Merging 2 Linked Lists in C

#include <stdio.h>
#include <conio.h>
#include <malloc.h>
#include <windows.h>
/* structure containing a data part and link part */
struct node
{
 int data ;
 struct node *link ;
} ;
void add ( struct node **, int ) ;
void display ( struct node * ) ;
int count ( struct node * ) ;
void merge ( struct node *, struct node *, struct node ** ) ;
int main( )
{
 struct node *first, *second, *third ;
 first = second = third = NULL ;  /* empty linked lists */
 add ( &first, 9 ) ;
 add ( &first, 12 ) ;
 add ( &first, 14 ) ;
 add ( &first, 17 ) ;
 add ( &first, 35 ) ;
 add ( &first, 61 ) ;
 add ( &first, 79 ) ;
 system ( "cls" ) ;
 printf ( "First linked list:\n" ) ;
 display ( first ) ;
 printf ( "No. of elements in Linked List: %d\n\n", count ( first ) ) ;
 add ( &second, 12 ) ;
 add ( &second, 17 ) ;
 add ( &second, 24 ) ;
 add ( &second, 36 ) ;
 add ( &second, 59 ) ;
 add ( &second, 64 ) ;
 add ( &second, 87 ) ;
 printf ( "Second linked list:\n" ) ;
 display ( second ) ;
 printf ( "No. of elements in Linked List: %d\n\n", count ( second ) ) ;
 merge ( first, second, &third ) ;
 printf ( "The merged list:\n" ) ;
 display ( third ) ;
 printf ( "No. of elements in Linked List: %d\n", count ( third ) ) ;
 return 0 ;
}
/* adds node to an ascending order linked list */
void add ( struct node **q, int num )
{
 struct node *r, *temp = *q ;
 r = ( struct node * ) malloc ( sizeof ( struct node ) ) ;
 r -> data = num ;
 /* if list is empty or if new node is to be inserted before the first node */
 if ( *q == NULL || ( *q ) -> data > num )
 {
  *q = r ;
  ( *q ) -> link = temp ;
 }
 else
 {
  /* traverse the entire linked list to search the position to insert the
      new node */
  while ( temp != NULL )
  {
   if ( temp -> data < num && ( temp -> link == NULL ||
          temp -> link -> data > num ))
   {
    r -> link = temp -> link ;
    temp -> link = r ;
    return ;
   }
   temp = temp -> link ;  /*go to next node */
  }
  r -> link = NULL ;
  temp -> link = r ;
 }
}
/* displays the contents of the linked list */
void display ( struct node *q )
{
 /* traverse the entire linked list */
 while ( q != NULL )
 {
  printf ( "%d ", q -> data ) ;
  q = q -> link ;
 }
 printf ( "\n" ) ;
}
/* counts the number of nodes present in the linked list */
int count ( struct node * q )
{
 int c = 0 ;
 /* traverse the entire linked list */
 while ( q != NULL )
 {
  q = q -> link ;
  c++ ;
 }
 return c ;
}
/* merges the two linked lists, restricting the common elements to occur only
    once in the final list */
void merge ( struct node *p, struct node *q, struct node **s )
{
 struct node *z ;
 z = NULL ;
 /* if both lists are empty */
 if ( p == NULL && q == NULL )
  return ;
 /* traverse both linked lists till the end. If end of any one list is reached
     loop is terminated */
 while ( p != NULL && q != NULL )
 {
  /* if node being added in the first node */
  if ( *s == NULL )
  {
   *s = ( struct node * ) malloc ( sizeof ( struct node ) ) ;
   z = *s ;
  }
  else
  {
   z -> link = ( struct node * ) malloc ( sizeof ( struct node ) ) ;
   z = z -> link ;
  }
  if ( p -> data < q -> data )
  {
   z -> data = p -> data ;
   p = p -> link ;
  }
  else
  {
   if ( q -> data < p -> data )
   {
    z -> data = q -> data ;
    q = q -> link ;
   }
   else
   {
    if ( p -> data == q -> data )
    {
     z -> data = q -> data ;
     p = p -> link ;
     q = q -> link ;
    }
   }
  }
 }
 /* if end of first list has not been reached */
 while ( p != NULL )
 {
  z -> link = ( struct node * ) malloc ( sizeof ( struct node ) ) ;
  z = z -> link ;
  z -> data = p -> data ;
  p = p -> link ;
 }
 /* if end of second list has not been reached */
 while ( q != NULL )
 {
  z -> link = ( struct node * ) malloc ( sizeof ( struct node ) ) ;
  z = z -> link ;
  z -> data = q -> data ;
  q = q -> link ;
 }
 z -> link = NULL ;
}