next = null;
    }
}
var head = null;

/* Function to pairwise swap
elements of a linked list */
function pairWiseSwap(head)
{

    // If linked list is empty or there
    //  is only one node in list
    if (head == null || head.next == null)
        return head;

    // Fix the head and its next explicitly to
    // avoid many if else in while loop
    var curr = head.next.next;
    var prev = head;
    head = head.next;
    head.next = prev;

    // Fix remaining nodes
    while (curr != null && curr.next != null)
    {
        prev.next = curr.next;
        prev = curr;
        var next = curr.next.next;
        curr.next.next = curr;
        curr = next;
    }

    prev.next = curr;
    return head;
}

/* Function to add a node at the
beginning of Linked List */
function push(new_data)
{
    var new_node = new Node();
    new_node.data = new_data;
    new_node.next = (head);
    (head) = new_node;
}

/* Function to print nodes
in a given linked list */
function printList(node)
{
    while (node != null)
    {
        document.write(node.data + " ");
        node = node.next;
    }
}

/* Driver code */
/* The constructed linked list is:
1->2->3->4->5->6->7 */
push( 7);
push( 6);
push( 5);
push( 4);
push(3);
push( 2);
push( 1);
document.write("Linked list before" +
            "calling pairWiseSwap() ");
printList(head);
var start = pairWiseSwap(head);
document.write("<br> Linked list after" +
            "calling pairWiseSwap() ");
printList(start);


</script>

Output

 Linked list before calling pairWiseSwap() 1 2 3 4 5 6 7 
 Linked list after calling pairWiseSwap() 2 1 4 3 6 5 7 

Time complexity : O(n)
Auxiliary Space: O(1)

Another approach: The approach here is to use the double pointers so that we need not update the head pointer during swap separately. 

Implementation:

C++

#include <bits/stdc++.h>
using namespace std;

// A nexted list node
struct Node {
  int data;
  struct Node* next;
};

/* Function to insert a node at the beginning */
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;
}

/* Utility function to print a singly linked list */
void printList(struct Node* head)
{
  struct Node* temp = head;
  while (temp != NULL) {
    cout << temp->data << " ";
    temp = temp->next;
  }
  cout << endl;
}

// Function to swap adjacent nodes
void swapPairs(struct Node** head)
{
  // Loop until we reach the last node
  while (*head && (*head)->next) {
    struct Node* one = *head;

    struct Node* two = one->next->next;

    *head = one->next;

    one->next->next = one;

    one->next = two;

    head = &one->next;
  }
}

// Driver program to test above functions
int main()
{
  struct Node* head = NULL;
  push(&head, 6);
  push(&head, 5);
  push(&head, 4);
  push(&head, 3);
  push(&head, 2);
  push(&head, 1);

  cout << ("Actual List:\n");
  printList(head);

  swapPairs(&head);

  cout << ("ModifiedLinked List:\n");
  printList(head);
}

// This code is contributed by garg28harsh.

C

#include <stdio.h>
#include <stdlib.h>

// A nexted list node
struct Node
{
    int data;
    struct Node *next;
};

/* Function to insert a node at the beginning */
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;
}

/* Utility function to print a singly linked list */
void printList(struct Node *head)
{
    struct Node *temp = head;
    while (temp != NULL)
    {
        printf("%d ", temp->data);
        temp = temp->next;
    }
    printf("\n");
}

// Function to swap adjacent nodes
void swapPairs(struct Node **head)
{
    //Loop until we reach the last node
    while(*head && (*head)->next)
    {
        struct Node *one = *head;

        struct Node *two = one->next->next;

        *head = one->next;

        one->next->next = one;

        one->next = two;

        head = &one->next;
    }
}

// Driver program to test above functions
int main()
{
    struct Node *head = NULL;
    push(&head, 6);
    push(&head, 5);
    push(&head, 4);
    push(&head, 3);
    push(&head, 2);
    push(&head, 1);

    printf("Actual List:\n");
    printList(head);

    swapPairs(&head);

    printf("ModifiedLinked List:\n");
    printList(head);

    getchar();
    return 0;
}

Java

/*package whatever //do not write package name here */

import java.io.*;
import java.util.Scanner;

class Node {
    int data;
    Node next;

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

class LinkedList {
    Node head;

    // Function to insert a node at the beginning
    void push(int new_data)
    {
        Node new_node = new Node(new_data);
        new_node.next = head;
        head = new_node;
    }

    // Utility function to print a singly linked list
    void printList()
    {
        Node temp = head;
        while (temp != null) {
            System.out.print(temp.data + " ");
            temp = temp.next;
        }
        System.out.println();
    }

    void swapPairs(Node head)
    {
        Node current = head;
        // Loop until we reach the last node
        while (current != null && current.next != null) {
            int temp = current.data;
            current.data = current.next.data;
            current.next.data = temp;
            current = current.next.next;
        }
    }

    public static void main(String[] args)
    {
        LinkedList list = new LinkedList();
        list.push(6);
        list.push(5);
        list.push(4);
        list.push(3);
        list.push(2);
        list.push(1);

        System.out.println("Actual List:");
        list.printList();

        list.swapPairs(list.head);

        System.out.println("Modified Linked List:");
        list.printList();
    }
}

C#

using System;
class Node {
    public int data;
    public Node next;

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

class LinkedList {
    public Node head;

    // Function to insert a node at the beginning
    public void push(int new_data)
    {
        Node new_node = new Node(new_data);
        new_node.next = head;
        head = new_node;
    }

    // Utility function to print a singly linked list
    public void printList()
    {
        Node temp = head;
        while (temp != null) {
            Console.Write(temp.data + " ");
            temp = temp.next;
        }
        Console.WriteLine();
    }

    public void swapPairs(Node head)
    {
        Node current = head;
        // Loop until we reach the last node
        while (current != null && current.next != null) {
            int temp = current.data;
            current.data = current.next.data;
            current.next.data = temp;
            current = current.next.next;
        }
    }

    public static void Main(string[] args)
    {
        LinkedList list = new LinkedList();
        list.push(6);
        list.push(5);
        list.push(4);
        list.push(3);
        list.push(2);
        list.push(1);

        Console.WriteLine("Actual List:");
        list.printList();

        list.swapPairs(list.head);

        Console.WriteLine("Modified Linked List:");
        list.printList();
    }
} // this code is contributed by devendra

Javascript

class Node {
  constructor(data) {
    this.data = data;
    this.next = null;
  }
}

// Function to insert a node at the beginning
function push(list, newData) {
  let newNode = new Node(newData);
  newNode.next = list.head;
  list.head = newNode;
}


// Utility function to print a singly linked list
function printList(head) {
  let temp = head;
  while (temp !== null) {
    console.log(temp.data);
    temp = temp.next;
  }
}

// Function to swap adjacent nodes
function swapPairs(list) {
  let head = list.head;
  let dummy = new Node(null);
  dummy.next = head;
  let prev = dummy;
  while (head && head.next) {
    let one = head;
    let two = one.next.next;
    prev.next = one.next;
    one.next.next = one;
    one.next = two;
    prev = one;
    head = one.next;
  }
  list.head = dummy.next;
}



let list = {head: null};
push(list, 6);
push(list, 5);
push(list, 4);
push(list, 3);
push(list, 2);
push(list, 1);

console.log("Actual List:");
printList(list.head);

swapPairs(list);

console.log("Modified Linked List:");
printList(list.head);

Python3

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None

    # Function to insert a node at the beginning
    def push(self, new_data):
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node

    # Utility function to print a singly linked list
    def printList(self):
        temp = self.head
        while temp:
            print(temp.data, end=" ")
            temp = temp.next
        print()

    def swapPairs(self, head):
        current = head
        # Loop until we reach the last node
        while current and current.next:
            temp = current.data
            current.data = current.next.data
            current.next.data = temp
            current = current.next.next


# Test
linked_list = LinkedList()
linked_list.push(6)
linked_list.push(5)
linked_list.push(4)
linked_list.push(3)
linked_list.push(2)
linked_list.push(1)

print("Actual List:")
linked_list.printList()

linked_list.swapPairs(linked_list.head)

print("Modified Linked List:")
linked_list.printList()
# this code is contributed by writer

Output

Actual List:
1 2 3 4 5 6 
ModifiedLinked List:
2 1 4 3 6 5 

Time complexity: O(n), where n is the number of nodes in the linked list. This is because the code iterates through each node in the linked list once and performs a constant amount of operations on each node.
Auxiliary Space: O(1)