#include <stdio.h>
#include <stdlib.h>
typedef struct node {
    int val;
    struct node *NEXT;
    struct node *PREV;
} node_t;

node_t *first, *last, *temp;

void create () {
    first = (node_t*) malloc (sizeof(node_t));
    last = (node_t*) malloc (sizeof (node_t));
    first = last = NULL;
}
int isEmpty() {
    if (first == NULL) return 1;
    else return 0;
}

void display () {
    temp = first;
    if (isEmpty ()) printf("The list is empty.");
    while (temp != NULL) {
        printf("%d ", temp->val);
        temp = temp->NEXT;
    }
    printf("\n");
}

void reverseDisplay() {
    temp = last;
    if (isEmpty ()) printf("The list is empty.");
    while (temp != NULL) {
        printf("%d ", temp->val);
        temp = temp->PREV;
    }
    printf("\n");
}

void insertFirst (int num) {
    temp = (node_t*) malloc (sizeof (node_t));
    temp->val = num;
    temp->NEXT = first;
    temp->PREV = NULL;
    if (first!=NULL) first->PREV = temp;
    else last = temp;
    first = temp;
}

void insertLast (int num) {
    temp = (node_t*) malloc (sizeof (node_t));
    temp->val = num;
    temp->PREV = last;
    temp->NEXT = NULL;
    if (last!=NULL )last->NEXT = temp;
    else first = temp;
    last = temp;
}


int popFirst() {
    int toBeReturned=0;
    if (!isEmpty()) {
        toBeReturned = first->val;
        temp = (node_t*) malloc (sizeof (node_t));
        temp = first->NEXT;
        if (first->NEXT == NULL) {
            first = last = NULL;
        }
        else {
            free(first);
            temp->PREV = NULL;
            first = temp;
        }
    }
    return toBeReturned;
}

int popLast () {
    int toBeReturned=0;
    if (!isEmpty()) {
        toBeReturned = last->val;
        temp = (node_t*) malloc (sizeof (node_t));
        temp = last->PREV;
        if (first->NEXT == NULL) {
            first = last = NULL;
        }
        else {
            free(last);
            temp->NEXT = NULL;
            last = temp;
        }
    }
    return toBeReturned;
}

int displayFirst() {
    if (isEmpty()) return 0;
    return first->val;
}

int displayLast() {
    if (isEmpty()) return 0;
    return last->val;
}
void menu() {
    while (1) {
        printf("Press 1 to insert at the beginning.\n");
        printf("Press 2 to insert at the end.\n");
        printf("Press 3 to pop an element from the beginning.\n");
        printf("Press 4 to pop an element from the end.\n");
        printf("Press 5 to show the first element.\n");
        printf("Press 6 to show the last element.\n");
        printf("Press 7 to display the list from first to last.\n");
        printf("Press 8 to display the list in reverse order.\n");
        printf("Press 0 to Exit.\n");
        int n;
        scanf("%d", &n);
        if (n==0) break;
        else if (n==1) {
            printf("Enter the number: ");
            int num;
            scanf("%d", &num);
            insertFirst(num);
        }
        else if (n==2) {
            printf("Enter the number: ");
            int num;
            scanf("%d", &num);
            insertLast(num);
        }
        else if (n==3) {
            if (!isEmpty()) printf("Popped value = %d\n", popFirst());
            else printf("The linked list is empty.\n");
        }
        else if (n==4) {
            if (!isEmpty()) printf("Popped value = %d\n", popLast());
            else printf("The linked list is empty.\n");
        }
        else if (n==5) {
            if (!isEmpty()) printf("The first element = %d\n", displayFirst());
            else printf("The linked list is empty.\n");
        }
        else if (n==6) {
            if (!isEmpty()) printf("The last element = %d\n", displayLast());
            else printf("The linked list is empty.\n");
        }
        else if (n==7) display();
        else if (n==8) reverseDisplay();
        else {
            printf("Invalid choice! Try again.\n");
        }
    }
}
int main() {
    create();
    menu();

    return 0;
}