### tibinyte's blog

By tibinyte, 11 months ago,
# Date
Problem
Contest
Difficulty
1 July 2021
Mordor Trip
Infoleague
Summer 2021
Round 1
1400*
First problem I ever set. It asks you to find the number of bubble sort iterations to sort a given array Good

2 July 2021
GCD
Infoleague
Summer 2021
Round 1
1700*
This problem is a very standard problem which asks to find sum of gcds across all subarrays Good

3 July 2021
Magic Digits
Infoleague
Summer 2021
Round 1
2000*
This is a quite ok digit dp problem which asks finding the number of numbers s.t. the maximum frequency of some digit — minimum frequency of some digit = $k$, where $k$ is given Good

4 July 2021
Anagrame
N/A
1300*
Very standard problem that asks to find how many subsets of $a$ are anagrams of $b$ Good

5 July 2021
Caesar Queries
Infoleague
Summer 2021
Training round
1800*
Very standard problem about stirling numbers of the second kind Good

6 July 2021
Array GCD
Infoleague
Summer 2021
Training round
1700*
Very standard problem asking for sum of gcds across all arrays of length $n$ and values in range $[1,m]$ Good

7 August 2021
Medians
Infoleague
Summer 2021
Round 2
2000*
Problem asking for number of subarrays with given median Good

8 August 2021
Caesar Legions
Infoleague
Summer 2021
Round 2
2100*
Problem asking for number of subarrays such that if it contains a color, it must contain every color of that kind Good

9 November 2021
Charity
Infoleague
Autumn 2021
Round 2
Div 2
1600*
A simple greedy problem Good

10 November 2021
Birthday Nim
Infoleague
Autumn 2021
Round 2
Div 1
1900*
I am only a coauthor in this task. The initial problem idea was by Gheal. A quite standard problem on PIE Good

11 December 2021
Find set
Infoleague
Autumn 2021
Training round
1500*
This problem is a math problem that turned into a troll problem with partial scoring. I don't take credit for its idea, only for making it appear in a contest :clown: Good

12 December 2021
àPaPdnarG
Infoleague
Autumn 2021
Training round
2100*
Educational problem on d&c dp optimization Good

13 January 2022
The revange of GrandPaPa
Infoleague
Winter 2022
Round 1
Div 1
2300*
A nice problem, but my sqrt optimizations were just overkill since very simple solutions were using just PIE. I felt bad after finding out all my work optimizing the dp calculations was useless :( Good

14 February 2022
Mex Pairs
CodeChef
CookOff
Medium-Hard
You set the cancer dp I couldn't solve.
First and hardest problem I set on codechef
Good

15 March 2022
Sussy Rooks
CodeChef
Lunchtime
Medium
A quite simple problem about both FFT and PIE. Since I didn't know FFT when I submited this problem on codechef, I'd like to mention Um_nik for improving my initial solution Good

16 March 2022
Xor or Floor?
Infoleague
Spring 2022
Round 1
Div 1
2300*
It's a quite simple dp problem with an optimization Good

17 March 2022
Floor or xor?
Infoleague
Spring 2022
Round 1
Div 2
1900*
Quite straightforward couting problem with sieve optimization Good

18 March 2022
The Hatchet
Infoleague
Spring 2022
Round 1
Div 2
1800*
Quite straightforward problem on monotonic stack Good

19 April 2022
CodeChef
Starters
35
Medium
Very cancer dp...
It's good to make your enemies hate you more tho...
Good

20 April 2022
CodeChef
Starters
34
Easy-Medium
I think it's one of my best easy problems Good

21 July 2022
Median Pairs
CodeChef
Lunchtime
Medium
Very similar statement to the one in Mex Pairs. Here we write an obvious dp and optimize it in a very interesting way Good

22 July 2022
Three Days Grace
Codeforces
Round #804
2600
First problem I set on codeforces. My initial solution was $O(vmax \cdot log^2)$, but thanks to valeriu, it was reduced to $O(vmax \cdot log)$. I think it's one of my best problems Good

23 November 2022
Maximum Substring
CodeTON
Round 3
800
In my opinion, it's a very good D2B problem, solution combines a simple observation with a not so trivial optimization to keep the balance Good

24 November 2022
Complementary XOR
CodeTON
Round 3
1400
I don't know why this problem was hated in the rate problem section, I think it's a good task that requires both some thinking and implementation Good

25 November 2022
Prefix GCD
CodeTON
Round 3
1800
This task was added one day before the competition. It looked a good difficulty jump from C, but the distance to E was a bit too large. I apologize for this Good

26 November 2022
Bracket cost
CodeTON
Round 3
2400
After realizing how to compute the cost in a smart and flexible way, you can easily use a BIT to solve it Good

27 November 2022
Majority
CodeTON
Round 3
2700
I think it's one of the best tasks in the contest, there are lots of solutions to it, mostly all at the same difficulty level. There also exists a $O(nlog^2n)$ solution, which I have implemented, but we used the $O(n^2)$ version to fit for $F$ position Good

28 November 2022
Doping
CodeTON
Round 3
3300
It has a hidden refference, but leaving that behind, I think it's a quite nice counting task, and the solution is very unexpected Good

29 November 2022
BinaryStringForces
CodeTON
Round 3
3500
First problem I set on codeforces that didn't have any solves during the official contest. It is supossed to be a hard problem with a quite long implementation, but looking at upsolving submissions, implementation can actually be quite short :) Good

30 March 2023
Sussy Perms
Starters 79
Medium
Finally setting on codechef again :) Yet another counting problem that almost went unsolved ( which is very surprising ). Am I too evil to ask variable mod FFT? Do people template variable mod FFT? I don't know. Orz jeroenodb first and only solve to this problem xD Good

31 May 2023
Array Merging
Codeforces Round #875
1000
A problem I created because we desperately needed a problem for D2B position. I don't think it's great but not bad either. Good

32 May 2023
The Third Grace
Codeforces Round #875
3200
A problem that was intended to be in CodeTON Round 3, but we didn't need it there so we used it here. I came up with this problem while I was in car xD Good

33 August 2023
Omida mincinoasa
Junior challenge 2023
2100*
Best statement of 2023. Find sum of $choose(n,i) \cdot i^k$, $n \le 10^9, k \le 5000$ Good

34 August 2023
Salutare Jupane
Junior challenge 2023
2300*
This problem was intended to be in CodeTON round 3 as D, but was removed few days before the competition. This version is however buffed. Good

• +144

 » 11 months ago, # |   +2 leafeon.
 » 11 months ago, # |   +1 What is quickest and the slowest amount of time you have taken to develop a problem? And is there any correlation with effort vs quality in developing problems?
•  » » 11 months ago, # ^ |   0 I think it took me like 3 days to code the solution for Mex Pairs. About shortest, I don't know what to say.Of course putting much effort into developing something makes it better.
 » 11 months ago, # | ← Rev. 2 →   0 What do you do when a problem idea of yours turns out to require a much harder solution than expected? When it's the contrary, I think it's usually not so bad, as this "easier solution" in this case is usually one that is much more interesting, but when it gets harder than expected, I often put it off to when I get a better understanding on what may be a good solution. However, as every work has to be done sometime, I know it can't be delayed infinitely, or else I'll forget the idea. So what would you do in this case?UPD: One more question. In problemsetting, do you make the solution after the problem, or the problem after the solution? This is a very constant dilemma that would be going on every problemsetter's heads, because usually the former makes fun problems that are hard to prove, and the latter makes problems that are easy to prove but is trivial/standard. Which approach do you take?
•  » » 11 months ago, # ^ |   +13 It's better to come up with the problem and then find the solution, it's more natural. Of course the reverse works, but it sometimes leads to an unnatural mess.I don't understand anything from the first paragraph to be honest, I think you're asking what am I doing when easier solutions exist than my intended one ?
•  » » » 11 months ago, # ^ |   -18 Nah, I mean when a problem idea (without a clear solution) started simple, but turned out to be much complex than what you imagined first. What would you do when this happens?
 » 11 months ago, # |   0 How can one write concise statement for Calorifer
•  » » 11 months ago, # ^ |   0 One does not do that. Only God can.
 » 11 months ago, # |   +44 How do you come up with such hard tasks with peak rating at 1600? I am 2100 rated, but the hardest task I invented was 2900.orz, you are a really talented problemsetter.
•  » » 11 months ago, # ^ | ← Rev. 3 →   +24 Bcs I do be trolling contests ( I don't even do contests on codeforces, mostly olympiad training ). I am also way better at solving tasks given more time to think rather than 2 hours. I could come up with the solution on my own to all of the aforementioned tasks.
•  » » » 11 months ago, # ^ |   +2 Admitting that you troll contests is a lie to anyone, you only trolled (so far) two (codeforces rounds)
•  » » 11 months ago, # ^ |   -28 I am not the OP, but I think I might be able to answer, I do be making problems above my ratings. I think problemsetting is basically a whole different skill compared to skills in contests. As tibinyte said, problemsetting is not done under a short time limit. So, they basically have a lot of time to think of new ideas and proofs for those ideas. Also important to problemsetting, would be an interest to a wide variety of algorithms/problems/concepts, sometimes ranging far above their own ratings. It would indeed be strange for a 1300-rated person to solve 2600-rated problems to practice for contests, but it is completely fine for one to think of the same problem deeply because it is interesting enough to research further, and these "interesting problems" are a lot of help later on in the problemsetting process. This help includes a whole problem idea, made from an existing one.So, yeah. A problemsetter can make problems, and the problems' difficulty isn't always correlated to the setters' ratings.
•  » » 11 months ago, # ^ |   0 The hardest task I invented is -inf rated :(
 » 11 months ago, # |   0 How can I submit a contest proposal when my rating isn't 2100+ ?
•  » » 11 months ago, # ^ |   0 By default you cannot, but if you were author in some other round, you gain permissions.
•  » » » 11 months ago, # ^ |   0 How can I be the author in "some other round"?
•  » » » » 11 months ago, # ^ |   0 Have friends that can propose contests.
•  » » » » » 11 months ago, # ^ |   0 Thanks!
 » 11 months ago, # |   +18 As a problem-setter myself, I find it very impressive that you are able to make an entire D1+2 round by yourself (except A). I know someone already asked you how you create such hard tasks as a green, but I am still in disbelief... You mentioned that you do not do Codeforces much, so what do you think you rating would be if you do Codeforces contests regularly. And how do you come up with such hard problems? For me, I find it very tough to make hard problems as when I come up with an idea, I will think that it is not solvable after some time and stop thinking about it. As the result, all the problems I set are those that I can solve in a shorter period of time, and hence easier. Can you share how you came up with the recent D1+2 H?
•  » » 11 months ago, # ^ |   +13 So what do you think you rating would be if you do Codeforces contests regularlyI've been really bored this summer so I did some virtual contests.Can you share how you came up with the recent D1+2 H?Well as you can tell, H is very similar to F. The main inspiration for both these task were Phoneix and computers and PWCA. H has a similar operation to the one in PWCA, but counting it for substrings is very interesting, and is much harder. Not to mention the implementation of H is a very hard task just by itself :)
 » 11 months ago, # |   0 Best wishes for you.