https://leetcode.cn/problems/kth-smallest-element-in-a-bst/description/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void preorder(TreeNode *root, vector<int> &res)
    {
        if(root == nullptr)
        {
            return;
        }
        res.push_back(root->val);
        preorder(root->left, res);
        preorder(root->right, res);
    }

    int kthSmallest(TreeNode* root, int k) 
    {
        vector<int> res;

        preorder(root, res);

        sort(res.begin(), res.end());

        return res[k - 1];

    }
};