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: Wrong Answer)

class List
  def initialize
    @first = nil
    @last = nil
  end

  def [] index
    current_item = @first

    # ?????????????????????
    index.times {|item|
      return nil if current_item.next.nil?
      current_item = current_item.next
    }

    current_item
  end

  def insert val
    new_item = ListItem.new val
    link new_item, @first
    @first = new_item
    @last = new_item if @first.nil?
  end

  def search target
    current_item = @first

    loop do
      return nil          if current_item.nil?
      return current_item if current_item.value == target
      current_item = current_item.next
    end
  end

  def delete target
    target_item = search target
    return false if target_item.nil?

    link target_item.prev, target_item.next

    # ??????????±????????????????
    @first = target_item.next if target_item.prev.nil?

    # ??????????±????????????????
    @last = target_item.prev if target_item.next.nil?

    return true
  end

  def delete_first
    return false if @first.nil?
    @first = @first.next
    link nil, @first
    @first = @last = nil if @first.nil?
    true
  end

  def delete_last
    return false if @last.nil?
    @last = @last.prev
    link @last, nil
    @first = @last = nil if @last.nil?
    true
  end

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

  private

  def link first, second
    first.next = second unless first.nil?
    second.prev = first unless second.nil?
  end

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

def main commands
  list = List.new

  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)