yuuki

def fact(n)
  n == 1 and return 1
  n*fact(n-1)
end

puts fact(40) / fact(20) / fact(20)

yuuki

MAXSIZE = 1000000
MEMO = Array.new(MAXSIZE)

def collatz_size(i)
  size = 0
  n = i
  while n != 1 do
    if n <= MAXSIZE && MEMO[n]
      size += MEMO[n]
      break

yuuki

def collatz(i)
  Enumerator.new do |y|
    n = i
    while n != 1 do
      n = n.even? ? n/2 : 3*n + 1
      y << n
    end
  end
end

yuuki

puts [
  37107287533902102798797998220837590246510135740250,
  46376937677490009712648124896970078050417018260538,
  74324986199524741059474233309513058123726617309629,
  91942213363574161572522430563301811072406154908250,
  23067588207539346171171980310421047513778063246676,
  89261670696623633820136378418383684178734361726757,
  28112879812849979408065481931592621691275889832738,
  44274228917432520321923589422876796487670272189318,
  47451445736001306439091167216856844588711603153276,

yuuki

r = (2..2000000).select do |i|
  !(2..Math.sqrt(i).floor).find {|j| i % j == 0 }  # 素数判定
end.inject(:+)
puts r
# 142913828922
# 30.40s with Core i5 mem 4G

yuuki

(1..1000).each do |a|
  (a..(1000 - a)).each do |b|
    c = 1000 - a - b
    puts a*b*c or exit if a**2 + b**2 == c**2
  end
end

# 31875000

yuuki

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

N = 731671765313306249192251196744265747423553491949349698352031277450632623957831801698480186947885184385861560789112949495459501737958331952853208805511125406987471585238630507156932909632952274430435576689664895044524452316173185640309871112172238311362229893423380308135336276614282806444486645238749303589072962904915604407723907138105158593079608667017242712188399879790879227492190169972088809377665727333001053367881220235421809751254540594752243525849077116705560136048395864467063244157221553975369781797784617406495514929086256932197846862248283972241375657056057490261407972968652414535100474821663704844031998900088952434506585412275886668811642717147992444292823086346567481391912316282458617866458359124566529476545682848912883142607690042242190226710556263211111093705442175069416589604080719840385096245544436298123098787992724428490918884580156166097919133875499200524063689912560717606058861164671094050775410022569831552000559357297257163626956188267042825248

yuuki

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

require 'prime'

Prime.each.with_index do |n, i|
  puts n or break if (i + 1) == 10001
end

# スクリプトキディ

yuuki

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

n = (1..100).each.inject {|sum, i| sum + i }
r = (1..100).each.inject {|sum, i| sum + i**2 }
puts n**2 - r

yuuki

#!/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