public class stack<A> implements StackI<A> {
    private listnode<A> head=null;

    public void push(A n) {
        head = new listnode<A>(n, head);
    }
    public A pop() {
        A v = head.val;
        head = head.next;
        return v;
    }
    public boolean isempty() {
        return head==null;
    }
    public String toString() {
        String s="";
        for (listnode<A> n=head; n!=null; n = n.next)
            s = n.val+" "+s;
        return s;
    }
    private class listnode<A> {
        private A val;
        private listnode<A> next=null;
        listnode(A v, listnode<A> l) {
            val = v;
            next = l;
        }
    }
}