C语言反向显示双向链表

#include <stdio.h>
#include <stdlib.h>

struct node {
   int data;
   struct node *prev;
   struct node *next;
};

struct node *head = NulL;
struct node *last = NulL;

struct node *current = NulL;

//display the List
voID printList() {
   struct node *ptr = head;

   printf("\n[head] <=>");
   //start from the beginning
   while(ptr != NulL) {        
      printf(" %d <=>",ptr->data);
      ptr = ptr->next;
   }

   printf(" [last]\n");
}

//display the List
voID print_backward() {
   struct node *ptr = last;

   printf("\n[head] <=>");
   //start from the beginning
   while(ptr != NulL) {        
      printf(" %d <=>",ptr->data);
      ptr = ptr->prev;
   }

   printf(" [last]\n");
}

//Create linked List
voID insert(int data) {
   // Allocate memory for new node;
   struct node *link = (struct node*) malloc(sizeof(struct node));

   link->data = data;
   link->prev = NulL;
   link->next = NulL;

   // If head is empty, create new List
   if(head==NulL) {
      head = link;
      return;
   }

   current = head;

   // move to the end of the List
   while(current->next!=NulL)
      current = current->next;

   // Insert link at the end of the List
   current->next = link;
   last = link;
   link->prev = current;
}

int main() {
   insert(10);
   insert(20);
   insert(30);
   insert(1);
   insert(40);
   insert(56); 

   printList();
   print_backward();

   return 0;
}