//public:
template<typename T>
void BST<T>::insert(T elem) {
    root = insert(elem, root);
}

//private:
    Node *insert(T elem, Node *node) {
        if (node == nullptr) {
            node = new Node(elem);
        } else if (node->data < elem) {
            node->right = insert(elem, node->right);
        } else {
            node->left = insert(elem, node->left);
        }
        return node;
    }