230. Kth Smallest Element in a BST(unsolved)

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

Note:
You may assume k is always valid, 1 ≤ k ≤ BST’s total elements.

Follow up:
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?

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    int priorsearch(TreeNode* root, int& k){        if(root)        {            int x=priorsearch(root->left,k);            if(k==0) return x;            else            {                k--;                if(k==0)                {                    return root->val;                }                else                    return priorsearch(root->right,k);            }        }        return 0;    }    int kthSmallest(TreeNode* root, int k) {        return priorsearch(root,k);    }};