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  1. /* C program to pairwise swap elements in a given linked list */
  2. #include<stdio.h>
  3. #include<stdlib.h>
  4. #define null NULL
  5. /* A linked list node */
  6. struct node
  7. {
  8. 	int data;
  9. 	struct node *next;
  10. };
  11.  
  12. struct node* swapPairs(struct node* head) {
  13.     struct node *prev = head;    
  14.     struct node *curr;
  15.     struct node *next = null, *tail = null;      
  16.     while(prev != null && prev->next != null) {
  17.         curr = prev->next;
  18.         next = curr->next;      
  19.         curr->next = prev;
  20.         if(tail != null) tail->next = curr;
  21.         else head = curr; //First two nodes are swapped. First node (previously second node) will be new head
  22.         tail = prev;      //Tail of lastly swapped nodes i.e. previously first one of them 
  23.         prev = next;      //Swapping done between curr and pre, now move to next node 
  24.     }
  25.     if(tail != null) {
  26.        tail->next = prev; 
  27.     }
  28.     return head;  
  29. }
  30.  
  31. /* Function to add a node at the begining of Linked List */
  32. void push(struct node** head_ref, int new_data)
  33. {
  34. 	/* allocate node */
  35. 	struct node* new_node =
  36. 			(struct node*) malloc(sizeof(struct node));
  37.  
  38. 	/* put in the data */
  39. 	new_node->data = new_data;
  40.  
  41. 	/* link the old list off the new node */
  42. 	new_node->next = (*head_ref);
  43.  
  44. 	/* move the head to point to the new node */
  45. 	(*head_ref) = new_node;
  46. }
  47.  
  48. /* Function to print nodes in a given linked list */
  49. void printList(struct node *node)
  50. {
  51. 	while (node != NULL)
  52. 	{
  53. 		printf("%d ", node->data);
  54. 	node = node->next;
  55. 	}
  56. }
  57.  
  58. int main()
  59. {
  60. 	struct node *start = NULL;
  61.  
  62. 	/* The constructed linked list is:
  63. 	2->3->4->5 */
  64. 	push(&start, 5);
  65. 	push(&start, 4);
  66. 	push(&start, 3);
  67. 	push(&start, 2);
  68. 	//push(&start, 1);
  69.  
  70. 	printf("Linked list before calling swapPairs()\n");
  71. 	printList(start);
  72.     start = swapPairs(start);
  73.  
  74. 	printf("\nLinked list after calling swapPairs()\n");
  75. 	printList(start);
  76.  
  77. 	return 0;
  78. }
  79.  
Success #stdin #stdout 0s 3468KB
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Linked list before calling swapPairs()

Linked list after calling swapPairs()
https://ideone.com/2zqWOA
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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