# Pairwise swap elements of a given linked list

Given a singly linked list, write a function to swap elements pairwise. For example, if the linked list is 1->2->3->4->5 then the function should change it to 2->1->4->3->5, 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.

## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

METHOD 1 (Iterative)
Start from the head node and traverse the list. While traversing swap data of each node with its next node’s data.

## C/C++

```/* C program to pairwise swap elements in a given linked list */
#include<stdio.h>
#include<stdlib.h>

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

/*Function to swap two integers at addresses a and b */
void swap(int *a, int *b);

/* Function to pairwise swap elements of a linked list */
void pairWiseSwap(struct Node *head)
{
struct Node *temp = head;

/* Traverse further only if there are at-least two nodes left */
while (temp != NULL && temp->next != NULL)
{
/* Swap data of node with its next node's data */
swap(&temp->data, &temp->next->data);

/* Move temp by 2 for the next pair */
temp = temp->next->next;
}
}

/* UTILITY FUNCTIONS */
/* Function to swap two integers */
void swap(int *a, int *b)
{
int temp;
temp = *a;
*a = *b;
*b = temp;
}

/* Function to add a node at the begining of Linked List */
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 */

/* move the head to point to the 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 */
push(&start, 5);
push(&start, 4);
push(&start, 3);
push(&start, 2);
push(&start, 1);

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

pairWiseSwap(start);

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

return 0;
}
```

## Java

```// Java program to pairwise swap elements of a linked list
{

/* Linked list Node*/
class Node
{
int data;
Node next;
Node(int d) {data = d; next = null; }
}

void pairWiseSwap()
{
Node temp = head;

/* Traverse only till there are atleast 2 nodes left */
while (temp != null && temp.next != null) {

/* Swap the data */
int k = temp.data;
temp.data = temp.next.data;
temp.next.data = k;
temp = temp.next.next;
}
}

/* Utility functions */

/* Inserts a new Node at front of the list. */
public void push(int new_data)
{
/* 1 & 2: Allocate the Node &
Put in the data*/
Node new_node = new Node(new_data);

/* 3. Make next of new Node as head */

/* 4. Move the head to point to new Node */
}

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

/* Driver program to test above functions */
public static void main(String args[])
{

/* Created Linked List 1->2->3->4->5 */
llist.push(5);
llist.push(4);
llist.push(3);
llist.push(2);
llist.push(1);

System.out.println("Linked List before calling pairWiseSwap() ");
llist.printList();

llist.pairWiseSwap();

System.out.println("Linked List after calling pairWiseSwap() ");
llist.printList();
}
}
/* This code is contributed by Rajat Mishra */
```

## Python

```
# Python program to swap the elements of linked list pairwise

# Node class
class Node:

# Constructor to initialize the node object
def __init__(self, data):
self.data = data
self.next = None

# Function to initialize head
def __init__(self):

# Function to pairwise swap elements of a linked list
def pairwiseSwap(self):

# There are no nodes in ilnked list
if temp is None:
return

# Traverse furthur only if there are at least two
#  left
while(temp is not None and temp.next is not None):

# Swap data of node with its next node's data
temp.data, temp.next.data = temp.next.data, temp.data

# Move temo by 2 fro the next pair
temp = temp.next.next

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

# Utility function to prit the linked LinkedList
def printList(self):
while(temp):
print temp.data,
temp = temp.next

# Driver program
llist.push(5)
llist.push(4)
llist.push(3)
llist.push(2)
llist.push(1)

print "Linked list before calling pairWiseSwap() "
llist.printList()

llist.pairwiseSwap()

print  "\nLinked list after calling pairWiseSwap()"
llist.printList()

# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

```

Output:
```Linked List before calling pairWiseSwap()
1 2 3 4 5
Linked List after calling pairWiseSwap()
2 1 4 3 5 ```

Time complexity: O(n)

METHOD 2 (Recursive)
If there are 2 or more than 2 nodes in Linked List then swap the first two nodes and recursively call for rest of the list.

```/* Recursive function to pairwise swap elements of a linked list */
void pairWiseSwap(struct node *head)
{
/* There must be at-least two nodes in the list */
if (head != NULL && head->next != NULL)
{
/* Swap the node's data with data of next node */

/* Call pairWiseSwap() for rest of the list */
}
}
```

Time complexity: O(n)

The solution provided there swaps data of nodes. If data contains many fields, there will be many swap operations. See this for an implementation that changes links rather than swapping data.

### Asked in: Amazon, Microsoft, Moonfrog Labs

Please write comments if you find any bug in above code/algorithm, or find other ways to solve the same problem.

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