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: Accepted)

class List
  attr_accessor :value, :prev, :next

  def initialize(value, prev, nxt)
    @value = value
    @prev = prev
    @next = nxt
  end
end

@bampei = List.new(nil, nil, nil)

def insert(x)
  list = List.new(x, nil, @bampei.prev)
  if(@bampei.prev.nil?)
    @bampei.prev = list
    @bampei.next = list
  else
    @bampei.prev.prev = list
    @bampei.prev = list
  end
end

def delete(x)
  target = @bampei.prev
  while(!target.nil?)
    if target.value == x
      if target.next && target.prev
        target.next.prev = target.prev
        target.prev.next = target.next
      elsif target.next
        target.next.prev = nil
        @bampei.prev = target.next
      elsif target.prev
        @bampei.next = target.prev
        target.prev.next = nil
      else
        @bampei.prev = nil
        @bambei.next = nil
      end
      break
    end
    target = target.next
  end
end

def deleteFirst
  @bampei.prev = @bampei.prev.next
  if @bampei.prev
    @bampei.prev.prev = nil
  end
end

def deleteLast
  if(@bampei.next&.prev)
    @bampei.next.prev.next = nil
    @bampei.next = @bampei.next.prev
  else
    @bampei.next = nil
    @bampei.prev = nil
  end
end

n = gets.to_i
n.times do
  input = gets.split(" ")
  if input.size == 2
    send(input[0], input[1].to_i)
  else
    send(input[0])
  end
end

target = @bampei.prev
outputs = []
while(!target.nil?)
  outputs << target.value
  target = target.next
end
puts outputs.join(' ')