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 {
    public class Node {
        private int _val;
        private Node _prev;
        private Node _next;

        public Node(int val, Node prev, Node next) {
            _val = val;
            _prev = prev;
            _next = next;
        }

        public int Value {
            get { return _val; }
            set { _val = value; }
        }

        public Node Next {
            get { return _next; }
            set { _next = value; }
        }

        public Node Prev {
            get { return _prev; }
            set { _prev = value; }
        }
    }

    public class DoublyLinkedList {
        private Node _dummy;

        public DoublyLinkedList() {
            _dummy = new Node(default(int), null, null);
            _dummy.Next = _dummy;
            _dummy.Prev = _dummy;
        }

        public Node First {
            get { return _dummy.Next; }
        }

        public Node Last {
            get { return _dummy.Prev; }
        }

        public Node End {
            get { return _dummy; }
        }

        public void AddAfter(int val) {
            Node n = ListSearch(val);
            AddAfter(n, val);
        }

        private void AddAfter(Node n, int val) {
            Node x = new Node(val, n, n.Next);
            n.Next.Prev = x;
            n.Next = x;
        }

        public void AddBefore(int val) {
            Node n = ListSearch(val);
            AddBefore(n, val);
        }

        private void AddBefore(Node n, int val) {
            Node x = new Node(val, n.Prev, n);
            n.Prev.Next = x;
            n.Prev = x;
        }

        public void Remove(int val) {
            Node n = ListSearch(val);
            Remove(n);
        }

        private void Remove(Node n) {
            if (n == _dummy)
                return;
            n.Prev.Next = n.Next;
            n.Next.Prev = n.Prev;
        }

        public void AddFirst(int val) {
            AddAfter(_dummy, val);
        }

        public void AddLast(int val) {
            AddBefore(_dummy, val);
        }

        public void RemoveFirst() {
            Remove(First);
        }

        public void RemoveLast() {
            Remove(Last);
        }

        private Node ListSearch(int val) {
            for (Node n = First; n != End; n = n.Next)
                if (n.Value == val)
                    return n;
            return _dummy;
        }

        public void Print() {
            StringBuilder sb = new StringBuilder();
            for (Node n = First; n != End; n = n.Next)
                sb.AppendFormat(" {0}", n.Value);
            Console.WriteLine(sb.ToString().TrimStart());
        }
    }

    class Program {
        static void Main(string[] args) {
            int n = int.Parse(Console.ReadLine());
            DoublyLinkedList dll = new DoublyLinkedList();
            for (int i = 0; i < n; i++) {
                string[] input = Console.ReadLine().Split(' ');
                switch (input[0]) {
                    case "insert":
                        dll.AddFirst(int.Parse(input[1]));
                        break;
                    case "delete":
                        dll.Remove(int.Parse(input[1]));
                        break;
                    case "deleteFirst":
                        dll.RemoveFirst();
                        break;
                    case "deleteLast":
                        dll.RemoveLast();
                        break;
                }
            }
            dll.Print();
        }
    }
}