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++11 (judge verdict: Accepted)

#include <cstdio>
#include <cstdlib>
#include <cstring>

using namespace std;

struct Node{
    int key;
    Node *next, *prev;
};

Node *nil;

Node* search(int key) {
    Node *cur = nil -> next;
    while (cur != nil && cur -> key != key) {
        cur = cur -> next;
    }
    return cur;
}

void init() {
    nil = (Node *)malloc(sizeof(Node));
    nil -> next = nil;
    nil -> prev = nil;
}

void printList() {
    Node *cur = nil -> next;
    int isf = 0;
    while (1) {
        if (cur == nil) break;
        if (isf++ > 0) printf(" ");
        printf("%d", cur -> key);
        cur = cur -> next;
    }
    printf("\n");
}

void deleteNode(Node *t) {
    if (t == nil) return;
    t -> prev -> next = t -> next;
    t -> next -> prev = t -> prev;
    free(t);
}

void deleteFirst() {
    deleteNode(nil -> next);
}

void deleteLast() {
    deleteNode(nil -> prev);
}

void deleteKey(int key) {
    deleteNode(search(key));
}

void insert(int key) {
    Node *x = (Node *)malloc(sizeof(Node));
    x -> key = key;
    x -> next = nil -> next;
    nil -> next -> prev = x;
    nil -> next = x;
    x -> prev = nil;
}

int main(void) {
    int key, n;
    int size = 0;
    char com[20];
    int np = 0, nd = 0;
    scanf("%d", &n);
    init();
    for (int i = 0; i < n; i++) {
        scanf("%s %d", com, &key);
        if (com[0] == 'i') {
            insert(key);
            np++;
            size++;
        } else if (com[0] == 'd') {
            if (strlen(com) > 6) {
                if (com[6] == 'F') deleteFirst();
                else if (com[6] == 'L') deleteLast();
            } else {
            deleteKey(key);
            nd++;
            }
            size--;
        }
    }

    printList();

    return 0;
}