Kth Smallest Element in a BST -- leetcode

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?

基本思路：

进行中序遍历。

/** * 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 kthSmallest(TreeNode* root, int k) {        stack<TreeNode *> s;        while (!s.empty() || root) {            while (root) {                s.push(root);                root = root->left;            }            root = s.top();            s.pop();            if (!--k)                return root->val;            root = root->right;        }        return 0;    }};