code: public static Trie<String> toStringTrie(List<String> words) {
          final TrieBuilder<String> builder = new TrieBuilder<>();
          for (String word : words) {
              builder.add(word, "");
          }
          return builder.build();
      }
docs: Sometimes a Trie's values aren't important, only the words matter.
  Convenience method that associates each word with an empty value.
  
  @param words Words to put in the Trie.
  @return A Trie where each word is associated to an empty value.