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
  class Cell
    attr_accessor :val, :prev, :next
    def initialize(v,p=nil,n=nil)
      @val = v
      @prev = p
      @next = n
    end
  end

  def insert(x)
    o = Cell.new(x)
    @top.prev = o unless @top == nil
    o.next = @top
    @top = o
    @bottom = o if @bottom == nil
  end

  def delete(x)
    p = @top
    until p == nil || p.val == x
      p = p.next
    end
    unless p == nil
      if p == @top
        @top = p.next
      else
        p.prev.next = p.next
      end
      if p == @bottom
        @bottom = p.prev
      else
        p.next.prev = p.prev
      end
    end
  end

  def deleteFirst
    unless @top == nil
      @top = @top.next
      @top.prev = nil
    end
  end

  def deleteLast
    unless @bottom == nil
      @bottom = @bottom.prev
      @bottom.next = nil
    end
  end

  def show
    p = @top
    until p == nil
      print " " unless p == @top
      print p.val
      p = p.next
    end
    print "\n"
  end
end

n = gets.to_i
list = List.new
for i in 1..n
  com,v = gets.split
  case com
  when "insert"
    list.insert(v.to_i)
  when "delete"
    list.delete(v.to_i)
  when "deleteFirst"
    list.deleteFirst
  when "deleteLast"
    list.deleteLast
  end
end
list.show