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 Kotlin (judge verdict: Accepted)

data class Node(
        val value: Int,
        var prev: Node?,
        var next: Node?
)

class DoublyLinkedList() {
    private var start: Node? = null
    private var last: Node? = null
    private var size: Int = 0

    fun insert(x: Int) {
        val node = Node(x, null, start)
        when (size) {
            0 -> {
                start = node
                last = node
            }
            else -> {
                start?.prev = node
                start = node
            }
        }
        size++
    }

    fun delete(x: Int) {
        if (size != 0) {
            var node = start
            while (node != null) {
                if (node.value == x) {
                    node.prev?.next = node.next
                    node.next?.prev = node.prev
                    if (node.prev == null) {
                        start = node.next
                    }
                    if (node.next == null) {
                        last = node.prev
                    }
                    size--
                    break
                }
                node = node.next
            }
        }
    }

    fun deleteFirst() {
        when (size) {
            0 -> return
            1 -> {
                start = null
                last = null
                size--
            }
            else -> {
                start = start?.next
                start?.prev = null
                size--
            }
        }
    }

    fun deleteLast() {
        when (size) {
            0 -> return
            1 -> {
                start = null
                last = null
                size--
            }
            else -> {
                last = last?.prev
                last?.next = null
                size--
            }
        }
    }

    override fun toString(): String {
        var node = start
        val b = mutableListOf<Int>()
        while (node != null) {
            b.add(node.value)
            node = node.next
        }
        return b.joinToString(" ")
    }
}

fun main(args: Array<String>) {
    val n = readLine()!!.toInt()
    val l = DoublyLinkedList()
    for (x in 1..n) {
        val ss = readLine()!!.split(" ")
        val command = ss[0]
        when (command) {
            "insert" -> l.insert(ss[1].toInt())
            "delete" -> l.delete(ss[1].toInt())
            "deleteFirst" -> l.deleteFirst()
            "deleteLast" -> l.deleteLast()
        }
    }
    println(l.toString())
}