Leetcode101: Kth Smallest Element in a BST

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:    void InOrder(TreeNode* Node, vector<int>& res)    {        if(Node->left)            InOrder(Node->left, res);        res.push_back(Node->val);        if(Node->right)            InOrder(Node->right, res);    }        int kthSmallest(TreeNode* root, int k) {        vector<int> res;        InOrder(root, res);        return res[k-1];    }};