// UndoStack is a variation of the Stack class that supports an undo operation
// for reversing the effect of push and pop operations.  The canUndo method
// indicates whether an undo is legal.

import java.util.*;

public class UndoStack<E> extends Stack<E> {
    private Stack undoStack;

    // post: constructs an empty UndoStack
    public UndoStack() {
        undoStack = new Stack();
    }

    // post: pushes and returns the given value on top of the stack
    public E push(E value) {
        super.push(value);
        undoStack.push("push");
        return value;
    }

    // post: pops and returns the value at the top of the stack
    public E pop() {
        E value = super.pop();
        undoStack.push(value);
        undoStack.push("pop");
        return value;
    }

    // post: returns whether or not an undo can be done
    public boolean canUndo() {
        return !undoStack.isEmpty();
    }

    // pre : canUndo() (throws IllegalStateException if not)
    // post: undoes the last stack push or pop command
    public void undo() {
        if (!canUndo()) {
            throw new IllegalStateException();
        }
        if (undoStack.pop().equals("push")) {
            super.pop();
        } else {
            E value = (E) undoStack.pop();         
            super.push(value);
        }
    }
}