Problem:
Doubly Linked List

Your task is to implement a double linked list.

Write a program which performs the following operations:

insert x: insert an element with key x into the front of the list.

delete x: delete the first element which has the key of x from the list. If there is not such element, you need not do anything.

deleteFirst: delete the first element from the list.

deleteLast: delete the last element from the list.

Input

The input is given in the following format:

n

command1

command2

...

commandn

In the first line, the number of operations n is given. In the following n lines, the above mentioned operations are given in the following format:

insert x

delete x

deleteFirst

deleteLast

Output

Print all the element (key) in the list after the given operations. Two consequtive keys should be separated by a single space.

Constraints

The number of operations ≤ 2,000,000

The number of delete operations ≤ 20

0 ≤ value of a key ≤ 109

The number of elements in the list does not exceed 106

For a delete, deleteFirst or deleteLast operation, there is at least one element in the list.

Sample Input 1

7
insert 5
insert 2
insert 3
insert 1
delete 3
insert 6
delete 5

Sample Output 1

6 1 2

Sample Input 2

9
insert 5
insert 2
insert 3
insert 1
delete 3
insert 6
delete 5
deleteFirst
deleteLast

Sample Output 2

1

Solution in C (judge verdict: Accepted)

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

struct node {
    struct node* prev;
    struct node* next;
    int key;
};

struct node* head = NULL;
struct node* tail = NULL;


static void insert(int key) {
    struct node *node;
    node = malloc(sizeof(struct node));
    node->key = key;
    
    if ( head == NULL ) {
        head = tail = node;
        node->prev = node->next = NULL;
    } else {
        node->next = head;
        head->prev = node;
        head = node;
    }
    return;
}

static void deleteFirst(void) {
    struct node* node;
    if ( !head ) {
        return;
    }
    if ( head == tail ) {
        node = head;
        head = tail = NULL;
    } else {
        head->next->prev = NULL;
        node = head;
        head = head->next;
    }

    free(node);

    return;
}

static void deleteLast(void) {
    struct node* node;
    if ( !tail ) {
        return;
    }
    if ( head == tail ) {
        node = head;
        head = tail = NULL;
    } else {
        tail->prev->next = NULL;
        node = tail;
        tail = tail->prev;
    }

    free(node);

    return;
}

static int delete(int key) {
    struct node* node;
    int ret = 0;

    for ( node = head; node != NULL; node = node->next) {
        if ( node->key == key ) {
            if ( node == head ) {
                deleteFirst();
            } else if ( node == tail ) {
                deleteLast();
            } else {
                node->prev->next = node->next;
                node->next->prev = node->prev;
                free(node);
            }
            return 1;
        }
    }

    return 0;
}

static void printAndDeleteList(void) {
    struct node* node;

    while ( head ) {
        printf("%d", head->key);
        deleteFirst();
        if ( head ) {
            printf(" ");
        }
    }

    printf("\n");

    return;
}
 
int main(int argc, char* argv[]) {
    int i, n;
    char buf[20];
    int key;

    scanf("%d\n", &n);

    for ( i = 0; i < n; i++ ) {
        scanf("%s %d\n", buf, &key);

        if ( strcmp("insert", buf) == 0 ) {
            insert(key);
        } else if ( strcmp("delete", buf) == 0 ) {
            delete(key);
        } else if ( strcmp("deleteFirst", buf) == 0 ) {
            deleteFirst();
        } else if ( strcmp("deleteLast", buf) == 0 ) {
            deleteLast();
        }
    }

    printAndDeleteList();

    return 0;
}