230. 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?BST的中序遍历为升序，找到第k个节点即可*//** * 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) {        int cnt=k;        int target;        inorder(root,cnt,target);        return target;    }    void inorder(TreeNode* root,int &k,int &target)    {        if(!root)return;        if(root->left)inorder(root->left,k);        k--;        if(k==0){            target=root->val;            return;        }        inorder(root->right,k);    }};