Kth Smallest Element in a BST

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) 
    {
        vector<int>l;
        preorderTraversal(root, l);
        sort(l.begin(),l.end());
        return l[k-1];
    }
    void preorderTraversal(TreeNode* root, vector<int> &l) 
    {
        if (root != NULL) {
            l.push_back(root->val);
            preorderTraversal(root->left, l);
            preorderTraversal(root->right, l);
        }
    }
};