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: Runtime Error)

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        try {
            int n = Integer.parseInt(sc.nextLine());
            Node head = new Node();
            Node tail = new Node();
            head.next = tail;
            tail.previous = head;
            while (sc.hasNext()) {
                String[] c_x = sc.nextLine().split(" ");
                if (c_x[0].equals("insert")) {
                    Node n1 = new Node(Integer.parseInt(c_x[1]));
                    Node n2 = head.next;
                    head.next = n1;
                    n2.previous = n1;
                    n1.previous = head;
                    n1.next = n2;
                }
                if (c_x[0].equals("delete")) {
                    Node n1 = head.next;
                    int t = Integer.parseInt(c_x[1]);
                    while (n1.key != -1) {
                        if (n1.key == t) {
                            break;
                        }
                        n1 = n1.next;
                    }
                    n1.previous.next = n1.next;
                    n1.next.previous = n1.previous;
                }
                if (c_x[0].equals("deleteFirst") && head.next != tail) {
                    Node n1 = head.next;
                    n1.previous.next = n1.next;
                    n1.next.previous = n1.previous;
                }
                if (c_x[0].equals("deleteLast") && head.next != tail) {
                    Node n1 = tail.previous;
                    n1.previous.next = n1.next;
                    n1.next.previous = n1.previous;
                }
            }
            Node n1 = head.next;
            StringJoiner sj = new StringJoiner(" ");
            while(n1.key != -1) {
                sj.add(Integer.toString(n1.key));
                n1 = n1.next;
            }
            System.out.println(sj.toString());
        } catch (NoSuchElementException
                | NumberFormatException e) {
            System.out.println(e);
        }
    }

    private static class Node {
        Node() {}
        Node(int key) {
            this.key = key;
        }
        int key = -1;
        Node next;
        Node previous;
    }
}