docs: public Tree ancestor(int height, Tree root) {
          if (height < 0) {
              throw new IllegalArgumentException("ancestor: height cannot be negative");
          }
          if (height == 0) {
              return this;
          }
          Tree par = parent(root);
          if (par == null) {
              return null;
          }
          return par.ancestor(height - 1, root);
      }