Arrays.sort(input) is mentioned as Quick Sort

[isarwarfp]

Say on FastInterstectionSol2.java class

   public void mergeSort(int[] input) {
        Arrays.sort(input);
    }

but Arrays.sort(input) is a quicksort
Sorts the specified array into ascending numerical order.
Implementation note: The sorting algorithm is a Dual-Pivot Quicksort by Vladimir Yaroslavskiy, Jon Bentley, and Joshua Bloch
as per the documentation.

[cutajarj]

Yep, you're right, it is quicksort. In previous versions of java it used to be merge sort, infact you can use the legacy merge sort if you use the java runtime args of -Djava.util.Arrays.useLegacyMergeSort=true.
It doesn't really matter if it's a merge sort or quick sort. Both algorithms are on average O(N log N). It is to demonstrate that there exists a faster way to perform an intersection between two list. Maybe the method should be renamed to "fastSort", in case java changes implementation again.

[isarwarfp]

Great !
Thanks for the clarification, "fastSort" is good but simple "sort" to make it simple because fastSort attract developer to dig whats new inside.