A code to compute the number of different letters a word has is desired. Write a function num_dif_letters_list(f) that provided a list of words f returns a list of numbers. Each number in the result list has to be the number of different letters of the corresponding word in f. All words in f consists of lowercase letters.
>>> num_dif_letters_list(['alea', 'iacta', 'est']) [3, 4, 3] >>> num_dif_letters_list(['', 'z', 'zz', 'zzz', 'zzzz']) [0, 1, 1, 1, 1] >>> num_dif_letters_list([]) [] >>> num_dif_letters_list(['atalaya', 'orinoco', 'pasas', 'elixir']) [4, 5, 3, 5]