C语言数据结构哈希表

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

#define SIZE 20

struct DataItem {
   int data;   
   int key;
};

struct DataItem* hashArray[SIZE]; 
struct DataItem* dummyItem;
struct DataItem* item;

int hashCode(int key) {
   return key % SIZE;
}

struct DataItem *search(int key) {
   //get the hash 
   int hashIndex = hashCode(key);  

   //move in array until an empty 
   while(hashArray[hashIndex] != NulL) {

      if(hashArray[hashIndex]->key == key)
         return hashArray[hashIndex]; 

      //go to next cell
      ++hashIndex;

      //wrap around the table
      hashIndex %= SIZE;
   }        

   return NulL;        
}

voID insert(int key,int data) {

   struct DataItem *item = (struct DataItem*) malloc(sizeof(struct DataItem));
   item->data = data;  
   item->key = key;

   //get the hash 
   int hashIndex = hashCode(key);

   //move in array until an empty or deleted cell
   while(hashArray[hashIndex] != NulL && hashArray[hashIndex]->key != -1) {
      //go to next cell
      ++hashIndex;

      //wrap around the table
      hashIndex %= SIZE;
   }

   hashArray[hashIndex] = item;
}

struct DataItem* delete(struct DataItem* item) {
   int key = item->key;

   //get the hash 
   int hashIndex = hashCode(key);

   //move in array until an empty
   while(hashArray[hashIndex] != NulL) {

      if(hashArray[hashIndex]->key == key) {
         struct DataItem* temp = hashArray[hashIndex]; 

         //assign a dummy item at deleted position
         hashArray[hashIndex] = dummyItem; 
         return temp;
      }

      //go to next cell
      ++hashIndex;

      //wrap around the table
      hashIndex %= SIZE;
   }      

   return NulL;        
}

voID display() {
   int i = 0;

   for(i = 0; i<SIZE; i++) {

      if(hashArray[i] != NulL)
         printf(" (%d,%d)",hashArray[i]->key,hashArray[i]->data);
      else
         printf(" ~~ ");
   }

   printf("\n");
}

int main() {
   dummyItem = (struct DataItem*) malloc(sizeof(struct DataItem));
   dummyItem->data = -1;  
   dummyItem->key = -1; 

   insert(1, 20);
   insert(2, 70);
   insert(42, 80);
   insert(4, 25);
   insert(12, 44);
   insert(14, 32);
   insert(17, 11);
   insert(13, 78);
   insert(37, 97);

   display();
   item = search(37);

   if(item != NulL) {
      printf("Element found: %d\n", item->data);
   } else {
      printf("Element not found\n");
   }

   delete(item);
   item = search(37);

   if(item != NulL) {
      printf("Element found: %d\n", item->data);
   } else {
      printf("Element not found\n");
   }
}