import java.util.*;
public class Stack {
    private List contents;
    public Stack() {
        contents = new ArrayList();  // or LinkedList or whatever
    }
    public void push(Object x) {
        contents.add(x); // adds to end
    }
    public Object pop() {
        if (isEmpty())
           return null;
        else {
           Object ans = contents.remove(contents.size()-1);
           return ans;
        }
    }
    public Object peek() {
        if (isEmpty())
           return null;
        else {
           Object ans = pop();
           push(ans);
           return ans;
        }
    }
    public boolean isEmpty() {
        return (contents.size() == 0);
    }
    public String toString() {
        if (isEmpty())
           return "empty stack";
        else
           return "stack " + contents;
    }
}