code: public int size()
      {
          int size = 0;
          if (value != null)
              size++;
          for (Trie<K, V> child : children.values())
              size += child.size();
          return size;
      }
docs: Returns the number of mappings in this {@link Trie}
  
  @return The number of mappings in this {@link Trie}