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++14 (judge verdict: Accepted)

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

using namespace std;

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

Node *nil;

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

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

void printAll(){
    Node *p = nil->next;
    int count = 0;
    while(p != nil){
        if(count > 0) printf(" ");
        printf("%d", p->key);
        p = p->next;
        count++;
    }
    printf("\n");
}

void insertNode(int num){
    Node *n = (Node *)malloc(sizeof(Node));
    n->key = num;
    n->next = nil->next;
    n->prev = nil;
    n->next->prev = n;
    nil->next = n;
}

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

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

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

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

int main() {
    int n;
    cin >> n;
    init();
    char command[20];
    int key;
    for (int i = 0; i < n; i++) {
        scanf("%s%d", command, &key);
        if(command[0] == 'i') insertNode(key);
        else if(strlen(command) <= 6) deleteKey(key);
        else if(command[6] == 'F') deleteFirstNode();
        else deleteLastNode();
    }

    printAll();
}