A few hours ago, the hacking phase for the round (Div. 3) ended and the system testing began. I wanted to check how many solutions failed the system tests, and I saw a huge number of TLEs on problem C. After looking at several solutions, I realized that they all had one thing in common — unordered_map.
I think many people know that unordered_map (hereafter UM) can work in both $$$O(1)$$$ and $$$O(n)$$$ time. Because of this, some people found a test case where UM runs in $$$O(n)$$$, which makes the overall complexity $$$O(n^2)$$$.
It got me wondering why people massively use UM when a regular map passes within the time limits. I asked ChatGPT to solve this problem, and it gave me a solution using UM.

#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    
    int t;
    if(!(cin >> t)) return 0;
    while(t--) {
        int n;
        ll k;
        cin >> n >> k;
        vector<ll> S(n), T(n);
        for(int i=0;i<n;i++) cin >> S[i];
        for(int i=0;i<n;i++) cin >> T[i];

        unordered_map<ll,int> cntS, cntT;
        cntS.reserve(n*2);
        cntT.reserve(n*2);

        auto rep = [&](ll x)->ll {
            ll r = x % k;
            if(r == 0) return 0;
            if (k % 2 == 0 && 2*r == k) return r; // специальный случай при k чётном
            return min(r, k - r);
        };

        for(ll x: S) cntS[rep(x)]++;
        for(ll x: T) cntT[rep(x)]++;

        if(cntS == cntT) cout << "YES\n";
        else cout << "NO\n";
    }
    return 0;
}

On the one hand, it’s kind of funny that people who copied from ChatGPT (or another LLM) got their solutions rejected.
On the other hand, there are people who wrote the code themselves but decided to use UM instead of map (although I think that’s their own fault because they could have just used a regular map).

Everyone knows that there are quite a few cheaters on rating rounds, but there were a lot of cheaters on the last round.
In the top 20 official users there are at least 10 users with a rating of less than 1200. This is a lot and the situation is terrible. Codeforces need a new anti-cheat system.
But I also want to say that competive programming (and not only, I think this applies to all industries) after the development and improvement of AI will move into a new era.