Lab 12: Merge and Quick Sort


In this week's lab, you'll implement two of the sorting algorithms that we learned about in lecture this week. In lecture, we focused on sorting arrays. In this lab, you'll instead focus on sorting linked lists, which requires some cleverness and a good understanding of how merge sort and quick sort operate.

All of the functions that you write will operate on the Princeton Queue Implementation, which implements a queue using a linked list. You should implement sorting using the public methods in the Queue class.

Merge Sort

Test driven development

In this week's lab, you'll practice test-driven-development by writing a test before writing any code. With test-driven-development, you start by writing a test that fails (because you haven't written any code yet!). After writing the relevant code, you re-run the test to make sure that it passes.

Today, you'll write a lightweight test by writing a main method in MergeSort. Your main method should create a Queue of unsorted objects and print that queue. Next, call MergeSort.mergeSort() on that queue, and print both the original queue (which should be unchanged) and the returned, sorted queue.

You can put any kind of object you like in your test queue, as long as the object implements the Comparable interface. It may work well to create a Queue of Strings, as in the code below:

Queue<String> students = new Queue<String>();

Try running your main method. Is the output what you expect, based on the implementation we provided of mergeSort()?


If you need to review how mergesort works, you may find the Merge sort demo from lecture or this Merge sort demo to be useful.

To help you implement merge sort, start by implementing two helper methods:

Once you've finished implementing these helper methods, use them to implement mergeSort. With the help of the two methods above, your mergeSort method should be short (fewer than 15 lines of code). Run the main method you wrote above to test whether your mergeSort implementation works!

Quick Sort

Test driven development

As you did for merge sort, begin by writing a main method in that creates an unsorted Queue, prints it, sorts it, and then prints the result.


If you need to review how quick sort works, take a look at slides 6 through 10 from this lecture. You'll be using the 3-way merge partitioning process described on slide 10. This partitioning approach, unfortunately, has no Hungarian dance demo (the dancers chose to partition based on the first item in the array, rather than on a random element.).

Begin by implementing the helper function partition(). The partition() method takes an unsorted queue called unsorted and an item to pivot on, and three empty queues called less, equal, and greater. When it returns, less should contain all items from unsorted that were less than the pivot, equal should contain all items from unsorted that were equal to the pivot, and greater should contain all items that were greater than the pivot.

Once you've implemented partition(), use it to implement the quickSort function. You may fund the getRandomItem() and catenate() methods that we've provided to be useful. Using these helper functions, your quickSort method should be short (fewer than 15 lines of code).


What does the <Item extends Comparable> syntax mean?

In this week's lab, many of the functions have syntax that looks something like:

public static <Item extends Comparable> Queue<Item> mergeSort(
        Queue<Item> items) {

Recall from lecture 13 that if a method operates on generic types, the generic type should be defined before the return type of the method. In the example above, the part of the function declaration that says <Item extends Comparable> means that the mergeSort function operates on generic type Item, which must extend Comparable (we need Item to extend Comparable so that we can use the compareTo method to compare items). In other words, you can interpret the declaration above as saying "the mergeSort function takes a Queue of things that implement the Comparable interface, and returns a Queue of those things in sorted order." If you're unsure how to write code in functions like this, take a look at the helper functions that we provided, which may be helpful examples.

My code works fine but the autograder fails with some sort of JSON error.

The issue is probably that your code is quadratic time instead of linearithmic. Your code should be able to easily handle collections of 10,000 items, even if there are lots of duplicates and/or the collection is in sorted order already.