Program to merge two linked list, restricting commomn elements to occur only once

#include <"stdio.h">
#include <"conio.h">
#include <"alloc.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 ** ) ;

void 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 ) ;

clrscr( ) ;
printf ( "First linked list : " ) ;
display ( first ) ;
printf ( "\nNo. of elements in Linked List : %d" , count ( first ) ) ;

add ( &second, 12 ) ;
add ( &second, 17 ) ;
add ( &second, 24 ) ;
add ( &second, 36 ) ;
add ( &second, 59 ) ;
add ( &second, 64 ) ;
add ( &second, 87 ) ;

printf ( "\n\nSecond linked list : " ) ;
display ( second ) ;
printf ( "\nNo. of elements in Linked List : %d" , count ( second ) ) ;

merge ( first, second, &third ) ;

printf ( "\n\nThe merged list : " ) ;
display ( third ) ;
printf ( "\nNo. of elements in Linked List : %d", count ( third ) ) ;
}

/* adds node to an ascending order linked list */
void add ( struct node **q, int num )
{
struct node *r, *temp = *q ;

r = 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 -> data > num ||
temp -> link == NULL ))
{
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 )
{
printf ( "\n" ) ;

/* traverse the entire linked list */
while ( q != NULL )
{
printf ( "%d ", q -> data ) ;
q = q -> link ;
}
}

/* 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 = malloc ( sizeof ( struct node ) ) ;
z = *s ;
}
else
{
z -> link = 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 = malloc ( sizeof ( struct node ) ) ;
z = z -> link ;
z -> data = p -> data ;
p = p -> link ;
}

/* if end of second list has been reached */
while ( q != NULL )
{
z -> link = malloc ( sizeof ( struct node ) ) ;
z = z -> link ;
z -> data = q -> data ;
q = q -> link ;
}
z -> link = NULL ;
}