n_queens.rb

  def northwest(file, rank)
    file + rank
  end

  def northeast(file, rank)
    file + @size - rank
  end

  def unsafe?(file, rank)
    @queens.any? do |f, r|
      r == rank ||
        northwest(f, r) == northwest(file, rank) ||
        northeast(f, r) == northeast(file, rank)
    end
  end

  def move!(file, rank)
    @queens.push([file, rank])
  end

  def unmove!(_file, _rank)
    @queens.pop
  end