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 it, 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(change: String): 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(): String?: 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(): String?: 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(): List<String>: This method returns a list of all applied changes in the order they were applied.

To implement these methods efficiently, we'll use two stacks. We use stacks to keep track of applied changes and undone changes because of their LIFO (Last In, First Out) property. The most recent change is always at the top, making it easy to undo and redo.

Let's break down each method's implementation and study how they interact with the stacks.

Let's implement the solution step-by-step. Firstly, we define our class and set up the initial state.

Here, DocumentHistory is the class, and we initialize changesStack as a MutableList to act as our stack of applied changes, and redoStack as another MutableList for undone changes.

Next, we put into effect the method to apply changes.

With applyChange, we take a string change and append it to the changesStack list. Additionally, we clear the redoStack to ensure that once a new change is applied, any previously undone changes cannot be redone.

Now, we will implement the method to undo the most recent change.

Here, undo checks if changesStack is not empty. If it's not, it removes the last item from the list, simulating a stack pop operation, adds it to redoStack, and returns the undone change. Otherwise, it returns null.

Now we'll create the method to redo the most recent undone change.

The redo method checks if redoStack is not empty. If it's not, it removes the last item, simulating a LIFO operation, adds it back to changesStack, and returns the redone change. Otherwise, it returns null.

Lastly, we implement the method to retrieve all applied changes.

The getChanges method simply returns a copy of the changesStack list, which includes all changes applied to the document in the order they were applied.

Here is the final implementation of our DocumentHistory class, combining all the methods we've discussed:

Let's test this with an example:

In today's lesson, we learned how to manage a document's editing history using stacks. We implemented methods to apply changes, undo the last change, redo the most recent undone change, and retrieve all applied changes. This exercise provided us with practical experience in using stacks to efficiently track and revert operations in a real-life scenario. Keep practicing similar challenges to deepen your understanding of data structures in various applications. Fantastic job today, and keep up the good work!