C++ Program For Pairwise Swapping Elements Of A Given Linked List By Changing Links
Last Updated :
13 Feb, 2023
Given a singly linked list, write a function to swap elements pairwise. For example, if the linked list is 1->2->3->4->5->6->7 then the function should change it to 2->1->4->3->6->5->7, and if the linked list is 1->2->3->4->5->6 then the function should change it to 2->1->4->3->6->5
This problem has been discussed here. The solution provided there swaps data of nodes. If data contains many fields, there will be many swap operations. So changing links is a better idea in general. Following is the implementation that changes links instead of swapping data.
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
class node
{
public:
int data;
node* next;
};
/* Function to pairwise swap elements
of a linked list. It returns head of
the modified list, so return value
of this node must be assigned */
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;
// Initialize previous and
// current pointers
node* prev = head;
node* curr = head->next;
// Change head before proceeding
head = curr;
// Traverse the list
while (true)
{
node* next = curr->next;
// Change next of current
// as previous node
curr->next = prev;
// If next NULL or next is the
// last node
if (next == NULL ||
next->next == NULL)
{
prev->next = next;
break;
}
// Change next of previous to
// next of next
prev->next = next->next;
// Update previous and curr
prev = next;
curr = prev->next;
}
return head;
}
/* Function to add a node at
the beginning of Linked List */
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 of the
// new node
new_node->next = (*head_ref);
// Move the head to point to
// the new node
(*head_ref) = new_node;
}
/* Function to print nodes
in a given linked list */
void printList(node* node)
{
while (node != NULL)
{
cout << node->data << " ";
node = node->next;
}
}
// Driver code
int main()
{
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);
cout << "Linked list before " <<
"calling pairWiseSwap() ";
printList(start);
// NOTE THIS CHANGE
start = pairWiseSwap(start);
cout << "Linked list after calling" <<
"pairWiseSwap() ";
printList(start);
return 0;
}
// This code is contributed by Manoj N
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: The time complexity of the above program is O(n) where n is the number of nodes in a given linked list. The while loop does a traversal of the given linked list.
Auxiliary Space: O(1)
Following is the recursive implementation of the same approach. We change the first two nodes and recur for the remaining list. Thanks to geek and omer salem for suggesting this method.
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
class node
{
public:
int data;
node* next;
};
/* Function to pairwise swap elements
of a linked list. It returns head
of the modified list, so return value
of this node must be assigned */
node* pairWiseSwap(node* head)
{
// Base Case: The list is empty or
// has only one node
if (head == NULL ||
head->next == NULL)
return head;
// Store head of list after two nodes
node* remaining = head->next->next;
// Change head
node* newhead = head->next;
// Change next of second node
head->next->next = head;
// Recur for remaining list and change
// next of head
head->next = pairWiseSwap(remaining);
// Return new head of modified list
return newhead;
}
/* Function to add a node at the
beginning of Linked List */
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 of the
// new node
new_node->next = (*head_ref);
// Move the head to point to
// the new node
(*head_ref) = new_node;
}
/* Function to print nodes in
a given linked list */
void printList(node* node)
{
while (node != NULL) {
cout << node->data << " ";
node = node->next;
}
}
/* Driver program to test above function */
int main()
{
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);
cout <<
"Linked list before calling pairWiseSwap() ";
printList(start);
// NOTE THIS CHANGE
start = pairWiseSwap(start);
cout <<
"Linked list after calling pairWiseSwap() ";
printList(start);
return 0;
}
// This code is contributed by rathbhupendra
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(n)
Please refer complete article on Pairwise swap elements of a given linked list by changing links for more details!
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