[leetcode] 230.Kth Smallest Element in a BST

题目：
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?
题意：
给一个二叉搜索树，寻找第k大元素。
思路：
结合二叉搜索树的特点，对二叉搜索树进行中序遍历的结果就是按照元素的升序排列。
所以采用中序遍历，并且保存遍历到当前位置的下标，如果到了k的位置，那么赋值，并且返回。
以上。
代码如下：

/** * 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) {     if(root == NULL)return 0;     int value = 0;     int index = 0;     InOrder(root, k, value, index);     return value;    }    void InOrder(TreeNode* root, int k, int& value, int& before) {        if(root == NULL)return;        InOrder(root->left, k, value, before);        before++;        if(before == k){            value = root->val;            before = k;            return;        }        else if(before > k)return;        InOrder(root->right, k, value, before);    }};