DSA using C – Doubly Linked List

Overview

Doubly Linked List is a variation of Linked list in which navigation is possible in both ways either forward and backward easily as compared to Single Linked List. Following are important terms to understand the concepts of doubly 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.

• Prev − Each Link of a linked list contain a link to previous link called Prev.

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

Doubly Linked List Representation

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

• Doubly LinkedList contains an link element called first and last.

• 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.

• Each Link is linked with its previous link using its prev link.

• Last Link carries a Link as null to mark the end of the list.

Basic Operations

Following are the basic operations supported by an list.

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

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

• Insert Last− add an element in the end of the list.

• Delete Last − delete an element from the end of the list.

• Insert After − add an element after an item of the list.

• Delete − delete an element from the list using key.

• Display forward − displaying complete list in forward manner.

• Display backward − displaying complete list in backward manner.

Insertion Operation

Following code demonstrate insertion operation at beginning in a doubly linked list.

//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()){
      //make it the last link
      last = link;
   } else {
      //update first prev link
      head->prev = link;
   }
   //point it to old first link
   link->next = head;
   
   //point first to new first link
   head = link;
}

Deletion Operation

Following code demonstrate deletion operation at beginning in a doubly linked list.

//delete first item
struct node* deleteFirst(){
   //save reference to first link
   struct node *tempLink = head;
   
   //if only one link
   if(head->next == NULL){
      last = NULL;
   } else {
      head->next->prev = NULL;
   }
   head = head->next;
   
   //return the deleted link
   return tempLink;
}

Insertion at End Operation

Following code demonstrate insertion operation at last position in a doubly linked list.

//insert link at the last location
void insertLast(int key, int data){
   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if(isEmpty()){
      //make it the last link
      last = link;
   } else {
      //make link a new last link
      last->next = link;     
      
      //mark old last node as prev of new link
      link->prev = last;
   }
   //point last to new last node
   last = link;
}

Example

DoublyLinkedListDemo.c

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

struct node {
   int data;
   int key;
   struct node *next;
   struct node *prev;
};
//this link always point to first Link 
struct node *head = NULL;

//this link always point to last Link 
struct node *last = NULL;
struct node *current = NULL;

//is list empty
bool isEmpty(){
   return head == NULL;
}
int length(){
   int length = 0;
   struct node *current;
   for(current = head; current!=NULL;
      current = current->next){
      length++;
   }
   return length;
}
//display the list in from first to last
void displayForward(){
   //start from the beginning
   struct node *ptr = head;
   
   //navigate till the end of the list
   printf("n[ ");
   while(ptr != NULL){        
      printf("(%d,%d) ",ptr->key,ptr->data);
      ptr = ptr->next;
   }
   printf(" ]");
}
//display the list from last to first
void displayBackward(){
   //start from the last
   struct node *ptr = last;
   
   //navigate till the start of the list
   printf("n[ ");
   while(ptr != NULL){    
      //print data
      printf("(%d,%d) ",ptr->key,ptr->data);
      
      //move to next item
      ptr = ptr ->prev;
      printf(" ");
   }
   printf(" ]");
}
//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()){
      //make it the last link
      last = link;
   } else {
      //update first prev link
      head->prev = link;
   }
   //point it to old first link
   link->next = head;
   
   //point first to new first link
   head = link;
}
//insert link at the last location
void insertLast(int key, int data){
   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if(isEmpty()){
      //make it the last link
      last = link;
   } else {
      //make link a new last link
      last->next = link;     
      
      //mark old last node as prev of new link
      link->prev = last;
   }
   //point last to new last node
   last = link;
}
//delete first item
struct node* deleteFirst(){
   //save reference to first link
   struct node *tempLink = head;
   
   //if only one link
   if(head->next == NULL){
      last = NULL;
   } else {
      head->next->prev = NULL;
   }
   head = head->next;
   //return the deleted link
   return tempLink;
}
//delete link at the last location
struct node* deleteLast(){
   //save reference to last link
   struct node *tempLink = last;
   
   //if only one link
   if(head->next == NULL){
      head = NULL;
   } else {
      last->prev->next = NULL;
   }
   last = last->prev;
   //return the deleted link
   return tempLink;
}
//delete a link with given key
struct node* delete(int key){
   //start from the first link
   struct node* current = head;
   struct node* previous = NULL;
   
   //if list is empty
   if(head == NULL){
      return NULL;
   }
   //navigate through list
   while(current->key != key){
      //if it is last node
      if(current->next == NULL){
         return NULL;
      } else {
         //store reference to current link
         previous = current;
         
         //move to next link
         current = current->next;             
      }
   }
   //found a match, update the link
   if(current == head) {
      //change first to point to next link
      head = head->next;
   } else {
      //bypass the current link
      current->prev->next = current->next;
   }    
   if(current == last){
      //change last to point to prev link
      last = current->prev;
   } else {
      current->next->prev = current->prev;
   }
   return current;
}
bool insertAfter(int key, int newKey, int data){
   //start from the first link
   struct node *current = head;      
   
   //if list is empty
   if(head == NULL){
      return false;
   }
   //navigate through list
   while(current->key != key){
      //if it is last node
      if(current->next == NULL){
         return false;
      } else {           
         //move to next link
         current = current->next;             
      }
   }
   //create a link
   struct node *newLink = (struct node*) malloc(sizeof(struct node));
   newLink->key = key;
   newLink->data = data;

   if(current==last) {
      newLink->next = NULL; 
      last = newLink; 
   } else {
      newLink->next = current->next;         
      current->next->prev = newLink;
   }
   newLink->prev = current; 
   current->next = newLink; 
   return true; 
}
main() {
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56); 

   printf("nList (First to Last): ");  
   displayForward();
   printf("n");
   printf("nList (Last to first): "); 
   displayBackward();

   printf("nList , after deleting first record: ");
   deleteFirst();        
   displayForward();

   printf("nList , after deleting last record: ");  
   deleteLast();
   displayForward();

   printf("nList , insert after key(4) : ");  
   insertAfter(4,7, 13);
   displayForward();

   printf("nList  , after delete key(4) : ");  
   delete(4);
   displayForward();
}

Output

If we compile and run the above program then it would produce following output −

List (First to Last): 
[ (6,56) (5,40) (4,1) (3,30) (2,20) (1,10) ]

List (Last to first): 
[ (1,10) (2,20) (3,30) (4,1) (5,40) (6,56) ]
List , after deleting first record: 
[ (5,40) (4,1) (3,30) (2,20) (1,10) ]
List , after deleting last record: 
[ (5,40) (4,1) (3,30) (2,20) ]
List , insert after key(4) : 
[ (5,40) (4,1) (4,13) (3,30) (2,20) ]
List , after delete key(4) : 
[ (5,40) (4,13) (3,30) (2,20) ]