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 <cstdlib>
#include <iostream>
#include <string>

using namespace std;

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

// ??????????????????????????????
// *??????????????£?¨?????????¨????????????????§????????????°?????????
// ????????????????????????????????¢?????¬??????????????????????????????????????°??????????????????????????§?????????
// nil = ?§????????????°?????¢?????¬???
Node *nil;

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

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

    // ???????????´??????????´??????????
    x -> next = nil -> next;//x's next = nil's next
    nil -> next -> prev = x;//nil's next's prev = x
    nil -> next = x;//nil's next = x
    x -> prev = nil;//x's prev = nil
}

Node* listSearch(int key) {
    Node *cur = nil -> next;//??????????¬????????´????????????????

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

void deleteNode(Node *t) {
    if (t == nil) return;//t??????????????¨?????????????????????
    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(listSearch(key));
}

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");
}

int main(){
    int n, m;
    string order;
    init();
    cin >> n;

    for (int i = 0; i < n; i++) {
        cin >> order;

        if (order == "insert") {
            cin >> m;
            insert(m);
        } else if (order == "delete") {
            cin >> m;
            deleteKey(m);
        } else if (order == "deleteFirst") {
            deleteFirst();
        } else if (order == "deleteLast") {
            deleteLast();
        } else {
            cout << "error" << endl;
        }
    }

    printList();

    return 0;
}