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: Runtime Error)

using System;

namespace Test
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            var n = int.Parse(Console.ReadLine());
            var list = new List();
            for (int i = 0; i < n; i++)
            {
                var command = Console.ReadLine().Split();
                var num = 0;
                switch(command[0]){
                    case "insert":
                        num = int.Parse(command[1]);
                        list.Insert(num);
                        break;
                    case "delete":
                        num = int.Parse(command[1]);
                        list.Delete(num);
                        break;
                    case "deleteFirst":
                        list.DeleteFirst();
                        break;
                    case "deleteLast":
                        list.DeleteLast();
                        break;
                }
            }
            var iterator=list.First;
            while (iterator!=list.Last)
            {
                Console.Write(iterator.Key);
                if (iterator.After == list.Last) break;
                Console.Write(" ");
                iterator = iterator.After;
            }
            Console.WriteLine();
            Console.Read();
        }
    }

    class Node
    {
        public Node Before { get; set; }
        public int Key { get; set; }
        public Node After { get; set; }
    }
    class List
    {
        public Node First { get; private set; }
        public Node Last { get; private set; }
        public List()
        {
            First = new Node();
            Last = First;
        }
        public void Insert(int x)
        {
            var Second = First;
            First = new Node { Key = x, After = Second};
            Second.Before = First;
        }
        public void Delete(int x)
        {
            var iterator = First;
            while (iterator != Last)
            {
                if (iterator.Key == x)
                {
                    if (iterator == First)
                    {
                        DeleteFirst();
                        break;
                    }
                    else
                    {
                        iterator.After.Before = iterator.Before;
                        iterator.Before.After = iterator.After;
                        break;
                    }
                }
                iterator = iterator.After;
            }
        }
        public void DeleteFirst()
        {
            First = First.After;
            First.Before = null;
        }
        public void DeleteLast()
        {
            Last.Before = Last.Before.Before;
            Last.Before.After = Last;
        }
    }
}