k-th smallest element in BST

Given a BST and an integer K. Find the Kth Smallest element in the BST.

• Time : O(k)
• Space : O(1)

c++

class Solution {
    int res = -1;
    void travel(struct Node* node, int k, int& counter) {
        if(!node) return;
        if(counter > k) return;
        travel(node->left, k, counter);
        if(k == counter) res = node->data;
        counter++;
        travel(node->right, k, counter);
    }

public:
    // Return the Kth smallest element in the given BST
    int KthSmallestElement(Node *root, int K) {
        res = -1;
        int counter = 1;
        travel(root, K, counter);
        return res;
    }
};