reverse.rb

#!/usr/bin/env ruby

def reverse(n, reversing = 0)
  if n == 0
    reversing
  else
    reverse(n / 10, n % 10 + reversing * 10)
  end
end

puts reverse(12345)



def reverse(n)
  reversed = 0

  while n / 10 > 0
    rem = n % 10

    reversed *= 10
    reversed += rem

    n /= 10
  end

  reversed  * 10 + n
end

puts reverse(12345)