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 Ruby (judge verdict: Runtime Error)

class List
  def initialize n
    @first = ListItem.new n
  end

  def [] index
    current_item = @first

    # ?????????????????????
    index.times {|item|
      if current_item.next.nil?
        raise '?????£'
      end
      current_item = current_item.next
    }

    current_item
  end

  def insert val
    new_item = ListItem.new val

    # ????????????????????????
    new_item.next = @first
    @first.prev = new_item

    @first = new_item
  end

  def delete target
    current_item = @first

    loop do

      if  current_item.nil?
        raise '????±??????????????????????'
      end

      if current_item.n == target
        # ????????????
        prev_item = current_item.prev
        next_item = current_item.next
        prev_item.next = next_item unless prev_item.nil?
        next_item.prev = prev_item unless next_item.nil?
        # ???????????????????????????
        if prev_item.nil?
          @first = next_item
        end
        return
      end

      current_item = current_item.next
    end

  end

  def delete_first
    @first = @first.next
    @first.prev = nil
  end

  def delete_last
    current_item = @first
    # ???????????§?????????
    current_item = current_item.next until current_item.next.nil?
    # ??????????????????????????¢??????????????????????????????
    current_item.prev.next = nil
  end

  def print delimiter = ' '
    print_items = []
    current_item = @first
    until current_item.nil?
      print_items << current_item.n
      current_item = current_item.next
    end
    puts print_items.join delimiter
  end

  class ListItem
    def initialize n
      @n = n
      @next = nil
      @prev = nil
    end
    attr_accessor :next, :prev, :n
  end
end

def main commands
  # ??¨???????????????????????????insert??¨?????????
  list = List.new commands.shift.split(' ')[1]

  commands.each do |raw_command|
    #list.print
    command, arg = raw_command.split ' '
    case command
    when 'insert'
      list.insert arg
    when 'delete'
      list.delete arg
    when 'deleteFirst'
      list.delete_first
    when 'deleteLast'
      list.delete_last
    else
      raise '??????'
    end
  end
  list.print
end


inputs = STDIN.read.split(/\n/)
inputs.shift

main(inputs)