Lesson 5

Welcome back! As we progress through our course on **Advanced Data Structures - Stacks and Queues in Java**, we focus on leveraging queues to crack algorithmic challenges often encountered in technical interviews. With their orderly structure, Queues are excellent for representing sequential processes and managing streaming data. In this lesson, we'll explore two problems highlighting complex queue manipulations. Let's get started and decode these intriguing interview problems, ensuring that the concepts are thoroughly understood with added examples and detailed explanations.

Let's begin with the concept of queue interleaving. Imagine you're orchestrating a dance sequence where dancers from two groups must perform in an alternating pattern. In a similar vein, our first computational task relates to a list of elements that we want to rearrange, ensuring that if we start with an order like $a_1$, $a_2$, ..., $a_{n/2}$, $b_1$, $b_2$, ..., $b_{n/2}$, we end up with a sequence $a_1$, $b_1$, $a_2$, $b_2$, ..., $a_{n/2}$, $b_{n/2}$. This organization method mirrors real-life situations, such as merging traffic from two lanes onto a single-lane road, ensuring each car takes its turn from each lane.

We will use two auxiliary queues, akin to having two sub-lines in the dance sequence or two lanes on the road, to hold the divided sections of the original queue. We maintain a clean and memory-efficient interleaving without needing extra arrays by systematically dequeuing elements from these and enqueuing them back into the original queue.

First, consider a queue constructed of dancers (or elements). We want to divide this queue into two groups, with the first half entering the `firstHalf`

queue and the second half into the `secondHalf`

queue. This way, we can alternately choose a dancer from each group and form a new, interleaved queue.

Let's construct our division:

Java`1Queue<Integer> firstHalf = new LinkedList<>(); 2Queue<Integer> secondHalf = new LinkedList<>(); 3 4while (queue.size() > n / 2) { 5 firstHalf.add(queue.remove()); 6} 7 8while (!queue.isEmpty()) { 9 secondHalf.add(queue.remove()); 10}`

By iterating over the original queue, we distribute the elements into two separate queues, simulating the splitting of dancers into two groups. With the first group ready, we proceed to the second, ensuring a balanced division.

With both groups lined up, we alternately take a member from each group, thus combining them into the interwoven order:

Java`1while (!firstHalf.isEmpty() || !secondHalf.isEmpty()) { 2 if (!firstHalf.isEmpty()) { 3 queue.add(firstHalf.remove()); 4 } 5 if (!secondHalf.isEmpty()) { 6 queue.add(secondHalf.remove()); 7 } 8}`

Imagine this as a dance coordinator calling out to each group in turn, forming a new sequence. This approach ensures no auxiliary arrays are needed, thus elegantly solving the problem using only the queues.

Now, let's shift our attention to the second problem: computing a moving average from a data stream. This problem finds its way into technical interviews and requires real-time decision-making, like a trader monitoring livestock prices for quick buying or selling decisions. Our task is to calculate the average of the last `k`

items in a stream of data, a critical operation for trend analysis in data analytics.

Consider a fitness tracking app that updates a user's average heart rate over the last 10 minutes. The app computes the average heart rate readings to showcase the most recent state of health, updating this information with each new reading received.

A naive method would involve storing all the data points and recalculating the average each time a new element arrives. However, this approach could be more suitable and efficient for a large dataset or an infinite stream. The computational overhead can become overwhelming as more data is processed.

A queue presents an efficient solution. Maintaining a sliding window of the most recent `k`

elements mimics our fitness app's ongoing cycle of heart rate readings, where fresh readings replace old data.

Imagine creating a class `MovingAverage`

that emulates our fitness tracking app's backend, tasked with dynamically providing the average of the last `k`

heart rate readings:

Java`1private int size; 2private Queue<Integer> window; 3private double sum; 4 5public MovingAverage(int size) { 6 this.size = size; 7 this.window = new LinkedList<>(); 8 sum = 0.0; 9} 10 11public double next(int val) { 12 if (window.size() == size) { 13 sum -= window.remove(); 14 } 15 16 window.add(val); 17 sum += val; 18 return sum / window.size(); 19}`

In the `next`

method, once the window reaches its maximum capacity (comparable to reaching the 10-minute mark in our app), we discard the oldest reading before adding the new one. We add the new value to our sum and then calculate the average by dividing the sum by the current window's size — much like updating the app display with the latest heart rate average.

Here is the complete code for you to test, envisioning live data streaming in and out, with the heartbeat average being updated after each new entry:

Java`1import java.util.LinkedList; 2import java.util.Queue; 3 4class MovingAverage { 5 private int size; 6 private Queue<Integer> window; 7 private double sum; 8 9 public MovingAverage(int size) { 10 this.size = size; 11 this.window = new LinkedList<>(); 12 sum = 0.0; 13 } 14 15 public double next(int val) { 16 if (window.size() == size) { 17 sum -= window.remove(); 18 } 19 window.add(val); 20 sum += val; 21 return sum / window.size(); 22 } 23} 24 25class Solution { 26 public static void main(String[] args) { 27 MovingAverage m = new MovingAverage(3); 28 System.out.println(m.next(1)); // returns 1.0 (like a single heart rate reading) 29 System.out.println(m.next(10)); // returns 5.5 (the average after a short burst of activity) 30 System.out.println(m.next(3)); // returns 4.66667 (normalizing after the burst) 31 System.out.println(m.next(5)); // returns 6.0 (the most recent average, taking into account the last three readings) 32 } 33}`

Try adding this code into a Java IDE, running it, and visualizing how the moving average changes dynamically, similar to our app's heart rate tracker.

Throughout this lesson, we've unlocked the potential of queues to streamline complex data manipulations, whether it’s mixing up queues for interleaving or keeping tabs on streaming data using a sliding window for moving averages. Both sets of problems allowed us to demonstrate how queues minimize redundancy and maximize efficiency—two highly prized qualities in technical interviews.

After mastering these techniques and understanding the rationale for using queues, you've added robust tactics to your coding repertoire, setting you up for success in practical scenarios. The use of real-life analogies, such as dance sequences and fitness apps, helps make these abstract concepts more tangible.

It's time to put these theories into practice. Your next challenge awaits with hands-on exercises where you'll apply our freshly-acquired queue knowledge. These exercises will be both a test and an opportunity to perfect the art of manipulating queues to solve algorithmic puzzles. Are you ready? Let the coding begin!