code: public V get(K[] key) {
          Node child = root.get(key[0]);
          if (child != null) {
              return child.getChild(key, 1);
          }
          return null;
      }
docs: Returns the associated value of a given key.
  Returns null if the key doesn't exist in the trie.
  @param key the key.
  @return the associated value or null.