/**

 * Definition for a binary tree node.

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

/**

BST中序遍历是从小到大排列的因此对其进行中序遍历然后取第k个元素；

**/

class Solution {

public:

    int kthSmallest(TreeNode* root, int k) {

        if(root==NULL) return ;

        int res=,i=;

        stack<TreeNode*> s;

        TreeNode* p=root;

        s.push(root);

        while(!s.empty()){

            while(p&&p->left){

                p=p->left;s.push(p);

            }

            p=s.top();s.pop();

            if(i==k) {res=p->val;break;}

            i++;

            p=p->right;

            if(p) s.push(p);

        }

        return res;

    }

};