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 Scala (judge verdict: Runtime Error)

import scala.io.StdIn.{readInt, readLine};
object Main {
    def main(args: Array[String]): Unit = {
        val n = readInt;
        val list = new DoublyLinkedList[Long]();
        for (i <- (0 until n)) {
            readLine.split(" ") match {
                case Array("insert", value) => list.insert(value.toLong);
                case Array("delete", value) => list.delete(value.toLong);
                case Array("deleteFirst") => list.deleteFirst;
                case Array("deleteLast") => list.deleteLast;
                case _ => ();
            }
        }
        println(list);
    }
}
class DoublyLinkedList[A]() {
    case class Node(key: Option[A], var prev: Node, var next: Node);
    private val head = Node(None, null, null);
    def isEmpty: Boolean = head.next == null && head.prev == null;
    override def toString: String = {
        def go(node: Node, ret: String): String = {
            node.key match {
                case Some(key) => go(node.next, ret + s" ${key}");
                case None => ret;
            }
        }
        if (isEmpty) "" else go(head.next, "").trim;
    }
    def insert(key: A): Unit = {
        if (isEmpty) {
            val node = Node(Some(key), head, head);
            head.next = node;
            head.prev = node;
        } else {
            val node = Node(Some(key), head, head.next);
            head.next.prev = node;
            head.next = node;
        }
    }
    
    def _delete(node: Node): Unit = {
        val prev = node.prev;
        val next = node.next;
        if (prev == head && next == head) {
            head.next = null;
            head.prev = null;
        } else {
            prev.next = next;
            next.prev = prev;
        }
    }
    
    def find(node: Node, key: A): Node = {
        if (node.key != None) {
            if (node.key.get == key) node else find(node.next, key);
        } else {
            null;
        }
    }
    
    def delete(key: A): Unit = {
        val target = find(head.next, key);
        if (target != null) _delete(target);
    }
    
    def deleteFirst: Unit = {
        _delete(head.next);
    }
    def deleteLast: Unit = {
        _delete(head.prev);
    }
}