DuyNgaDocTon1410 is an AI cheater.
Firstly, let's look at their performance. In his first Div. 2 round, they place 22nd. In the next one, they places 2nd (as a pupil), solving every single question! The same day, they take another Div. 2 contest, and fail to even solve two questions, ending at to 6690th place, despite attempting B, C, and D multiple times. Remember, they just got 2nd place a few hours earlier. It is not possible for a human to vary this much in performance on the same day.
Now, lets look at their submissions. One thing immediately caught my attention: out of his 100 most recent submissions, 33 of them are WA on (pre)test 1, and 5 are compilation errors. So this person doesn't test their code before submitting, and doesn't even check if it compiles. Very common among AI copy-pasters.
If we look at the code in these submissions, it is not immediately clear that he uses AI. However, there are too many signs of AI for this to be a coincidence. For example, most of his submissions have no comments, while others such as 310106426 have some of the most detailed and perfect comments I have ever seen. Another discrepancy is his header usage: some of his solutions use bits/stdc++.h, while others include each header individually, and one of his solutions even uses both (which is completely useless). There is even more evidence when you look at his coding style.
Altogether, DuyNgaDocTon1410 is obviously using AI to cheat. Nothing else can explain this amount of evidence. Please ban him and all other AI cheaters so that Codeforces remains fair.
Unrelated: while looking at several of his submissions my account got flagged as being used to crawl, but I was just browsing through his AI submissions.
