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 *link;
struct node {
    int item;
    link next;
    link prev;
};

void printList(link);

int main(void) {
    int n, i = 0, data;
    link head = NULL, last = NULL, t;
    char command[12];
    scanf("%d", &n);
    while (i < n) {
        scanf("%s", command);
        if (!strcmp(command, "insert")) {
            scanf("%d", &data);
            t = malloc(sizeof(*t));
            t->item = data;
            t->next = head;
            if (t->next != NULL) t->next->prev = t;
            t->prev = NULL;
            head = t;
            if (t->next == NULL) last = t;
        } else if (!strcmp(command, "delete")) {
            scanf("%d", &data);
            t = head;
            while (t != NULL) {
                if (t->item == data) {
                    if (t->prev != NULL) t->prev->next = t->next;
                    else head = t->next;
                    if (t->next != NULL) t->next->prev = t->prev;
                    else last = t->prev;
                    free(t);
                    break;
                } else t = t->next;
            }
        } else if (!strcmp(command, "deleteFirst")) {
            t = head;
            if (head == last) last = NULL;
            head = head->next;
            if (t->next != NULL) t->next->prev = NULL;
            free(t);
        } else if (!strcmp(command, "deleteLast")) {
            t = last;
            if (last == head) head = NULL;
            last = last->prev;
            if (t->prev != NULL)t->prev->next = NULL;
            free(t);
        } else printf("Invalid Command.\n");
        i++;
    }
    printList(head);
    return 0;
}

void printList(link l) {
    link t = l;
    while (t->next != NULL) {
        printf("%d ", t->item);
        t = t->next;
    }
    if (t != NULL) printf("%d\n", t->item);
}