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D is interesting and easy to code.

ABC were easy

D was just chinese for me. Hope to learn and be better for next contest. Learnt about the reverse the Summation Technique.

Thanks for the problems :)

I see that problems so hard, and editorial too

Can someone explain the solution to E in simpler words? "preceding bits", "next bit" are context-less. And what is meant by "Numbers not on this step are certainly useless"?

Hope you guys can check out the solution explanations I wrote — I've shared some of my thoughts on these problems and approaches to solving them.

My solution of Problem A, D, E, F

why in problem C don't check if res&l==0 what if res is larger but still has bit equal to l

Could someone make an editorial for the editorial of D please? What is the meaning behind mentioned segments? What are the $$$l_i$$$ and $$$r_i$$$ stands for?..

Why can't i output l when k <= 2 in C?

in problem C ,if the choice for an or an-1 exceeds r then why dont we try to find the array with less number of terms equal to l and allowing more freedom of choice for subsequent terms?

I think the gap between problem C and problem D difficulties was way big.

Can anyone help me with B?

Hi, could someone explain the other solutions to Problem D? I noticed different approaches, and some of the code looks quite interesting. I'd love to understand the reasoning behind them.

Such as following solution

	vector<num> dp(N+1, 0);
	dp[0] = 1;
	for(int x = N; x >= 1; x--){
		vector<num> ndp = dp;
		for(int i = 0; i < N; i++) ndp[i+1] += dp[i];
		dp = ndp;
		for(int i = 1; i <= N; i++){
			ndp[i-1] += ndp[i] * num(i) * num(x);
		}
		dp = ndp;
	}
	cout << dp[0] << '\n';

Did anyone solve B by managing rings that are covered as we add a's gradually? For example, if a=[5,3,...] after step 2 we know that every point with distance [5-3, 5+3] is covered, simply by looking at how circle centered at distance 5 and radius 3 is moving around the circle with distance 5.

One mindless solution to Problem A is to apply Dijkstra's algorithm, whose complexity is acceptable.

How is the answer 37 for n=3 in D? I am getting 36 in 24 combinations Please help

~~~~~~~~~~~~~~~~

include

using namespace std;

define ll long long

int main() { int t; cin >> t; while(t--){

ll n, l, r, k , ans;  
cin >> n >> l >> r >> k;
bool found = 0;

if(n == 2){
  cout << -1 << "\n";
}else if(n % 2 == 1){
  cout << l << "\n";
}else{ // n even
  if(k <= n - 2){
     cout << l << "\n";
  }else{

    for(ll i = l + 1; i <= r; i++ ){
      if((i & l) == 0){
          ans = i;
         found = true;
         break;
      }

    }

    if(found){
      cout << ans << "\n";

    }else{
      cout << -1 << "\n" ;
    }
  }


}

}

}

~~~~~~~~~~~~~~~~~~~~~~ Why is this code wrong on test 3997? How can I know this test case?

D was a really cool problem. It took a lot of time to understand, but it was definitely worth it, a very well-crafted and insightful problem.

For Problem C, can somebody explain why the tactic that a1 exor a2 exor .... an-2 = l doesn't work then why aren't we checking the same thing for l+1,l+2, ... r?

Can somebody explain why does $$$s_1 \leq \frac{S - m}{2} + \frac{\max(a_i)}{2} \leq m + s_2$$$ hold in B?

C was a good problem! I was able to guess the solution instantly but proving was satisfying.

That shows why i love Codeforces. It has a difficult thinking but easy coding. Thanks..

I received a message saying my solution 327573503 for problem 2119A coincided with others (mokshitha_156). I admit that I shared my code with a friend because she was struggling, and I didn't fully understand the consequences.

I now realize this is a violation of Codeforces rules. I sincerely apologize and accept disqualification from the round. This was my first and last mistake.I will never repeat it.

Please consider this as a one‑time error. Thank you for understanding.

Hi, I'm appealing the flag on solution 327610026 (problem 2119C). I regularly use Ideone without knowing that the default setting makes code public, which unintentionally allowed others to see and copy it. I didn’t share links or collaborate—I simply didn't realize I needed to manually switch it to private. Today after getting an alert, i realised about this feature... I sincerely apologize for the unintentional leak. This was not deliberate plagiarism.

Hello Codeforces team,

I have received the notification that my solution for Problem 2119A coincided with other submissions in this round.

I want to clarify that I did not intentionally share my code with anyone, nor did I copy it from anyone else. I understand that my solution matching others so closely may still be considered a violation of the rules.

I fully respect the rules of fair competition, and I apologize for any inconvenience caused. I will make sure to be more careful in the future to avoid any similar issues.

Thank you for your understanding and for your hard work organizing these contests.

[deleted]

Dear Codeforces team,

I am appealing the skipped verdicts on my submissions for both Problem A (Add or Xor) and Problem B (Line Segment) in Round 1035 (Div. 2). I want to clearly state that I did not cheat, collaborate, or share my code with anyone. Below is a detailed explanation of my approaches and supporting context:

Problem A (2119A – Add or Xor): The task is to transform a to b using +1 (cost x) and XOR with 1 (cost y) at minimum cost. I modeled this as a shortest path problem, where each integer is a node and operations define the edges with associated costs. This directly maps to Dijkstra's algorithm , I used my own Dijkstra template, which I regularly use in contests.

My code uses a priority_queue and visited-distance tracking, and I limited the state space to a fixed size. I now understand that many others independently arrived at a similar Dijkstra solution, which likely caused this mass flagging. But the logic and template used are part of my personal style, not copied from any contestant.

Problem B (2119B – Line Segment): My approach for this problem was if the actual dist is in between the max dist from the source and min dist from the source then we can reach the terminal point so It would say Yes else No.

This approach was derived independently during the contest . It's a clean mathematical solution and not something I copied. And my code style is always the same .

I kindly request a manual review of my submissions. I understand the difficulty of handling mass similarity cases, but I hope my explanation clarifies that I competed honestly, with my own logic and coding style.

Submission IDs:

A:327563324

B:327606925

I am new to Codeforces and was not fully aware of the rules regarding code sharing and plagiarism. In a recent contest, I mistakenly copied a solution from Ideone without understanding that this violated the Codeforces regulations. I now realize this was a serious mistake.

I sincerely apologize for this action. It was not done with any malicious intent, but rather out of ignorance. I assure you that I have learned from this experience and will never repeat such behavior again. I respectfully request your understanding and a chance to continue participating on the platform while following all the rules properly from now on.

Thank you for your time and consideration.

Есть ли у задачи B более простое решение? Или можно ли упростить авторское решение?

For 2119D - Token Removing, here's my solution summary with visual explanation https://www.youtube.com/live/O0qbgoDUj0g?t=1962s

I have a feeling like nowadays the problem in the round have a trends going observation heavy,i don't think observation in B is popular,i would doubt that is the author put the thing he learn on school math class to the problem?Just like I put a integration by part calculus in the problem then say from some observation we can use defination of integration to prove the formula.For C,D i think is good,nice logic light observation and at least can learn something.

Hello Codeforces team,

I have received the notification that my solution 327604665 for Problem 2119C coincided with other submissions in this round. I would like to tell that you can check my previous submission as well. when i got to know that it is causing TLE then i optimized it using this approach and took hints from google as well. The entire logic was mine. Hopefully will not happen next time.

Really liked problem D and F.

Good questions!

can someone tell that in D why does counting the number of sequences work?because we were asked for number of ways right?

I see A B C D all four problems are interesting and challenging as well and qsn D takes more than four hours to understand the eaxactly the solution after attempting.

In problem F, one can compensate for the inability to make a few observations by bashing with an ugly centroid decomposition

The definition of $$$f(i, j)$$$ in the editorial of D is very weird to me. Isn't it more intuitive to define $$$i$$$ as the length of the token sequence and $$$j$$$ as the lower bound of the last token index? I get that the author's solution is perfectly valid, but it is weird to me that $$$i-j$$$ represents the length of the token sequence.