foo.rb

input = ["A/B/C/", "A/B/D/", "A/B/E/", "A/F/G/"]
mut = {}

def f(hash, str)
  left, rest = str.split("/", 2)

  return if left.empty?

  hash[left] ||= {}
  f(hash[left], rest) unless rest.empty?
end

input.each { |str| f(mut, str) }

puts mut.inspect