C语言数据结构循环链表

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

struct node {
   int data;
   int key;

   struct node *next;
};

struct node *head = NulL;
struct node *current = NulL;

bool isEmpty() {
   return head == NulL;
}

int length() {
   int length = 0;

   //if List is empty
   if(head == NulL) {
      return 0;
   }

   current = head->next;

   while(current != head) {
      length++;
      current = current->next;   
   }

   return length;
}

//insert link at the first location
voID insertFirst(int key, int data) {

   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;

   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {
      //point it to old first node
      link->next = head;

      //point first to new first node
      head = link;
   }    
}

//delete first item
struct node * deleteFirst() {

   //save reference to first link
   struct node *templink = head;

   if(head->next == head) {  
      head = NulL;
      return templink;
   }     

   //mark next to first link as first 
   head = head->next;

   //return the deleted link
   return templink;
}

//display the List
voID printList() {

   struct node *ptr = head;
   printf("\n[ ");

   //start from the beginning
   if(head != NulL) {

      while(ptr->next != ptr) {     
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }

   printf(" ]");
}

voID main() {
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56); 

   printf("Original List: "); 

   //print List
   printList();

   while(!isEmpty()) {            
      struct node *temp = deleteFirst();
      printf("\nDeleted value:");  
      printf("(%d,%d) ",temp->key,temp->data);
   }   

   printf("\nList after deleting all items: ");
   printList();   
}