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.

C++

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/* 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;
    }
}
  
/* Druver 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;
}

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Java

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/* 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.

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C#

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/* 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

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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


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