C语言从双向链表中删除数据

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

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

voID print_backward() {
   struct node *ptr = last;

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

   printf(" [head]\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;
}

voID remove_data(int data) {
   int pos = 0;
   struct node *pre_node;

   if(head==NulL) {
      printf("linked List not initialized");
      return;
   } 

   if(head->data == data) {
      if(head->next != NulL) {
         head->next->prev = NulL;
         head = head->next;
         return;
      } else {
         head = NulL;
         printf("List is empty Now");
         return;
      }
   } else if(head->data != data && head->next == NulL) {
      printf("%d not found in the List\n", data);
      return;
   }

   current = head;

   while(current->next != NulL && current->data != data) {
      pre_node = current;
      current = current->next;
   }        

   if(current->data == data) {
      pre_node->next = pre_node->next->next;

      if(pre_node->next != NulL) {          // link back
         pre_node->next->prev = pre_node;
      } else
         last = pre_node;

      free(current);
   } else
      printf("%d not found in the List.", data);

}

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

   printList();
   remove_data(20);

   printList();
   print_backward();

   return 0;
}