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: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;
 
class Program
{
    public class DL<T> : IEnumerable<T>
    {
        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            var n = First;
            while (n != null)
            {
                yield return n.Value;
                n = n.Next;
            }
        }
        public IEnumerator<T> GetEnumerator()
        {
            var n = First;
            while (n != null)
            {
                yield return n.Value;
                n = n.Next;
            }
        }
        IEnumerable<Node> Nodes()
        {
            var n = First;
            while (n != null)
            {
                yield return n;
                n = n.Next;
            }
        }
        Node First;
        Node Last;
        class Node
        {
            internal T Value;
            internal Node Next {get; set;}
            internal Node Prev {get; set;}
        }
        public DL()
        {
        }
        public void InsertFirst(T item)
        {
            var node = new Node { Value = item, Prev = null, Next = First};
            if (First == null)
            {
                First = Last = node;
            }
            else
            {
                First.Prev = node;
                First = node;
            }
        }
        void DeleteNode(Node node)
        {
            var p = node.Prev;
            var n = node.Next;
            if (p != null)
                node.Prev.Next = n;
            if (n != null)
                node.Next.Prev = p;
            if (First == node)
            {
                First = n;
            }
            if (Last == node)
            {
                Last = p;
            }
        }
        public void Delete(T item)
        {
            foreach (var n in Nodes())
            {
                if (Object.Equals(n.Value, item))
                {
                    DeleteNode(n);
                    break;
                }
            }
        }
        public void DeleteFirst()
        {
            DeleteNode(First);
        }
        public void DeleteLast()
        {
            DeleteNode(Last);
        }
    }
    static void Main(string[] args)
    {
        var c = int.Parse(Console.ReadLine());
        var la = new DL<int>();
        while (c-- > 0)
        {
            var sac = Console.ReadLine().Split(' ');
            switch (sac[0])
            {
                case "insert":
                {
                    la.InsertFirst(int.Parse(sac[1]));
                    break;
                }
                case "delete":
                {
                    la.Delete(int.Parse(sac[1]));
                    break;
                }
                case "deleteFirst":
                {
                    la.DeleteFirst();
                    break;
                }
                case "deleteLast":
                {
                    la.DeleteLast();
                    break;
                }
            }
        }
        Console.WriteLine(string.Join(" ", la.Select(x => x.ToString()).ToArray()/*AOJ!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!*/));
    }
}