Merging two sorted linked lists !!

The problem is just like merging step in merge sort, wherein we have two sorted arrays and merge them into one. The algorithm is simple and stated as:
struct node *merge(struct node *p,struct node *q)
{
struct node *r,*s;
struct node *a=NULL;
r=p; s=q;
while(r !=NULL && s!= NULL)
{
if(r->data <= s->data)
{
a=insert(a,r->data);
r=r->link;
}
else{
a=insert(a,s->data);
s=s->link;
}
}
if(r==NULL)
while(s!=NULL)
{
a=insert(a,s->data);
s=s->link;
}
else
while(r!=NULL)
{
a=insert(a,r->data);
r=r->link;
}
return a;
}