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 JAVA (judge verdict: Time Limit Exceeded)

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Stream;
import static java.util.stream.Collectors.joining;

class Main {
    static Node nil = new Node();

    static class Node {
        Integer key;
        Node next;
        Node prev;
    }

    static void insert(Integer key) {
        Node n = new Node();
        n.key = key;
        n.next = nil.next;
        nil.next = nil.next.prev = n;
        n.prev = nil;
    }

    static void delete(Integer key) {
        for (Node n = nil.next; n != nil; n = n.next) {
            if (n.key.equals(key)) {
                n.prev.next = n.next;
                n.next.prev = n.prev;
                return;
            }
        }
    }

    static void deleteFirst() {
        nil.next = nil.next.next;
        nil.next.prev = nil;
    }

    static void deleteLast() {
        nil.prev.prev.next = nil;
        nil.prev = nil.prev.prev;
    }

    static List<Integer> keys() {
        List<Integer> keys = new ArrayList<Integer>();
        for (Node n = nil.next; n != nil; n = n.next) {
            keys.add(n.key);
        }
        return keys;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        nil.prev = nil.next = nil;
        int n = sc.nextInt();

        for (int i = 0; i < n; i++) {
            char[] command = sc.next().toCharArray();
            switch(command[0]) {
            case 'i':
                  insert(sc.nextInt());
                  break;
            default :
                switch(command.length) {
                case 6:
                    delete(sc.nextInt());
                    break;
                case 11:
                    deleteFirst();
                    break;
                case 10:
                    deleteLast();
                    break;
                }
            }
        }

        for (Node node = nil.next;; node = node.next) {
            if (node.next != nil) {
                System.out.print(node.key + " ");
            } else {
                System.out.println(node.key);
                break;
            }
        }

    }
}