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)

fun main(args: Array<String>) {
    val sc = System.`in`.bufferedReader()
    val n = Integer.parseInt(sc.readLine())
    val list = MyLinkedList<Int>()
    repeat(n) {
        val command = sc.readLine().split(" ")
        when (command[0]) {
            "insert" -> {
                list.insert(Integer.parseInt(command[1]))
            }
            "delete" -> {
                list.delete(Integer.parseInt(command[1]))
            }
            "deleteFirst" -> {
                list.deleteFirst()
            }
            "deleteLast" -> {
                list.deleteLast()
            }
        }
    }
    println(list)
}

class MyLinkedList<T> {
    private val nil: Element = Element(null)
    init {
        nil.next = nil
        nil.prev = nil
    }

    fun insert(value: T) {
        val element = Element(value)
        element.next = nil.next
        nil.next?.prev = element
        nil.next = element
        element.prev = nil
    }

    fun delete(value: T) {
        var element = nil.next
        while (element != nil) {
            if (element?.value == value) {
                element?.prev?.next = element?.next
                element?.next?.prev = element?.prev
                return
            }
            element = element?.next
        }
    }

    fun deleteFirst() {
        val element = nil.next
        nil.next = element?.next
        element?.next?.prev = nil
    }

    fun deleteLast() {
        val element = nil.prev
        nil.prev = element?.prev
        element?.prev?.next = nil
    }

    override fun toString(): String {
        var element = nil.next
        val strBuilder = StringBuilder()
        while (element != nil) {
            strBuilder.append(element?.value)
            strBuilder.append(" ")
            element = element?.next
        }
        strBuilder.deleteCharAt(strBuilder.lastIndex)
        return strBuilder.toString()
    }

    inner class Element(val value: T?) {
        var prev: Element? = null
        var next: Element? = null
    }
}