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Merge Sort

int[] merge(int[] first, int[] second) {
return null;

This lesson continues our exploration of sorting by examining a new approach. We’ll start with a simple but powerful observation, and then examine how to build on it to create a complete sorting algorithm. This will also represent our first sorting algorithm that achieves best-case sorting performance! What are we waiting for?


We’ll begin with an observation.

Next, let’s implement merge on two int arrays and confirm our hunch about its performance.

// Implement merge


Next let’s consider how to design a sorting algorithm that utilize our merge method. We’ll also use this as a chance to point out how we can apply recursive algorithms on arrays, rather than trees, which we’ve used in the past.

Finally, let’s analyze the performance of Mergesort. This is an interesting case! Let’s walk through it carefully.

Homework: Mergesort

Created By: learncs.online Staff
/ Version: 2020.11.0

Create a public class named Mergesort that provides a single instance method (this is required for testing) named mergesort. mergesort accepts an array of ints and returns a sorted (ascending) array. You should not modify the passed array. If the array that is passed is null you should throw an IllegalArgumentException.

Mergesort should extend Merge, and its parent provides several helpful methods:

  • int[] merge(int[] first, int[] second): this merges two sorted arrays into a second sorted array. If either array is null it throws an IllegalArgumentException, so don't call it on null arrays.
  • int[] copyOfRange(int[] original, int from, int to): this acts as a wrapper on java.util.Arrays.copyOfRange, accepting the same arguments and using them in the same way.

(You can't use java.util.Arrays in this problem for reasons that will become obvious if you inspect the rest of the documentation...)

Note that you do need to use merge and call it the correct number of times. This will be tested during grading. You should use an array of size 1 or 0 as your base case.

More Practice

Need more practice? Head over to the practice page.