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)

command_num = STDIN.gets.to_i
commands = STDIN.read.split("\n")

list_head = nil
list_tail = nil

commands.each do |command|
  func, arg =command.split(' ')
  case func
  when 'insert'
    if list_head.nil?
      # list is empty
      list_head = {value: arg, prev: nil, next: nil}
      list_tail = list_head
    else
      list_second = list_head
      list_head = {value: arg, prev: nil, next: list_second}
      list_second.update({prev: list_head})
    end
  when 'delete'
    current = list_head
    until current.nil? do
      if current[:value] == arg
        if current[:prev].nil? and current[:next].nil?
          # list becomes empty
          list_tail = nil 
          list_head = nil
        elsif current[:prev].nil?
          # delete first element
          list_head = current[:next]
          list_head.update({prev: nil})
        elsif current[:next].nil?
          # delete last element
          list_tail = current[:prev]
          list_tail.update({next: nil})
        else
          current[:prev].update({next: current[:next]})
          current[:next].update({prev: current[:prev]})
        end
        break
      else
        current = current[:next]
      end
    end
  when 'deleteFirst'
    unless list_head.nil?
      list_head = list_head[:next]
      list_head.update({prev: nil})
    end
  when 'deleteLast'
    unless list_tail.nil?
      list_tail = list_tail[:prev]
      list_tail.update({next: nil})
    end
  end
end
current = list_head
until current.nil? do
  print("#{current[:value]}")
  current = current[:next]
  print(" ") unless current.nil?
end
print("?\n")