str_rev.rb

#!/usr/bin/env ruby

str = "abcde"
cnt = str.length - 1
rev = []

str.split(//).each { |c|
  rev[cnt] = c
  cnt = cnt - 1
}
puts rev.to_s

str_rev2.rb

#!/usr/bin/env ruby

def reverse(str,rev = [])
  rev << str.pop
  reverse(str,rev) if !str.empty?
  rev.to_s
end

str = "abcde".split(//)
str_rev = reverse(str)
puts str_rev