Kth Smallest Element in a BST

class Solution {
public:
    int kthSmallest(TreeNode* root, int k) {
        int num = 0;
        TreeNode*pNode=NULL;
        visit(root, k, num, pNode);

        return pNode->val;
    }
    
private:
    void visit(TreeNode *root, int k, int &num, TreeNode*&pNode)
    {
        if (root == NULL)
        {
            return;
        }

        visit(root->left, k, num, pNode);
        
            
            if (++num == k)
            {
                pNode=root;
                return;
            }
        
        visit(root->right, k, num, pNode);
    }
};