code: @Nullable
      @VisibleForTesting
      Node<V> findNode(String path, boolean exact) {
          requireNonNull(path, "path");
          return findNode(root, path, 0, exact);
      }
docs: Returns a {@link Node} which is mapped to the given {@code path}.
  If {@code exact} is {@code true}, internally-added node may be returned.