Given a binary search tree, write a function
kthSmallest to find the kth smallest element in it.
You may assume k is always valid, 1 ≤ k ≤ BST’s total elements.
Input: root = [3,1,4,null,2], k = 1
Input: root = [5,3,6,2,4,null,null,1], k = 3
What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?
借助两个全局变量，一个存放结果，一个存放当前还需要遍历的节点。中序遍历，直到节点遍历完 k 个，返回结果。