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>

typedef struct node list;

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

list *head, *tail;

void insert(int key) {
    list *new;
    new = (list*)malloc(sizeof(list));
    new->key = key;
    new->next = head->next;
    new->prev = head;
    head->next = new;
    new->next->prev = new;
}

void del(int key) {
    list *t = head->next, *prev = head;
    while(t != tail && t->key != key) {
        prev = t;
        t = t->next;
    }

    if (t != tail) {
        prev->next = t->next;
        t->next->prev = prev;
        free(t);
    }
}

void del_first() {
    list *first = head->next;
    head->next = first->next;
    first->next->prev = head;
    free(first);
}

void del_last() {
    list *last = tail->prev;
    tail->prev = last->prev;
    last->prev->next = tail;
    free(last);
}

void show() {
    list *t = head->next;
    while(t != tail) {
        printf("%d%c", t->key, ((t->next == tail) ? '\n' : ' '));
        t = t->next;
    }
}

int main() {
    int i, n;
    char command[20];
    int key;

    scanf("%d", &n);

    head = (list*)malloc(sizeof(list));
    tail = (list*)malloc(sizeof(list));
    head->key = tail->key = -1;
    head->next = tail;
    tail->prev = head;
    head->prev = tail->next = NULL;

    for(i = 0; i < n; i++) {
        scanf("%s", command);
        if (strcmp(command, "insert") == 0) {
            scanf("%d", &key);
            insert(key);
        } else if (strcmp(command, "delete") == 0) {
            scanf("%d", &key);
            del(key);
        } else if (strcmp(command, "deleteFirst") == 0) {
            del_first();
        } else if (strcmp(command, "deleteLast") == 0) {
            del_last();
        }
    }

    show();
    return 0;
}