q2.rb

module Q2
  PATTERN = [
    [1], [2], [3], [4], [5], [6], [7], [8], [9],
    [1, 2],
    [2, 3],
    [3, 6],
    [6, 9],
    [9, 8],
    [8, 7],
    [7, 4],
    [4, 1]
  ]

  def self.hit(board, target)
    # いずれかが存在しなければ不正な投球
    target.each { |n| return 0 unless board.include?(n) }

    hit_board = board - target

    # 空になったら終わり
    return 1 if hit_board.empty?

    # 残りのパターンを網羅
    return PATTERN.map { |target| self.hit(hit_board, target) }.inject(:+)
  end

  def self.strike
    PATTERN.map { |target| self.hit((1..9).to_a, target) }.inject(:+)
  end
end

puts "answer : #{Q2.strike}"