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
  def initialize
    @head = Node.new(nil)
    @tail = Node.new(nil)
    @head.next = @tail
    @tail.prev = @head
  end

  def insert(x)
    v = Node.new(x)
    left, right = @head, @head.next
    left.next, right.prev = v, v
    v.prev, v.next = left, right
  end

  def deleteFirst
    @head.next.unlink
  end

  def deleteLast
    @tail.prev.unlink
  end

  def delete(x)
    u = @head.next
    until u == @tail
      v = u.next
      if u.value == x
        u.unlink 
        return
      end
      u = v
    end
  end

  def to_s
    a = []
    v = @head.next
    until v == @tail
      a << v.value
      v = v.next
    end
    a * " "
  end

  class Node
    def initialize(x)
      @value = x
      @next = @prev = nil
    end

    attr_accessor :value, :next, :prev

    def unlink
      left, right = @prev, @next
      left.next = right
      right.prev = left
    end
  end
end

n = gets.to_i
list = List.new
n.times do
  command, x = gets.chomp.split
  if x 
    list.send(command, x.to_i)
  else
    list.send(command)
  end
end
puts list