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 C# (judge verdict: WA: Presentation Error)

using System;

namespace Elementary_data_structures
{
    class Doubly_Linked_List
    {
        static void Main(string[] args)
        {
            var n = int.Parse(Console.ReadLine());
            var dllist = new DLinkedList<string>();
            for (int i = 0; i < n; i++)
            {
                var token = Console.ReadLine().Split(' ');
                switch (token[0])
                {
                    case "insert":
                        dllist.Insert(token[1]);
                        break;
                    case "delete":
                        dllist.Delete(token[1]);
                        break;
                    case "deleteFirst":
                        dllist.DeleteFirst();
                        break;
                    case "deleteLast":
                        dllist.DeleteLast();
                        break;
                }
            }
            Console.WriteLine(dllist);
        }

        public class DLinkedList<T>
        {
            public class Node
            {
                T key;
                public T Key { get { return key; } }
                public Node Prev { get; internal set; }
                public Node Next { get; internal set; }

                public Node(T key, Node prev, Node next)
                {
                    this.key = key;
                    Prev = prev;
                    Next = next;
                }
            }

            Node dummy;
            public Node First { get { return dummy.Next; } }
            public Node Last { get { return dummy.Prev; } }
            public Node EndOfList { get { return dummy; } }

            public DLinkedList()
            {
                dummy = new Node(default(T), null, null);
                dummy.Next = dummy;
                dummy.Prev = dummy;
            }

            public void Insert(T key)
            {
                Node n = new Node(key, dummy, First);
                First.Prev = n;
                dummy.Next = n;
            }

            public void DeleteNode(Node n)
            {
                if (n == dummy) return;
                n.Prev.Next = n.Next;
                n.Next.Prev = n.Prev;
                return;
            }

            public void Delete(T key)
            {
                for (Node n = First; n != EndOfList; n = n.Next)
                {
                    if (n.Key.Equals(key))
                    {
                        DeleteNode(n);
                        return;
                    }
                }
            }

            public void DeleteFirst()
            {
                DeleteNode(First);
            }

            public void DeleteLast()
            {
                DeleteNode(Last);
            }

            public override string ToString()
            {
                var s = "";
                for (Node n = First; n != EndOfList; n = n.Next)
                {
                    s += n.Key.ToString() + " ";
                }
                s.Trim();
                return s;
            }
        }
    }
}