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

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

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

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

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

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

yuuki

digits = Enumerator.new do |y|
  n = 2**1000
  while n > 0 do
    y << n % 10
    n /= 10
  end
end
puts digits.inject(:+)
# 1366

yuuki

print(sum((int(s) for s in list(str(2**1000)))))
# 1366

yuuki

data = %w/
75
95 64
17 47 82
18 35 87 10
20 04 82 47 65
19 01 23 75 03 34
88 02 77 73 07 63 67
99 65 04 28 06 16 70 92
41 41 26 56 83 40 80 70 33

yuuki

use 5.010;

# チート
use Date::Simple ('ymd');
use Date::Range;

my $range = Date::Range->new(ymd(1901, 1, 1), ymd(2000, 12, 31));
my @dates = grep { $_->day == 1 and $_->day_of_week == 0 } $range->dates;
say $#dates + 1;