This is my first blog post, so it probably will end up being scuffed. ==============================

The problem

So I was working on CF 472D, Design Tutorial: Inverse the Problem and it requires an MST (sorry for spoiling) and I chose to use Kruskals MST to do this (just note that I could've avoided this issue by using any other MST alg, but that's not the point here). So I just coded vanilla Kruskals where I std::sort'ed the edges and used a DSU with path compression and union by size. And I submitted the code and lo and behold, it was TLE on tc 39! I entered into a state of depression and decided to change my std::sort to stable sort and wow, it ac'ed (with 1934ms on a 2 second TL)! Well I was talking some people on Discord later and I this up and one of them told me that the only reason std::sort was slow was since I had used c++11 (which I did) instead of c++17 where std::sort was upgraded. So I submitted it again with std::sort and c++17 and yay it passed (with 1466ms)! and ~470ms faster than my last submission! But to satisfy my curiosity I submitted it again with c++17 and std::stable_sort and woah, it passed with 1154ms, some 310ms faster than the last one.

Please do not judge my template/coding style as that is not the point here, but if you look at my code, the only differences between the submissions are the submission settings and a different sort on line 65 (or the second line of the kruskals function).

the submissions from each correlate to:

• 1154ms, c++17 with stable_sort

• 1466ms, c++17 with sort

• 1934ms, c++11 with stable_sort

• TLE tc 39, c++11 with sort

also note I did not benchmark memory since that was not something that bothered me in all of this

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The resolution?

So here is one (of probably very few) situations where std::stable_sort outperformed std::sort. The problem that I have right now is that I do not want to miss a submission in a future contest over which sort I chose, since there are definitely problems out here where this problem cannot be avoided by something just as simple as using Prim instead of Kruskal.

If anyone has a good suggestion of which to use (std::stable_sort or std::sort) or a simple impl of a sorting alg that I can write up a little more time for similar results, please link it. I will probably just end up using stable_sort from now on since that was what made a difference here unless there is a really convincing argument or implementation that'll make me switch. Thanks for reading and I apologize for any typos, unclear sections, and overall scuffedness.