求第k大的树通过伸展树的特点，每次查询后自动伸展将目标节点通过左旋，右旋直至根节点，这样下一个最大值就可以通过左子树的最右端子树求得。(The largest K tree can be extended automatically after each query, and the target node is left, right to the root node, so that the next maximum value can be obtained through the most right terminal tree of the left subtree.)