public class intstack implements IntStackI {
    private intlistnode head=null;

    public void push(int n) {
        head = new intlistnode(n, head);
    }
    public int pop() {
        int v = head.val;
        head = head.next;
        return v;
    }
    public boolean isempty() {
        return head==null;
    }
    public String toString() {
        String s="";
        for (intlistnode n=head; n!=null; n = n.next)
            s = n.val+" "+s;
        return s;
    }
    private class intlistnode {
        private int val=0;
        private intlistnode next=null;
        intlistnode(int v, intlistnode l) {
            val = v;
            next = l;
        }
    }
}