修改下面的程序，使其既能以正序又能以逆序显示电影列表。一种方法是修改链表定义以使链表能被双向遍历；另一种方法是使用递归。
/* films2.c -- 使用结构链表 */
#include <stdio.h>
#include <stdlib.h> /* 提供malloc()原型 */
#include <string.h> /* 提供strcpy()原型 */
#define TSIZE 45 /* 存放片名的数组大小 */
struct film {
char title[TSIZE]；
int rating;
struct film * next; /* 指向链表的下一个结构 */
};
int main (void)
{
struct film * head = NULL;
struct film * prev, * current;
char input[TSIZE];
/* 收集并存储信息 */
puts ("Enter first movie title: ");
while (gets (input) != NULL && input[0] != '\0')
{
current = (struct film *)malloc(sizeof (struct film));
if (head == NULL) /* 第一个结构 */
head = currend;
else
prev->next = current;
current->next = NULL;
strcpy (current->title, input);
puts ("Enter your rating <0-10>: ");
scanf ("%d", ¤t->rating);
while (getchar () != '\n')
continue;
puts ("Enter next movie title (empty line to stop): ");
prev = current;
}
/* 给出电影列表 */
if (head = NULL)
printf("No data entered. ");
else
printf ("Here is the movie list: \n");
current = head;
while (current != NULL)
{
printf("Movie: %s Rating: %d\n", current->title, current->rating);
current = current->next;
}
/* 任务已经完成，因此释放所分配的内存 */
current = head;
while (current != NULL)
{
free (current);
current = current->next;
}
printf("Bye!\n");
return 0;
}
#include <stdio.h> #include <stdlib.h> #include <string.h> #define TSIZE 45 struct movie{ char name[TSIZE]; int rating; struct movie *next; struct movie *former; }; void CreateMovies(struct movie **phead, struct movie **pend); void DisplayOriginal(struct movie *head, struct movie *end); void DisplayReverse(struct movie *head, struct movie *end); void FreeMoives(struct movie *head, struct movie *end); int main(void) { struct movie *head = NULL, *end = NULL; CreateMovies(&head ,&end); DisplayOriginal(head,end); DisplayReverse(head,end); FreeMoives(head,end); return 0; } void CreateMovies(struct movie **phead, struct movie **pend) { char input[TSIZE]; puts("Enter first movie title:"); while(gets(input) != NULL && input[0] != '\0') { if(*phead == NULL) { *phead = malloc( sizeof(struct movie) ); (*pend) = *phead; } else { (*pend)->next = malloc( sizeof(struct movie) ); (*pend)->next->former = (*pend); (*pend) = (*pend)->next; } strcpy((*pend)->name, input); puts("Enter your rating :"); scanf("%d",&(*pend)->rating); getchar(); puts("Enter next movie title (empty line to stop):"); } } void DisplayOriginal(struct movie *head, struct movie *end) { if( head == NULL ) { printf("No movies in the list\n"); return; } printf("display in the original order:\n"); while(head != end) { printf("%s\t\t%d\n",head->name,head->rating); head = head->next; } printf("%s\t\t%d\n",head->name,head->rating); } void DisplayReverse(struct movie *head, struct movie *end) { if( end == NULL ) { printf("No movies in the list\n"); return; } printf("display in the reverse order:\n"); while(end != head) { printf("%s\t\t%d\n",end->name,end->rating); end = end->former; } printf("%s\t\t%d\n",end->name,end->rating); } void FreeMoives(struct movie *head, struct movie *end) { struct movie *previous; if( head == NULL ) return; while(head != end) { previous = head; //注意：先得到下一个指针，再释放前一个指针 head = head->next; free(previous); } free(head); }