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}
case class Node[T](data: Option[T], var prev: Node[T], var next: Node[T])
case class DoublyLinkedList[T]() {
    val n = new Node[T](None, null, null)
    def isEmpty(): Boolean = if (n.prev == null && n.next == null) true else false
    def insert(e: T): Unit = {
        if (isEmpty) {
            val newNode = new Node(Option(e), n, n)
            n.next = newNode
            n.prev = newNode
        } else {
            val newNode = new Node(Option(e), n, n.next)
            n.next.prev = newNode
            n.next = newNode
        }
    }
    def delete(e: T): Unit = {
        def find(node: Node[T]): Node[T] = node.next match {
            case a if a.data.isEmpty => null
            case a if a.data.get == e => a 
            case a => find(a)
        }
        if (!isEmpty) {
            val target = find(n)
            if (target != null) {
                val targetNext = target.next
                val targetPrev = target.prev
                if (targetNext eq targetPrev) {
                    n.prev = null
                    n.next = null
                } else {
                    targetPrev.next = targetNext
                    targetNext.prev = targetPrev
                }
            }
        }
    }
    def deleteFirst(): Unit = {
        if (!isEmpty) {
            val targetNext = n.next.next
            if (n eq targetNext) {
                n.prev = null
                n.next = null
            } else {
                n.next = targetNext
                targetNext.prev = n
            }
        }
    }
    def deleteLast(): Unit = {
        if (!isEmpty) {
            val targetPrev = n.prev.prev
            if (n eq targetPrev) {
                n.prev = null
                n.next = null
            } else {
                n.prev = targetPrev
                targetPrev.next = n
            }
        }
    }
    def printList(): Unit = {
        if (isEmpty) {
            println()
        } else {
            def go(node: Node[T], list: List[T]): List[T] = node.prev match {
                case a if a.data.isEmpty => list
                case a => go(a, a.data.get :: list)
            }
            println(go(n, Nil).mkString(" "))
            
        }
    }
}
object Main {
    def main(args: Array[String]): Unit = {
        val list = DoublyLinkedList[String]()
        for (i <- (1 to readInt)) {
            val input = readLine.split(" ") 
            if (input(0) == "insert") list.insert(input(1))
            else if (input(0) == "delete") list.delete(input(1))
            else if (input(0) == "deleteFirst") list.deleteFirst()
            else if (input(0) == "deleteLast") list.deleteLast()
        }
        list.printList()
    }
}