module NQueen
  def expand(size, &block)
    size.times do |row|
      nqueen(size, [], row, &block)
    end
  end

  def nqueen(size, board, row, &block)
    board.each_with_index do |v, c|
      check = v - row
      return if check == 0 || check.abs == board.length - c
    end

    board = board + [row]

    if board.length == size
      yield(board)
      return
    end

    size.times do |r|
      nqueen(size, board, r, &block)
    end
  end
  module_function :expand, :nqueen
end

if __FILE__ == $0
  size = (ARGV.shift || '5').to_i
  nq = NQueen.expand(size) do |x|
    puts x.join(" ")
  end
end