Welcome! Today, we will delve into managing a document's editing history using stacks. Imagine building a text editor; you would need to handle actions like adding text, undoing, and redoing those changes. We will see how these features can be efficiently implemented using stacks. By the end of this lesson, you will possess an in-depth understanding of applying stacks in practical scenarios.

Before starting the coding portion, let's dissect the methods we will implement. These methods will manage a document's edit history, allowing us to apply changes, undo them, and redo them effectively.

• applyChange(String change): This method applies a change to the document. The change, represented as a string, is stored in a way that allows us to remember the order of applied changes. Any previously undone changes are discarded.
• undo(): This method undoes the most recent change and allows us to store it for a possible redo. It returns the change that was undone. If there are no changes available to undo, it returns null.
• redo(): This method redoes the most recent undone change, making it active again. It returns the change that was redone. If there are no changes available to redo, it returns null.
• getChanges(): This method returns an array of all applied changes in the order they were applied.