Doubly_linked_list.c

/* 
 Doubly linked list
 Navigation possible in both directions
 */

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

struct node {
  int data;
  int key;

  struct node *next;
  struct node *prev;
}

struct node *first = NULL;
struct node *last = NULL;
struct node *current = NULL;

book isEmpty() {
  return head == NULL;
}

int length() {
   int length = 0;
   struct node *current;
	
   for(current = first; current != NULL; current = current->next){
      length++;
   }
	
   return length;
}

struct node* returnListDisplayForward() {
   struct node* node_current = first;
   struct node* doubly_linked_list_items = (struct node*) malloc(sizeof(struct node));
   int n = 0;
   while(node_current != NULL) {
      doubly_linked_list_items[n].data = node_current->data;
      doubly_linked_list_items[n].key = node_current->key;
      node_current->next;
      n=n+1;
   }
   
   return doubly_linked_list_items;
}

struct node* returnListDisplayBackward() {
   struct node* node_current = last;
   struct node* doubly_linked_list_items = (node*)calloc(length(), sizeof(struct node));
   int n = length()-1;
   while(node_current != NULL) {
      doubly_linked_list_items[n].data = node_current->data;
      doubly_linked_list_items[n].key = node_current->key;
      node_current->prev;
      n=n+1;
   }
   
   return doubly_linked_list_items;
}

//insert node at the first index
void insertFirst(int key, int data) {
   //create a node
   struct node* node_item = (struct node*) malloc(sizeof(struct node));
   node_item->key = key;
   node_item->data = data;
	
   if(isEmpty()) {
      //make it the last node
      last = node_item;
   } else {
      //update first prev node
      first->prev = node_item;
   }

   //point next to first
   node_item->next = first;
	
   //assign first to new first node_item
   first = node_item;
}

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

   //point last to new last node
   last = node_item;
}

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

//delete link at the last location
struct node* deleteLast() {
   //save reference to last link
   struct node *tempNode = last;
	
   //if only one link
   if(first->next == NULL) {
      first = NULL;
   } else {
      last->prev->next = NULL;
   }
	
   last = last->prev;
	
   //return the deleted node
   return tempNode;
}

//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(first == 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 == first) {
      //change first to point to next link
      first = first->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 = first; 
	
   //if list is empty
   if(first == 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 = newKey;
   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; 
}

void 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();
}