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.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ALDS_1_3_C
{
    public class Node
    {
        int value;
        Node prev;
        Node next;

        public Node(int value, Node prev, Node next)
        {
            this.value = value;
            this.prev = prev;
            this.next = next;
        }

        public int Value
        {
            get { return this.value; }
            set { this.value = value; }
        }

        public Node Next
        {
            get { return this.next; }
            set { this.next = value; }
        }

        public Node Prev
        {
            get { return this.prev; }
            set { this.prev = value; }
        }
    }

    public class DoubleLinkedList
    {
        Node dummy;

        public DoubleLinkedList()
        {
            this.dummy = new Node(0, null, null);
            this.dummy.Next = this.dummy;
            this.dummy.Prev = this.dummy;
        }

        public Node First
        {
            get { return this.dummy.Next; }
        }

        public Node Last
        {
            get { return this.dummy.Prev; }
        }

        //連結リストの先頭にvalueを持つ要素を継ぎ足す
        public void InsertFirst(int value)
        {
            Node insertNode = new Node(value, this.dummy, this.dummy.Next);
            this.dummy.Next.Prev = insertNode;
            this.dummy.Next = insertNode;
        }

        // keyを持つ最初の要素をリストから削除する
        public void delete(int key)
        {
            int i = 0;
            for (Node n = this.First; n != this.dummy; n = n.Next)
            {
                if (n.Value == key)
                {
                    n.Prev.Next = n.Next;
                    n.Next.Prev = n.Prev;
                    return;
                }
                i++;
            }
        }

        public void deleteFirst()
        {
            this.First.Next.Prev = this.dummy;
            this.dummy.Next = this.First.Next;
        }

        public void deleteLast()
        {
            this.Last.Prev.Next = this.dummy;
            this.dummy.Prev = this.Last.Prev;
            
        }

        public void PrintLinkedList()
        {
            for(Node n = this.First; n != this.dummy; n = n.Next)
            {
                Console.Write(n.Value);
                if (n != this.Last)
                {
                    Console.Write(" ");
                }
                else
                {
                    Console.WriteLine();
                }
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            int command_num;
            DoubleLinkedList dll = new DoubleLinkedList();

            command_num = int.Parse(Console.ReadLine());

            if(command_num > 2000000)
            {
                Console.WriteLine("command_num is over 2000000!");
                return;
            }

            for(int i = 0; i < command_num; i++)
            {
                string command_line = Console.ReadLine();
                string[] sp_command_line = command_line.Split(' ');

                string command = sp_command_line[0];

                if(command == "insert")
                {
                    int val = int.Parse(sp_command_line[1]);
                    dll.InsertFirst(val);
                }
                else if(command == "delete")
                {
                    int val = int.Parse(sp_command_line[1]);
                    dll.delete(val);
                }
                else if(command == "deleteFirst")
                {
                    dll.deleteFirst();
                }
                else if(command == "deleteLast")
                {
                    dll.deleteLast();
                }
            }
            dll.PrintLinkedList();
        }
    }
}