# Description

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.

The original problem is here.

The original code is here.

# My Solution

I solve this problem in C++, as below:

``````/*
*Kth Smallest Element in a BST
*Author: shuaijiang
*Email: zhaoshuaijiang8@gmail.com
*/
#include<iostream>
#include<stack>
#include<stdlib.h>
using namespace std;

/**
* 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*> myStack;
vector<int> result;
if(root==NULL)
return 0;
myStack.push(root);

while(!myStack.empty()){
TreeNode * node = myStack.top();
myStack.pop();
if(node->left == NULL && node->right == NULL)
result.push_back(node->val);
else{
if(node->right != NULL)
myStack.push(node->right);
TreeNode *left = node->left;
node->left = NULL;
node->right = NULL;
myStack.push(node);
if(left != NULL)
myStack.push(left);
}
if(result.size()==k)
return result[k-1];
}
}
};
``````

# Note

To solve the problem, I traversal the BST use a stack, and pop the stack from samll element to large. Finally, return the kth smallest element.