Why I think the problems are easier after Sept. begins? Or just my skill grew?

In Codeforces Round 1124 (Div.2), I might found it to easy the problems A-D; In near 2 Atcode Beginner Contest, I solve five problems, one prob. more than summer, and my spring ABCs also got five.

Who can tell me please?

I'm very happy that i got -9 contrib, please give me more downvote so i can got enough anger to destroy the world!!!

If you give me an upvote, please comment and i will be so angry that give your every blog an upvote!!!

For Chinese Version, click this

Special thanks to Codeforces for the evaluation resources!

To avoid any suspicion of cheating, I used problems I have already ACed for testing.

I don't think Doubao can pass the second round (

Each AI gets one revival chance (unless it does nothing or cannot be used for free)

Round 1

Let's start with an easy *800 problem!

CF2248A

It's a super simple greedy problem, both players obviously delete from the very front.

Analysis: Perplexity gave a perfectly logical explanation, but the submitted code ended up wrong. DeepSeek R1 crashed mid-thinking and gave no answer. AstronClaw had an invalid output format.

Round 2

Let's move on to a trickier *800 problem.

CF2250A

Extremely tricky! Let's see if the AI is careful enough. I myself got it wrong many times.

Analysis: So far domestic Chinese AIs seem to take the lead. DeepSeek R1 works extremely well, but it has to be used online otherwise it crashes. Qwen and iFlytek Spark perform nicely, while other AIs fail to notice the details and require carefully crafted extra prompts.

Sorry it's a "Chinese-style" problem with tons of tiny details, but it's a great test anyway — everyone can go try it out themselves!

Round 3

*900 Math Problem CF2238B

Now the top 8 top 7 are selected, the finalists list:

Now the Challenge Round

Challenge Problem: CF689D

An RMQ + Binary Search problem. *2100

For Chinese Version, click this

Special thanks to Codeforces for the evaluation resources!

To avoid any suspicion of cheating, I used problems I have already ACed for testing.

I don't think Doubao can pass the second round (

Each AI gets one revival chance (unless it does nothing or cannot be used for free)

Round 1

Let's start with an easy *800 problem!

CF2248A

It's a super simple greedy problem, both players obviously delete from the very front.

Analysis: Perplexity gave a perfectly logical explanation, but the submitted code ended up wrong. DeepSeek R1 crashed mid-thinking and gave no answer. AstronClaw had an invalid output format.

Round 2

Let's move on to a trickier *800 problem.

CF2250A

Extremely tricky! Let's see if the AI is careful enough. I myself got it wrong many times.

Analysis: So far domestic Chinese AIs seem to take the lead. DeepSeek R1 works extremely well, but it has to be used online otherwise it crashes. Qwen and iFlytek Spark perform nicely, while other AIs fail to notice the details and require carefully crafted extra prompts.

Sorry it's a "Chinese-style" problem with tons of tiny details, but it's a great test anyway — everyone can go try it out themselves!

Round 3

*900 Math Problem CF2238B

Now the top 8 top 7 are selected, the finalists list:

Now the Challenge Round

Challenge Problem: CF689D

An RMQ + Binary Search problem. *2100

For Chinese version, click here

This competition will feature 17 free AI models competing against each other, with the top three strongest AIs ultimately selected.

Competition Format

• Qualifier Round: Select 8 AIs from the total of 17. Starting with Codeforces problems rated *800, the difficulty increases progressively to eliminate participants until no more than 8 AIs remain. The remaining eliminated AIs can participate in a challenge round; if they successfully solve a problem rated *2100, they earn one resurrection chance.
• Knockout Round: After the qualifiers, the 8 AIs engage in one-on-one PKs with progressively increasing difficulty. The first AI to provide an incorrect answer is eliminated. If an eliminated AI has a resurrection chance, it can consume one to advance to the next round alongside the winner.
• It is evident that the knockout round will consist of at least 3 rounds and at most 4 rounds.

Participating AIs

P.S. For first five models marked *, I used https://chatai.org/ and can't know which models they are. Sorry. I will research out it.

Problem link

We find an odd nodes tree has only one centroid. So if $$$n$$$ is odd, we don't change it. If $$$n$$$ is even, we delete an leaf and make it odd. Then we insert the deleted leaf to the centroid. And sorry for I met many bugs. Hope you can't meet them. Becareful.

1st Wrong Answer Okay, we need max to be min.

2nd Wrong Answer No, we just calculated the deleted leaf in!!!

3rd Wrong Answer It's not finished though. The nodes after delete is $$$n-1$$$ not $$$n$$$.

AC link

#include <bits/stdc++.h>
#define N 100005
using namespace std;
int T,n;
vector<int> g[N];
int s[N],mx[N],C,Leaf;
int leaf(int x,int fa){//O(depth) find a leaf
    if(g[x].size()==1&&fa)return x;//Not the root so fa!=0
    for(int y:g[x]){
        if(y==fa)continue;
        return leaf(y,x);
    }
}
void dfs(int x,int fa){//Calculate all subtree node numbers
    s[x]=1;mx[x]=0;//Init
    for(int y:g[x]){
        if(y==fa||y==Leaf)continue;
        dfs(y,x);//Calculate the subtree
        mx[x]=max(mx[x],s[y]);
        s[x]+=s[y];
    }
    mx[x]=max(mx[x],n-1-s[x]);//Caution: As x is the root, then the origin father is follow x's subtree. So we have another subtree with its father and uncles.
    //     (1)
    //     / \
    //   (x) (3)
    //    |
    //   (2)
    //Like up graph and at X we have subtree (2) and also (1)-(3).
}
void solve(){
    cin>>n;
    for(int i=1;i<=n;i++)g[i].clear();//clear the graph
    for(int i=1,u,v;i<n;i++){//input the graph
        cin>>u>>v;
        g[u].push_back(v),g[v].push_back(u);
    }
    if(n&1){//N is odd and the tree can only have one centroid. Just randomly delete an edge and add it back.
        cout<<1<<" "<<g[1][0]<<"\n";
        cout<<1<<" "<<g[1][0]<<"\n";
        return;
    }
    //Now N is even. We need delete an edge and connect it with our Subtree's centroid.
    Leaf=leaf(1,0);
    cout<<Leaf<<" "<<g[Leaf][0]<<"\n";
    C=0;mx[C]=0x3f3f3f3f;
    dfs(1,0);
    //Now we find the centroid.
    for(int i=1;i<=n;i++){
        if(i==Leaf)continue;
        if(mx[i]<mx[C]){
            //cout<<i<<" is better:"<<mx[i]<<'\n';
            C=i;
        }
    }
    //Now centroid is C.
    cout<<Leaf<<" "<<C<<'\n';
}
int main(){
    ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);//Make input faster
    cin>>T;while(T--)solve();
    return 0;
}