# 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.

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