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Author: Robert Baxter

Assume there is a class StringStack that provides an implementation of a stack of String objects:

Method/Constructor Description
public StringStack() constructs an empty StringStack
public void push(String s) pushes s onto the top of the stack
public String pop() removes and returns the top of the stack
public boolean isEmpty() true if stack is empty, false otherwise
public int size() returns the number of elements in the stack

Write a class UndoStack that extends the functionality of the StringStack class to allow an "undo" operation. Any push or pop can be undone by a call to undo. If several push and/or pop operations have been performed, it should be possible to perform several undos in a row, each reversing the most recent operation. If no push or pop operations have been performed, or if they have all been undone, there is nothing to undo. Provide a mechanism for checking whether or not there is something left to undo.

Method/Constructor Description
public UndoStack() constructs an empty UndoStack
public void undo() undoes most recent push/pop that hasn't already been undone (throws IllegalStateException if nothing to undo)
public boolean canUndo() returns true if there is something to undo, false otherwise

You may use your own data structures, but you must rely on the existing class to do basic stack operations.

Type your solution here:

This is an inheritance problem. Write a Java class using inheritance. (You do not need to write any import statements.)

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