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//swap nodes without swapping data
/* This program swaps the nodes of linked list rather
than swapping the field from the nodes.*/

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

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

/* Function to swap nodes x and y in linked list by
changing links */
void swapNodes(struct Node** head_ref, int x, int y)
{
	// Nothing to do if x and y are same
	if (x == y)
		return;

	// Search for x (keep track of prevX and CurrX
	struct Node *prevX = NULL, *currX = *head_ref;
	while (currX && currX->data != x) {
		prevX = currX;
		currX = currX->next;
	}

	// Search for y (keep track of prevY and CurrY
	struct Node *prevY = NULL, *currY = *head_ref;
	while (currY && currY->data != y) {
		prevY = currY;
		currY = currY->next;
	}

	// If either x or y is not present, nothing to do
	if (currX == NULL || currY == NULL)
		return;

	// If x is not head of linked list
	if (prevX != NULL)
		prevX->next = currY;
	else // Else make y as new head
		*head_ref = currY;

	// If y is not head of linked list
	if (prevY != NULL)
		prevY->next = currX;
	else // Else make x as new head
		*head_ref = currX;

	// Swap next pointers
	struct Node* temp = currY->next;
	currY->next = currX->next;
	currX->next = temp;
}

/* Function to add a node at the beginning of 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 */
	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(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 swapNodes() ");
	printList(start);

	swapNodes(&start, 4, 3);

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

	return 0;
}





//delete alternate nodes of a linked list
// C program to remove alternate nodes of a linked list
#include<stdio.h>
#include<stdlib.h>

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

/* deletes alternate nodes of a list starting with head */
void deleteAlt(struct Node *head)
{
	if (head == NULL)
		return;

	/* Initialize prev and node to be deleted */
	struct Node *prev = head;
	struct Node *node = head->next;

	while (prev != NULL && node != NULL)
	{
		/* Change next link of previous node */
		prev->next = node->next;

		/* Free memory */
		free(node);

		/* Update prev and node */
		prev = prev->next;
		if (prev != NULL)
			node = prev->next;
	}
}

/* UTILITY FUNCTIONS TO TEST fun1() and fun2() */
/* Given a reference (pointer to pointer) to the head
of a list and an int, push a new node on the front
of the 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 */
	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(struct Node *node)
{
	while (node != NULL)
	{
		printf("%d ", node->data);
		node = node->next;
	}
}

/* Driver program to test above functions */
int main()
{
	/* Start with the empty list */
	struct Node* head = NULL;

	/* Using push() to construct below list
	1->2->3->4->5 */
	push(&head, 5);
	push(&head, 4);
	push(&head, 3);
	push(&head, 2);
	push(&head, 1);

	printf("\nList before calling deleteAlt() \n");
	printList(head);

	deleteAlt(head);

	printf("\nList after calling deleteAlt() \n");
	printList(head);

	return 0;
}