Kth Largest Element in a BST - Given a binary search tree, write a function Largest to find the kth Largest element in it.
类似 Kth Smallest Element in a BST https://leetcode.com/problems/kth-smallest-element-in-a-bst/discuss/63660/3-ways-implemented-in-JAVA-(Python)%3A-Binary-Search-in-order-iterative-and-recursive ，instead of 先左后右，代码改成先右后左就行了