Merge sort - linked list

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 test[] = {8, 3, 2, 6, 1, 5, 4, 7, 9, 0};
int i;

head = NULL;
/* insert some numbers into the linked list */
for(i = 0; i < 10; 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;
}