#include "Vstack.h"
#include <assert.h>
template<class T>
    class Link_stack : public Gstack<T> {
    	struct node {
    		T data;
    		node* next;
    		node(T d, node* n) 
			    { data = d; next = n; }
    	};
    	node* top;
    public:
    	Link_stack( ) 
		    { top = NULL; }
    	void push(T x) 
		    { top = new node(x, top); }
    	T pop() {
        		assert( !isempty() );
        		T t = top -> data;
        		node* oldtop = top;
        		top = top -> next;
        		delete oldtop;
        		return t;
    	}

    	bool isempty() 
		    { return( top == NULL ); }
    	bool isfull() 
		    { return( FALSE ); }
    };