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: Wrong Answer)

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
	Scanner scan = new Scanner(System.in);
	int n = Integer.parseInt(scan.nextLine());
	MyDoublyLinkedList list = new MyDoublyLinkedList();
	for (int i = 0; i < n; i++) {
	    String[] input  = scan.nextLine().split(" ");
	    String command = input[0];
	    System.out.println(i+1 +"??????: " + command);
	    switch (command) {
	    case "insert":
		list.insert(Integer.parseInt(input[1]));
		break;
	    case "delete":
		list.delete(Integer.parseInt(input[1]));
		break;
	    case "deleteFirst":
		list.deleteFirst();
		break;
	    case "deleteLast":
		list.deleteLast();
		break;
	    }
	}
	list.printList();
    }
}

class Node {
    Node next;
    Node prev;
    int key;
    Node (int key) {
	this.key = key;
    }
}

class MyDoublyLinkedList {
    Node head;
    int size;
    MyDoublyLinkedList () {
	head = new Node(-1);
	head.next = head;
	head.prev = head;
	this.size = 0;
    }
    void insert (int x) {
	Node n = new Node(x);
	n.next = head.next;
	n.prev = head;
	head.next.prev = n;
	head.next = n;
	size++;
    }
    void delete (int x) {
	Node n = head;
	int index = 0;
	while (n.next.key != x && index < size){
	    n = n.next;
	    index++;
	}
	if (index < size) {
	    n.next = n.next.next;
	    n.next.next.prev = n;
	    size--;
	}
    }
    void deleteFirst () {
	Node n = head;
	n.next = n.next.next;
	n.next.next.prev = n;
	size--;
    }
    void deleteLast () {
	Node n = head.prev.prev;
	n.next = head;
	n.next.next.prev = n;
	size--;
    }
    void printList () {
	Node n = head.next;
	for (int i = 0; i < size - 1; i++){
	    System.out.print(n.key + " ");
	    n = n.next;
	}
	System.out.println(n.key);
    }
}