Swap all pairwise nodes in a linked list

In this lesson, I will show you an iterative program to swap all pairwise nodes of a linked list.

Are we swapping the pairwise data of nodes or the complete node?

We'll we swapping the nodes, as a result the data will get swapped. Swapping only the element does not make any sense.

For example, if the linked list is 1-2-3-4-5-6 then pairwise swapping all nodes will give us the linked list 2-1-4-3-6-5.

If the linked list is 1-2-3-4-5 then pairwise swapping all nodes will give us the linked list 2-1-4-3-5.

Observe the representation above, the address and data of the nodes.

The following program code require the understanding of insertAfter() function and printList() function.

[code language="C"]
/Pairwise swap all elements of linked list.

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

struct Node{
	int data;
	struct Node *next;
};

void printList(struct Node*);
struct Node* insertAfter(struct Node**, int); //returns adress of last node
void pairWiseSwap(struct Node**);

int main()
{
	struct Node *head;
	struct Node *endMarker;
	head = (struct Node*)malloc(sizeof(struct Node));
	head->data = 1;
	head->next = NULL;
	endMarker = insertAfter(&head, 2);
	endMarker = insertAfter(&endMarker, 3);
	endMarker = insertAfter(&endMarker, 4);
	endMarker = insertAfter(&endMarker, 5);
	endMarker = insertAfter(&endMarker, 6);

	pairWiseSwap(&head);

	printList(head);

	return 0;
}

void pairWiseSwap(struct Node **head){
	struct Node *t1, *t2, *beforeT1 = NULL;
	int count = 0;

	//init
	t1 = *head;
	t2 = (*head)->next;

	while(t1 && t2){

		t1->next = t2->next;
		t2->next = t1;

		if(count == 0)
			(*head) = t2;
		else{
			beforeT1->next = t2;
		}

		beforeT1 = t2;

		if(t2->next == NULL)
			t1 = NULL;
		else
			t1 = t1->next; //t1 = t1->next;

		if(t1 == NULL)
			t2 = NULL;
		else
			t2 = t1->next;

		count++;
		beforeT1 = beforeT1->next;
	}

}

void printList(struct Node* node){
	int count=0;
	while(node!=NULL){
		printf("\t%d", node->data);
		node = node->next;
		count++;
	}
	printf("\nTotal nodes printed=\t%d\n", count);
}

struct Node* insertAfter(struct Node **node, int data){
    if( (*node)==NULL) {
        printf("Node Does Not Exists\n");
        return NULL;
    }
    struct Node *new = (struct Node*)malloc(sizeof(struct Node));
    new->data = data;
    new->next = (*node)->next;
    (*node)->next = new;
    return new;
}

[/code]