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.Text;


namespace ALDS1_3_C
{
    class program
    {
        static void Main(string[] args)
        {
            BDLlist BDL = new BDLlist();
            string input = Console.ReadLine();

            int N = int.Parse(input);

            for (int i = 0; i < N; i++)
            {
                input = Console.ReadLine();
                string[] ope = input.Split(' ');
                int num = 0;
                if (ope.Length >= 2)
                {
                    num = int.Parse(ope[1]);
                }

                switch (ope[0])
                {
                    case "insert":
                        BDL.InsertFirst(num);
                        break;

                    case "delete":
                        BDL.DeleteValue(num);
                        break;

                    case "deleteFirst":
                        BDL.DeleteFirst();
                        break;

                    case "deleteLast":
                        BDL.DeleteLast();
                        break;
                }
            }

            Console.WriteLine(BDL.ListUp());

            while (true)
            {
                input = Console.ReadLine();
                if (string.IsNullOrEmpty(input)) break;
            }
        }
    }

    public class BDLlist
    {
        public class Node
        {
            //field
            int val;
            Node next;
            Node previous;

            //constructor
            internal Node(int val, Node previous, Node next)
            {
                this.val = val;
                this.next = next;
                this.previous = previous;
            }

            //property
            public int Value
            {
                get { return this.val; }
                set { this.val = Value; }
            }

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

            public Node Previous
            {
                get { return this.previous; }
                internal set { this.previous = value; }
            }
        }

        //field
        Node dummy;

        //constructor
        public BDLlist()
        {
            this.dummy = new Node(default(int), null, null);
            this.dummy.Next = this.dummy;
            this.dummy.Previous = this.dummy;
        }

        //property
        public Node First
        {
            get { return this.dummy.Next; }
        }
        public Node Last
        {
            get { return this.dummy.Previous; }
        }
        public Node End
        {
            get { return this.dummy; }
        }

        public int Count
        {
            get
            {
                int i = 0;
                for (Node n = this.First; n != this.End; n = n.Next)
                {
                    ++i;
                }
                return i;
            }
        }

        public string ListUp()
        {
            StringBuilder sb = new StringBuilder();
            Node current = this.First;
            for (; current != this.End; current = current.Next)
            {
                sb.Append(current.Value);
                if (current.Next != this.End) sb.Append(" ");
            }

            return sb.ToString();
        }

        public void InsertFirst(int elem)
        {
            this.InsertAfter(this.dummy, elem);
        }

        public void InsertAfter(Node n, int elem)
        {
            Node m = new Node(elem, n, n.Next);
            n.Next.Previous = m;
            n.Next = m;
        }

        public void Delete(Node n)
        {
            if (n == this.dummy)
            {
                return;
            }
            n.Previous.Next = n.Next;
            n.Next.Previous = n.Previous;
        }

        public void DeleteFirst()
        {
            this.Delete(this.First);
        }

        public void DeleteLast()
        {
            this.Delete(this.Last);
        }

        public void DeleteValue(int n)
        {
            Node current = this.First;
            if (current == null) return;
            for (; current.Next != this.End; current = current.Next)
            {
                if (current.Value == n)
                {
                    this.Delete(current);
                    return;
                }
            }
            if (current.Value == n)
            {
                this.Delete(current);
            }
        }
    }
}