Merge Sort using Linked List

Write a C Program to implement Merge Sort using Linked List. Here’s simple C Program to implement Merge Sort using Linked List in C Programming Language.

A linked list cannot be accessed randomly and because of this slow access time, sorting algorithms like quick sort cannot be applied to it.

So we use merge sort for this purpose. It works on divide and conquer technique. Time complexity is O(nlogn).

Here is the source code of the C Program to implement Merge Sort using Linked List. The C program is successfully compiled and run(Codeblocks) on a Windows system. The program output is also shown below.

SOURCE CODE : :

/*  C Program to implement Merge Sort using Linked List  */

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

struct node {
 int number;
 struct node *next;
};

/* add a node to the linked list */
struct node *addnode(int number, struct node *next);
/* preform merge sort on the linked list */
struct node *mergesort(struct node *head);
/* merge the lists.. */
struct node *merge(struct node *head_one, struct node *head_two);

int main(void)
{
 struct node *head;
 struct node *current;
 struct node *next;
 int n,i;

 printf("Enter No. of elements u want to sort : ");
 scanf("%d",&n);

 int test[n];

 printf("Enter elements : \n");
for(i=0;i<n;i++)
{
printf("Enter %d element : ",i+1);
scanf("%d",&test[i]);
}

 head = NULL;
 /* insert some numbers into the linked list */
 for(i = 0; i < n; i++)
  head = addnode(test[i], head);

 /* sort the list */
 head = mergesort(head);

 /* print the list */
 printf(" before  after\n"), i = 0;
 for(current = head; current != NULL; current = current->next)
  printf("%4d\t%4d\n", test[i++], current->number);

 /* free the list */
 for(current = head; current != NULL; current = next)
  next = current->next, free(current);

 /* done... */
 return 0;
}

/* add a node to the linked list */
struct node *addnode(int number, struct node *next) {
 struct node *tnode;

 tnode = (struct node*)malloc(sizeof(*tnode));

 if(tnode != NULL) {
  tnode->number = number;
  tnode->next = next;
 }

 return tnode;
}

/* preform merge sort on the linked list */
struct node *mergesort(struct node *head) {
 struct node *head_one;
 struct node *head_two;

 if((head == NULL) || (head->next == NULL))
  return head;

 head_one = head;
 head_two = head->next;
 while((head_two != NULL) && (head_two->next != NULL)) {
  head = head->next;
  head_two = head->next->next;
 }
 head_two = head->next;
 head->next = NULL;

 return merge(mergesort(head_one), mergesort(head_two));
}

/* merge the lists.. */
struct node *merge(struct node *head_one, struct node *head_two) {
 struct node *head_three;

 if(head_one == NULL)
  return head_two;

 if(head_two == NULL)
  return head_one;

 if(head_one->number < head_two->number) {
  head_three = head_one;
  head_three->next = merge(head_one->next, head_two);
 } else {
  head_three = head_two;
  head_three->next = merge(head_one, head_two->next);
 }

 return head_three;
}

OUTPUT : :

/*  C Program to implement Merge Sort using Linked List  */

Enter No. of elements u want to sort : 7

Enter elements :

Enter 1 element : 5
Enter 2 element : 2
Enter 3 element : 1
Enter 4 element : 4
Enter 5 element : 7
Enter 6 element : 9
Enter 7 element : 0

 before  after
   5       0
   2       1
   1       2
   4       4
   7       5
   9       7
   0       9

Above is the source code for C Program to find area and circumference of circle which is successfully compiled and run on Windows System.The Output of the program is shown above .

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