#ifndef old_h
#define old_h

#include <cstdlib>  // malloc, free
#include <optional> // std::optional

template<typename T> class stack
{
public:
    ~stack() { while (pop()); }

    void push(T t)
    {
        struct node *new_node = (struct node *)malloc(sizeof(struct node));
        new_node->next = head;
        new_node->data = t;
        head = new_node;
    }

    std::optional<T> pop()
    {
        if (head != NULL)
        {
            struct node *copy = head;
            head = copy->next;
            T data = copy->data;
            free(copy);
            return std::optional<T>(data);
        }
        else
        {
            return std::nullopt;
        }
    }

private:
    struct node
    {
        struct node *next;
        T data;
    };

    struct node *head = NULL;
};

#endif // old_h