Today's lesson will take you on an exciting journey through Stacks, a powerful tool in Kotlin. In programming, Stacks are fundamental data structures utilized in various applications. Our goal for this lesson is to understand the concept of Stacks, learn how to implement and manipulate them in Kotlin, and delve deep into their complexities. Let's get started!

First and foremost, let's understand what a Stack is. Imagine a stack of plates where you can only remove the top plate at any given time. That's precisely what a Stack is: a Last-In, First-Out (LIFO) structure. Stacks are used in memory management, backtracking algorithms, and more. The key operations involved are push (adding an element to the top of the stack), pop (removing the topmost element), and peek (looking at the topmost element without removing it).

Kotlin allows us to implement Stacks in a variety of ways. One straightforward method is using an array for storage. This array-based stack utilizes a fixed-size array, providing quick access but is limited by its capacity. Let's look into creating a Stack using an array in Kotlin:

Here, top represents the index in the stackArray that is currently the top element of the stack. It is initialized to -1, indicating the stack is empty, and there is no valid index for the top element.

In an array-based stack, the push operation adds a new element at the top of the Stack. Here’s how to write a push function in Kotlin:

Before adding an element, the method checks if there is space in the stack by comparing top with size-1 (maximum array index). If room is available, the element is inserted; otherwise, a "Stack Overflows" message is displayed. The ++top operation increments the top variable by one and then uses this new value as an index to add the data element into the stackArray.

The pop operation removes the topmost element from the Stack:

This method removes and returns the top item of the stack. It checks if the stack is empty by verifying if top is greater than -1. If true, it decrements top and returns the element just removed. Notice that the element is not removed from the array; the top variable decrement prevents access to the popped element.

Lastly, the peek operation returns the topmost element without removing it from the Stack:

While it's valuable to understand how a stack is constructed, manually implementing it is not always necessary. Kotlin does not have a direct Stack class like some other languages, but you can use the ArrayDeque class, which provides functionality similar to a stack. ArrayDeque allows for efficient stack operations, including push(), pop(), and peek().

Here’s an example using ArrayDeque in Kotlin:

The output will be:

This example demonstrates creating a stack using ArrayDeque and utilizing operations analogous to push(), pop(), and peek().

Congratulations, you've completed the lesson on Stacks in Kotlin! You will now proceed to the practice sessions using the concepts learned here. These hands-on practice exercises will help solidify your knowledge and improve your problem-solving skills. So, get ready to roll up your sleeves and dive right in!