samuelbanya / phase-4-data-structures-queue-from-stack

Create a Queue from Two Stacks Lab

Learning Goals

• Implement a MyQueue class using two Stacks

Introduction

Recall in an earlier lesson we talked about the difference between a Stack and a Queue. Basically, they are very similar, except that Stacks process records in "First In, Last Out" (FILO) order, while Queues process records in "First In, First Out" (FIFO) order. We used as a real-world example of a Queue a line of airline passengers waiting to pass through the security checkpoint: the passenger who joined the line first will be the first to pass through security (most of the time, anyway).

Because the behavior of a Queue is a bit different from that of a Stack, the method names that are used (by convention) are a bit different as well. Rather than including #push and #pop methods, a Queue implementation generally has an #add method, which adds an element to the end of the Queue, and a #remove method, which removes the first element in the Queue. (You might also see these methods called #enqueue and #dequeue.) Finally, like a Stack, a Queue usually includes a #peek method, but, in this case, #peek returns the element that went into the Queue first rather than the one that went in most recently.

In this lab, you will implement a MyQueue class, but you will be doing it a bit differently than you did for the Stack class. Specifically, you will use Stack as the underlying data structure rather than an array. We have included the code in lib/stack.rb. We have also required that file in lib/queue_from_stack.rb, where you will be coding your solution.

Instructions

Start by creating the MyQueue class, and set it up to initialize two new, empty instances of the Stack class; in order for the tests to pass, you will need to name these @s1 and @s2. You should not initialize an empty array for your MyQueue instance.

Once you have the first test passing, build out the following methods:

• MyQueue#add(value): adds an element to the end of the Queue
• MyQueue#remove: removes and returns the first element in the Queue
• MyQueue#peek: returns the first element in the Queue without removing it

Note: in solving this challenge, you can only use the four methods included in the Stack implementation — #push, #pop, #peek, and #empty? — and the two instances of Stack that are created when the MyQueue instance is initialized.

Hint: If you're having a little trouble figuring out how to approach this problem, give some thought to what will happen if you pop each element off of one Stack instance and push it onto another. You should be able to use this information to set up your add method such that the order of the elements is correct for using #push and #pop.

Conclusion

As you can probably imagine, implementing a Queue using a Stack as the underlying data structure is not the the most efficient way to go. Hopefully, however, building out this class gave you a good feel for the difference in how these two data structures process records.