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: Accepted)

object Main extends App {
  class Node(val key: Int, var prev: Node, var next: Node) {
    def insert(key: Int): Unit = {
      val n = new Node(key, this, this.next)
      this.next.prev = n
      this.next = n
    }

    def listSearch(key: Int): Node =
      Stream
        .iterate(this.next)(_.next)
        .takeWhile(_.key != -1)
        .find(_.key == key)
        .getOrElse(null)

    def deleteNode(n: Node): Unit =
      if (n != null) {
        n.prev.next = n.next
        n.next.prev = n.prev
      }

    def deleteFirst(): Unit    = deleteNode(this.next)
    def deleteLast(): Unit     = deleteNode(this.prev)
    def delete(key: Int): Unit = deleteNode(listSearch(key))

    def show: String =
      Stream
        .iterate(this.next)(_.next)
        .takeWhile(_.key != -1)
        .map(_.key)
        .mkString(" ")

    def action(line: String): Unit = line match {
      case "deleteFirst"                  => this.deleteFirst()
      case "deleteLast"                   => this.deleteLast()
      case _ if line.startsWith("insert") => this.insert(line.replaceFirst("insert ", "").toInt)
      case _                              => this.delete(line.replaceFirst("delete ", "").toInt)
    }
  }

  def init(): Node = {
    val n: Node = new Node(-1, null, null)
    n.next = n
    n.prev = n
    n
  }

  val a = init()

  (1 to readInt)
    .foreach(_ => a.action(readLine))

  println(a.show)
}