Data Structure - Linked List

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Linked List Basics

A linked-list is a sequence of data structures which are connected together via links.

Linked List is a sequence of links which contains items. Each link contains a connection to another link. Linked list the second most used data structure after array. Following are important terms to understand the concepts of Linked List.

• Link − Each Link of a linked list can store a data called an element.

• Next − Each Link of a linked list contain a link to next link called Next.

• LinkedList − A LinkedList contains the connection link to the first Link called First.

Linked List Representation

As per above shown illustration, following are the important points to be considered.

• LinkedList contains an link element called first.
• Each Link carries a data field(s) and a Link Field called next.
• Each Link is linked with its next link using its next link.
• Last Link carries a Link as null to mark the end of the list.

Types of Linked List

Following are the various flavours of linked list.

• Simple Linked List − Item Navigation is forward only.

• Doubly Linked List − Items can be navigated forward and backward way.

• Circular Linked List − Last item contains link of the first element as next and and first element has link to last element as prev.

Basic Operations

Following are the basic operations supported by a list.

• Insertion − add an element at the beginning of the list.

• Deletion − delete an element at the beginning of the list.

• Display − displaying complete list.

• Search − search an element using given key.

• Delete − delete an element using given key.

Insertion Operation

Insertion is a three step process −

• Create a new Link with provided data.
• Point New Link to old First Link.
• Point First Link to this New Link.

//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;
	
   //point it to old first node
   link->next = head;
	
   //point first to new first node
   head = link;
}

Deletion Operation

Deletion is a two step process −

• Get the Link pointed by First Link as Temp Link.
• Point First Link to Temp Link's Next Link.

//delete first item
struct node* deleteFirst(){
   //save reference to first link
   struct node *tempLink = head;
	
   //mark next to first link as first 
   head = head->next;
	
   //return the deleted link
   return tempLink;
}

Navigation Operation

Navigation is a recursive step process and is basis of many operations like search, delete etc. −

• Get the Link pointed by First Link as Current Link.
• Check if Current Link is not null and display it.
• Point Current Link to Next Link of Current Link and move to above step.

Note −

//display the list
void printList(){
   struct node *ptr = head;
   printf("\n[ ");
	
   //start from the beginning
   while(ptr != NULL){        
      printf("(%d,%d) ",ptr->key,ptr->data);
      ptr = ptr->next;
   }
	
   printf(" ]");
}

Advanced Operations

Following are the advanced operations specified for a list.

• Sort − sorting a list based on a particular order.

• Reverse − reversing a linked list.

Sort Operation

We've used bubble sort to sort a list.

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;                        
      }
   }   
}

Reverse Operation

Following code demonstrate reversing a single linked list.

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;
}

To see linked-list implementation in C programming language, please click here.