階乗、順列の数、組み合わせの数を求める

IntegerMath.rb

class Integer
  def combination(k)
    self.factorial/(k.factorial*(self-k).factorial)
  end

  def permutation(k)
    self.factorial/(self-k).factorial
  end

  def factorial
    return 1 if self == 0
    (1..self).inject(:*)
  end
end