euler5.rb

#!/usr/bin/env ruby
# -*- coding: utf-8 -*-

require 'prime'

factories_count = {}
Prime.each(20) {|i| factories_count[i] = 0}
(2..20).each do |n|
  factories_count.each_key do |k|  # 素因数分解
    break if n < k
    t = n # copy
    count = 0
    while t >= k do
      count += 1 if t % k == 0
      t /= k
    end
    factories_count[k] = count if factories_count[k] < count
  end
end
puts factories_count.map { |k, v| k.to_i * v }.inject(1) { |product, i| product * i }