Project Euler Problem 34

問題

145 is a curious number, as 1! + 4! + 5! = 1 + 24 + 120 = 145.

Find the sum of all numbers which are equal to the sum of the factorial of their
digits.

Note: as 1! = 1 and 2! = 2 are not sums they are not included.

ソース

class Integer
	@@fact_cache = [1]
	def fact
		f = @@fact_cache[self]
		return f unless f.nil?
		@@fact_cache[self] = (1..self).inject(:*)
	end
end

puts (3..100000).inject(0){|s, i|
	s + (i == i.to_s.scan(/./).inject(0){|s, c| s + c.to_i.fact} ? i : 0)
}

解答

40730

感想

コレもそのまま組むだけで、特に難しいところはない。（ループ回数はいつものごとく決め打ちだが。。。
factは0-9のみなのでキャッシュしておくと速い