Passing a variable by reference to a recurring function

Question

Is an argument to a function which is a reference to variable treated static in recursive function? Below is a function for finding kth smallest root in a BST.

int findNode(TreeNode* root, int &k) {
    if(root == NULL)
        return -1;
    // We do an inorder traversal here.
    int k1 = findNode(root->left, k);
    if(k == 0) return k1; // left subtree has k or more elements.
    k--;
    if(k == 0) return root->val; // root is the kth element.
    return findNode(root->right, k); // answer lies in the right node.
}

int kthsmallest(TreeNode* root, int k) {
    return findNode(root, k);  // Call another function to pass k by reference.
}

The function kthsmallest returns the kth smallest node's value.

Node definition:

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
}

My question is why is k passed by reference.

Answer 1

The meaning of k is relevant to the overall algorithm, not the individual call to findNode. It is like a countdown timer; the algorithm terminates when k reaches 0. All the recursive calls contribute to the same countdown.

Passing a reference to a variable in a calling scope solves a similar problem as static, but it's generally considered a superior technique in software engineering. Globals (e.g. static) limit a program's scalability.

The moral of the story is not to use names like k. Call it something like remaining_nodes.