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.io.StdIn.{readInt,readLine}
import scala.collection.mutable.ArrayBuffer

class Node(val data:Int) {
  var prev:Node = null
  var next:Node = null
}

class DList {
  var first = new Node(-1)
  var last = new Node(-1)
  first.next = last
  first.next.prev = first

  def insert(data:Int) = {
    var newNode = new Node(data)
    newNode.next = first.next
    if(first.next!=null) first.next.prev = newNode
    first.next = newNode
    newNode.prev = first
  }

  def search(data:Int) = {
    var node = first.next
    while(node != last && node.data != data) {
      node = node.next
    }
    node
  }

  def delete(node:Node):Unit = {
    if(node!=last) {
      node.prev.next = node.next
      node.next.prev = node.prev
    }
  }

  def delete(data:Int):Unit = delete(search(data))
  def deleteFirst() = delete(first.next)
  def deleteLast() = delete(last.prev)

  def print() = {
    val b = new ArrayBuffer[Int]
    var node = first.next
    while(node != last) {
      b += node.data
      node = node.next
    }
    println(b.toArray.mkString(" "))
  }
}

object Main {
  def main(args:Array[String]) = {
    val dlist = new DList()
    val n = readInt
    for(i<-1 to n) {
      val s = readLine.split(" ")
      if(s(0)=="insert") dlist.insert(s(1).toInt)
      else if(s(0)=="delete") dlist.delete(s(1).toInt)
      else if(s(0)=="deleteFirst") dlist.deleteFirst
      else if(s(0)=="deleteLast") dlist.deleteLast
    }
    dlist.print()
  }
}