Array#reverse

def reverse!(n)
  (n.length / 2).times { |i| n[-(i + 1)], n[i] = n[i], n[-(i + 1)] }
  n
end

def reverse(n)
  n.length.downto(1).each_with_object([]) { |i, lst| lst << n[i - 1] }
end

p reverse! [1, 2, 3, 4]
p reverse! [1, 2, 3, 4, 5]
p reverse [1, 2, 3, 4]
p reverse [1, 2, 3, 4, 5]

FizzBuzz

def fizzbuzz(n)
  if n % 15 == 0
    "FizzBuzz"
  elsif n % 5 == 0
    "Buzz"
  elsif n % 3 == 0
    "Fizz"
  else
    n
  end
end

100.times do |i|
  puts fizzbuzz i
end

フィボナッチ数列

def fib(num)
  case num
  when 0, 1
    num
  else
    fib(num - 1) + fib(num - 2)
  end
end

素数

def sosuu
  (2..100).to_a.each_with_object([]) { |i, sosuus| sosuus << i if sosuus.none? { |j| (i % j).zero? } }
end

p sosuu