为什么程序清单17.2中的链表只能沿一个方向遍历？怎样修改s

/* 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", &current->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;
}