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;
using System.Text;

namespace AOJ
{
    class Program
    {
        static void Main(string[] args)
        {
            var input = Console.ReadLine().Split();
            var n = int.Parse(input[0]);
            var list = new List<string>();

            for(var i = 0; i < n; i++){
                input = Console.ReadLine().Split();
                if(input[0] == "insert"){
                    list.insert(input[1]);
                }else if(input[0] == "delete"){
                    list.delete(input[1]);
                }else if(input[0] == "deleteFirst"){
                    list.deleteFirst();
                }else if(input[0] == "deleteLast"){
                    list.deleteLast();
                }
            }
            Console.WriteLine(list.ToString());
        }
    }


    class List<T>
    {
        private Cell<T> _head;
        private Cell<T> _tail;

        public List(){
            _head = new Cell<T>();
            _tail = new Cell<T>();
            _head._next = _tail;
            _tail._pred = _head;
        }

        public void insert(T data){
            var c = new Cell<T>(data, _head, _head._next);
            _head._next._pred = c;
            _head._next = c;
        }

        public void delete(T data){
            var c = search(data);
            del(c);
        }

        public void deleteFirst(){
            var first = _head._next;
            del(first);
        }

        public void deleteLast(){
            var last = _tail._pred;
            del(last);
        }

        public override string ToString(){
            var current = _head._next;
            var result = "";
            if(current != _tail){
                var builder = new StringBuilder(current._data.ToString());
                current = current._next;
                while(current != _tail){
                    builder.Append($" {current._data}");
                    current = current._next;
                }

                result = builder.ToString();
            }

            return result;
        }

        private Cell<T> search(T data){
            var current = _head._next;
            while((current != _tail) && (!current._data.Equals(data))){
                current = current._next;
            }
            return current;
        }

        private void del(Cell<T> c){
            var pred = c._pred;
            var next = c._next;

            pred._next = next;
            next._pred = pred;
        }

        class Cell<Tdata>
        {
            public Cell<Tdata> _next;
            public Cell<Tdata> _pred;
            public Tdata _data;

            public Cell() { }
            public Cell(Tdata data, Cell<Tdata> pred, Cell<Tdata> next)
            {
                _data = data;
                _pred = pred;
                _next = next;
            }

        }
    }
}