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
    @head = Node.new
    @tail = Node.new(@head)
    @head.nxt = @tail
  end

  def insert(val)
    nxt_old = @head.nxt
    node = Node.new(@head, nxt_old, val)
    @head.nxt = node
    nxt_old.prv = node
  end

  def delete(val)
    node = @head.nxt
    while node.val
      if node.val == val
        node.prv.nxt = node.nxt
        node.nxt.prv = node.prv
        return
      end
      node = node.nxt
    end
  end

  def delete_first
    raise 'List is mpety.' if @head.nxt == @tail
    @head.nxt = @head.nxt.nxt
    @head.nxt.prv = @head
  end

  def delete_last
    raise 'List is mpety.' if @head.nxt == @tail
    @tail.prv = @tail.prv.prv
    @tail.prv.nxt = @tail
  end

  def to_s
    vals = []
    node = @head.nxt
    while node.val
      vals << node.val
      node = node.nxt
    end
    vals.join(' ')
  end

  class Node
    attr_accessor :prv, :nxt, :val
    def initialize(prv = nil, nxt = nil, val = nil)
      @prv = prv
      @nxt = nxt
      @val = val
    end
  end
end

list = List.new

STDIN.read.split(/\n/)[1..-1].each do |line|
  ary = line.split(/\s/)
  command = ary[0]
  #start = Time.now
  case command
  when 'insert' then
    list.insert ary[1].to_i
  when 'delete' then
    list.delete ary[1].to_i
  when 'deleteFirst' then
    list.delete_first
  when 'deleteLast' then
    list.delete_last
  end
end