Kth Smallest in bst

class Solution {
public:
    void f(TreeNode* root, int k, int &counter, int &ans){
        if(root==NULL)return;

        f(root->left, k, counter,ans);
        counter++;
        if(counter==k){
            ans = root->val;
        }
        f(root->right, k,counter,ans);

    }
    int kthSmallest(TreeNode* root, int k) {
        int counter = 0;
        int ans;
        f(root, k, counter, ans);

        return ans;

    }
};