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)

import scala.collection.mutable.ListBuffer
import scala.io.StdIn

object Main extends App {

  sealed abstract class Node {
    val key: Int
    var prev:Node
    var next: Node
  }
  
  case class Elements(key: Int, var prev:Node, var next: Node) extends Node {
    override def toString: String = s"Elements($key)"
  }
  
  case object MyNil extends Node{
    val key: Int = 0
    var prev: Node = MyNil
    var next: Node = MyNil
    override def toString: String = s"MyNil"
  }
  
  def insert(newVal: Int): Unit = {
    var x = Elements(newVal, MyNil, MyNil.next)
    MyNil.next.prev = x
    MyNil.next = x
  }
  
  def listSearch(searchVal: Int): Node = {
    var target = MyNil.next
    while(target.key != searchVal && target != MyNil){
      target = target.next
    }
    target
  }
  
  // nullを設定可能か？
  def deleteNode(target: Node): Unit = {
    if(target != MyNil){
      target.prev.next = target.next
      target.next.prev = target.prev
    }
  }

  def deleteFirst(): Unit = deleteNode(MyNil.next)

  def deleteLast(): Unit = deleteNode(MyNil.prev)

  def deleteKey(target: Int): Unit = deleteNode(listSearch(target))
  
  
  val n = StdIn.readLine().trim.toInt
  case class Command(cmd: String, key: Option[Int])
  
  (0 until n).map{ _ =>
    val line = StdIn.readLine().trim.split(' ')
    line.toList match {
      case a::Nil if a == "deleteFirst" => deleteFirst()
      case a::Nil if a == "deleteLast" => deleteLast()
      case a::b::Nil if a == "insert" => insert(b.toInt)
      case a::b::Nil if a == "delete" => deleteKey(b.toInt)
      case _ => 
    }
  }
  //println(commandList)
  val buff = ListBuffer.empty[Int]
  var current = MyNil.next
  while(current != MyNil){
    buff += current.key
    current = current.next
  }
  println(buff.mkString(" "))
}