class Queens
  def initialize(nqueens = 8)
    @col = []
    @qpos = []
    @up = []
    @down = []
    @nqueens = nqueens
    (0...@nqueens).each { |i|
      @col[i] = @qpos[i] = 0
    }
    (-@nqueens...@nqueens).each { |i|
      @up[i+@nqueens] = 0
      @down[i+@nqueens] = 0
    }
  end

  def out(block)
    if block then
      block.call(@qpos)
    else
      puts((0...@nqueens).collect { |i| @qpos[i].to_s }.join(" "))
    end
  end
  
  def expand(n,block)
    (0...@nqueens).each { |c|
      if @col[c] == 0 && @up[n+c] == 0 &&
          @down[n-c+@nqueens] == 0 then
        @qpos[n] = c
        if n+1 >= @nqueens then
          out(block)
        else
          @col[c] = @up[n+c] = @down[n-c+@nqueens] = 1
          expand(n+1,block)
          @col[c] = @up[n+c] = @down[n-c+@nqueens] = 0
        end
      end
    }
  end
  
  def run(&block)
    while true do
      expand(0,block)
    end
  end
end

if __FILE__ == $0 then
  Queens.new.run
end