c program for double link list

#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();
}