Linked list

#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;
void printList() {
   struct node *ptr = head;
   printf("\n[ ");
   while(ptr != NULL) {
      printf("(%d,%d) ",ptr->key,ptr->data);
      ptr = ptr->next;
   }	
   printf(" ]");
}
void insertFirst(int key, int data) {
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   link->next = head;
   head = link;
}
struct node* deleteFirst() {
   struct node *tempLink = head;
   head = head->next;
   return tempLink;
}
bool isEmpty() {
   return head == NULL;
}
int length() {
   int length = 0;
   struct node *current;
   for(current = head; current != NULL; current = current->next) {
      length++;
   }
   return length;
}
struct node* find(int key) {
   struct node* current = head;
   if(head == NULL) {
      return NULL;
   }
   while(current->key != key) {
      if(current->next == NULL) {
         return NULL;
      } else {
         current = current->next;
      }
   }      
   return current;
}
struct node* delete(int key) {
   struct node* current = head;
   struct node* previous = NULL;
   if(head == NULL) {
      return NULL;
   }
   while(current->key != key) {
      if(current->next == NULL) {
         return NULL;
      } else {
         previous = current;
         current = current->next;
      }
   }
   if(current == head) {
      head = head->next;
   } else {
      previous->next = current->next;
   }    
   return current;
}
void sort() {
   int i, j, k, tempKey, tempData;
   struct node *current;
   struct node *next;	
   int size = length();
   k = size ;
   for ( i = 0 ; i < size - 1 ; i++, k-- ) {
      current = head;
      next = head->next;	
      for ( j = 1 ; j < k ; j++ ) {   

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

            tempKey = current->key;
            current->key = next->key;
            next->key = tempKey;
         }
			
         current = current->next;
         next = next->next;
      }
   }   
}
void reverse(struct node** head_ref) {
   struct node* prev   = NULL;
   struct node* current = *head_ref;
   struct node* next;
	
   while (current != NULL) {
      next  = current->next;
      current->next = prev;   
      prev = current;
      current = next;
   }
   *head_ref = prev;
}
void main() {
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56); 
   printf("Original 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();
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   printf("\nRestored List: ");
   printList();
   printf("\n");  
   struct node *foundLink = find(4);	
   if(foundLink != NULL) {
      printf("Element found: ");
      printf("(%d,%d) ",foundLink->key,foundLink->data);
      printf("\n");  
   } else {
      printf("Element not found.");
   }
   delete(4);
   printf("List after deleting an item: ");
   printList();
   printf("\n");
   foundLink = find(4);	
   if(foundLink != NULL) {
      printf("Element found: ");
      printf("(%d,%d) ",foundLink->key,foundLink->data);
      printf("\n");
   } else {
      printf("Element not found.");
   }	
   printf("\n");
   sort();	
   printf("List after sorting the data: ");
   printList();	
   reverse(&head);
   printf("\nList after reversing the data: ");
   printList();
}