template<class T>
class List<T> {
    class Node{
        T data;
        Node * next;
        ...
    };
    Node * head;
    ...
    ~List() {/*iterating through all nodes and deleting them*/}
    ...
};

template<class T>
class Tree<T> {
    class Node {
        Node * next;
        ...
        T key;
        List<int> internal_list;
    };
    destroy_node(Node * x) { delete x; }
};