public class Stack {
    CircularList contents;
    Stack() {
        contents = new CircularList();
    }
    public void push(Object x) {
        contents.insert(x);
    }
    public Object pop() {
        if (isEmpty())
           return null;
        else {
           Object topItem = contents.head();
           contents.delete();
           return topItem;
        }
    }
    public Object peek() {
        if (isEmpty())
           return null;
        else
           return contents.head();
    }
    public boolean isEmpty() {
        return (contents.isEmpty());
    }
    public String toString() {
        if (isEmpty())
           return "empty stack";
        else
           return "stack " + contents;
    }
}