public class Stack {
    ListItem contents;
    Stack() {
        contents = null;
    }
    public void push(Object x) {
        contents = new ListItem(x,contents);
    }
    public Object pop() {
        if (isEmpty())
           return null;
        else {
           Object topItem = contents.item;
           contents = contents.next;
           return topItem;
        }
    }
    public Object peek() {
        if (isEmpty())
           return null;
        else
           return contents.item;
    }
    public boolean isEmpty() {
        return (contents == null);
    }
    public String toString() {
        if (isEmpty())
           return "empty stack";
        else
           return "stack " + contents;
    }
}