code: private Node<T> get(Node<T> x, long key) {
          while (x != null) {
          	if (x.value == key) return x;
          	if (key < x.value) x = x.left;
          	else x = x.right;
          }
          return null;
      }
docs: value associated with the given key in subtree rooted at x; null if no such key