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

object Main {
  def main(args: Array[String]): Unit = {
    val num = StdIn.readInt
    var list = ListBuffer.empty[Int]

    for (i <- 1 to num) {
      val query = StdIn.readLine().split(" ")
      val value = if (query.length == 2) query(1).toInt else 0
      query(0) match {
        case "insert" => list = ListBuffer(value) ++ list
        case "delete" => 
          val index = list.indexOf(value)
          if (0 <= index) list.remove(index)
        case "deleteFirst" => list = list.tail
        case "deleteLast" => list = list.init
      }
    }
    println(list.mkString(" "))
  }
}