eight-queen.rb

require 'set'

COL_NUM = 8

def queen()
  place([], 1, COL_NUM).select{ |l| l.length == 8 }
end

def place(accum, col, col_num)
  available_squares = find_available_squares(accum, col_num)
  return available_squares.map{ |square| accum + [square] } if col == col_num
  return [accum] if available_squares.empty?
  return available_squares.flat_map{ |square| place(accum + [square], col+1, col_num) }
end

def find_available_squares(selected_squares, col_num)
  squares = (1..col_num).to_a
  current_col = selected_squares.length + 1
  selected_squares.each_with_index do |place, i|
    diff_between_cols = current_col - (i + 1)
    squares.delete(place + diff_between_cols) # 斜め下方向のセルを排除
    squares.delete(place - diff_between_cols) # 斜め上方向のセルを排除
    squares.delete(place)                     # 同一行のセルを排除
  end
  squares
end

puts "組み合わせ数: #{queen().length}"
queen().each {|pat| p pat}