Đảo ngược 1 dãy số

#include <stdio.h>

typedef int element_type;
struct node {
element_type element;
struct node *next;
};

void khoi_tao_ds(struct node **first, struct node **last)
{
*first = *last = NULL;
}

void insert(element_type e, struct node **first, struct node **last)
{
struct node *tmp;
tmp = (struct node*) malloc(sizeof(struct node));
tmp->element = e;
tmp->next = NULL;
if (*first == NULL)
*first = *last = tmp;
else
{
(*last)->next = tmp;
(*last) = (*last)->next;
}
}

void xoa_ds(struct node **first, struct node **last)
{
struct node *tmp;
tmp = *first;
while (tmp != NULL)
{
tmp = (*first)->next;
free(*first);
*first = tmp;
}
*first = *last = NULL;
}

void nhap_ds(struct node **first, struct node **last)
{
element_type e;
printf("\nNhap cac gia tri so (-1) de ket thuc : ");
do {
scanf("%d", &e);
if (e != -1)
insert(e, first, last);
} while (e != -1);
}

void print_ds(struct node *first)
{
struct node *tmp;

tmp = first;
while (tmp != NULL)
{
printf("%d ", tmp->element);
tmp = tmp->next;
}
}

void daonguocds(struct node **first, struct node **last)
{
struct node *before = NULL, *after, *cursor ;

if (*first != NULL)
{
after = *first;
cursor = after->next;
*last = *first;
while (cursor != NULL)
{
after->next = before;
before = after;
after = cursor;
cursor = cursor->next;
}
after->next = before;
*first = after;
}
}

void main()
{
struct node *first, *last;

khoi_tao_ds(&first, &last);
nhap_ds(&first, &last);
printf("Danh sach ban dau : ");
print_ds(first);
daonguocds(&first, &last);
printf("\nSau khi dao nguoc : ");
print_ds(first);
xoa_ds(&first, &last);
getch();
}