Every Problem I've Ever Set

#	User	Rating
1	tourist	3985
2	jiangly	3814
3	jqdai0815	3682
4	Benq	3529
5	orzdevinwang	3526
6	ksun48	3517
7	Radewoosh	3410
8	hos.lyric	3399
9	ecnerwala	3392
9	Um_nik	3392

#	User	Contrib.
1	cry	169
2	maomao90	162
2	Um_nik	162
4	atcoder_official	161
5	djm03178	158
6	-is-this-fft-	157
7	adamant	155
8	awoo	154
8	Dominater069	154
10	luogu_official	150

As some of you might already know, this blog is inspired by my very good friend tibinyte's post, which you can find here.

The golden problems are my personal favourites, the green problems are pretty good and the red problems are not worth your time in my opinion.

Problem difficulty legend:

1600? : the rating shown is just my estimate, and most likely innacurate
2100* : the rating shown is the average of all the votes cast on the editorial regarding the difficulty of the problem
2500 : the problem has this exact rating on codeforces

Infoleague problems

#	Date	Problem	Contest	Difficulty	Tags	Problem Feedback	Additional comments
1	November 20th 2021	MCLS	Infoleague Autumn 2021 Round 2, Div. 2	1200?	dp	N/A, I would give it a solid 6/10	This is the first problem I've ever set in a public contest, extremely standard.
2	November 20th 2021	Bordered Subarrays	Infoleague Autumn 2021 Round 2, Div. 1	1800?	data structures, binary search	N/A, I would give it a solid 9/10	Quite standard, but a very enjoyable problem with a really clean solution.
3	November 20th 2021	Birthday Nim	Infoleague Autumn 2021 Round 2, Div. 1	1900?	combinatorics, dp	N/A, I would give it a solid 8/10	I came up with the idea for this problem, and tibinyte solved it.
4	December 4th 2021	Mountains	Infoleague Winter 2021 Training Round	1500?	data structures	N/A, I would give it a solid 7/10	A neat, simple problem. Good practice problem for difference arrays.
5	December 4th 2021	Updating Inversions	Infoleague Winter 2021 Training Round	2200?	data structures	N/A, but I wouldn't recommend this problem ever/10	Boring, standard data structures problem with a stupidly low time limit.
6	December 4th 2021	Rubik String	Infoleague Winter 2021 Training Round	900?	greedy/dp	N/A, I would give it a solid 6/10	Meh
7	December 4th 2021	Xor Plains	Infoleague Winter 2021 Training Round	1600?	greedy, constructive algorithms	N/A, I would give it a solid 8.5/10	I initially came up with the idea for this problem back in 2020, probably one of my best problems from that time.
8	January 8th 2022	Make Sum Great Again	Infoleague Winter 2022 Round 1, Div. 2	1400?	binary search	N/A, I would give it a solid 7/10	Good practice problem for binary searching the answer.
9	January 8th 2022	Binary Search Search	Infoleague Winter 2022 Round 1, Div. 2	1800?	math	N/A, I would give it a solid 9.5/10	Magic solution, quite a fun problem to think about.
10	January 8th 2022	Plates	Infoleague Winter 2022 Round 1, Div. 2	1900?	bitmasks, brute force, dp	N/A, I would give it a solid 6/10	Almost a direct application of bitmask dp.
11	January 8th 2022	Touch	Infoleague Winter 2022 Round 1, Div. 2	2200?	greedy, constructive algorithms	N/A, I would give it a solid 9/10	This problem is deceivingly difficult and I would highly recommend you giving it a try.
17	March 30th 2022	Yet Another Constructive Problem	Infoleague Spring 2022 Round, Div. 2	1600*	greedy, constructive algorithms	N/A, I would give it a solid 7/10	I remember that writing the generator for this problem was horrible :)
18	March 30th 2022	Jump	Infoleague Spring 2022 Round, Div. 1	2000*	data structures, combinatorics	N/A, I would give it a solid 9/10	Generally an awesome problem
19	March 30th 2022	Bamboo Coloring	Infoleague Spring 2022 Round, Div. 1	2500*	trees, data structures, constructive algorithms	N/A, 9.25/10	Legendary problem, deceivingly difficult, also quite difficult to implement. Mega orz to anyone who manages to solve it using decremental connectivity (because I couldn't).
51	March 13th 2023	Adece	Infoleague PreOJI 11-12th Grades	2000?	combinatorics, math	N/A, 8.5/10	Pretty interesting problem, inspired by this.
52	March 13th 2023	Aproapecoliniare	Infoleague PreOJI 11-12th Grades	2100?	data structures, implementation, sortings	N/A, 7/10	Mainly an implementation task.
53	March 13th 2023	Retrotrees	Infoleague PreOJI 11-12th Grades	1600?	dsu, greedy, sortings, trees	N/A, 8.75/10	Fun problem.

Peppers++ and Moisil++

Without going into much detail, these are contests for middle and high school students from Romania.

The following problems are available in this group. The statements are unfortunately only in Romanian, in the contest materials section of each problem.

#	Date	Problem	Contest	Difficulty	Tags	Problem Feedback	Additional comments
12	February 23rd 2022	an	Peppers++ 2022, 5th Grade	800?	math	N/A, I would give it an 8/10	This problem is basically a math question.
13	February 23rd 2022	nkspells	Peppers++ 2022, 5-6th Grades	800?	math, brute force	N/A, I would give it a 6/10	This problem is identical to this one, except that, in my version, $$$a_{n+1}=a_n-\text{Min} \cdot \text{Max}$$$
14	February 23rd 2022	pdist	Peppers++ 2022, 6th Grade	1700?	math, brute force	N/A, 3/10	Trust me when I say that this problem is the most atrocious thing ever created.
15	February 23rd 2022	ardei	Peppers++ 2022, 7-8th Grades	1600?	greedy, sortings	N/A, 9.5/10	Brief summary of the statement: Split an array $$$a_1,a_2,\ldots,a_n$$$ into $$$k$$$ subsequences such that their total cost is minimal. The cost of a subsequence is equal to: $$$\text{Max(subsequence)}-\text{Min(subsequence)}$$$. $$$1 \le k \le n \le 5 \cdot 10^5$$$, $$$1 \le a_i \le 10^9$$$
16	February 23rd 2022	lacul	Peppers++ 2022, 7-8th Grades	1700?	dp, implementation, brute force, data structures	N/A, 6.5/10	Standard-ish 2-d prefix sum problem.
34	December 8th 2022	Cifra Pierduta	Moisil++ 2022, 9th Grade	800?	math, constructive algorithms	N/A, 5/10	Very easy problem for beginners. *If you do not have access to see the problem, join the archive group. The statements are in the bottom right corner of the screen, under contest materials.*
35	December 8th 2022	Match	Moisil++ 2022, 9th Grade	1000?	binary search, two pointers	N/A, 6.5/10	Classical problem designed to check whether the students know the basics of binary search.
36	December 8th 2022	Matematica	Moisil++ 2022, 9th Grade	1800?	number theory, math, implementation	N/A, 6/10	The intended complexity is $$$O(n \cdot \frac{\sqrt{x_i}}{log(\sqrt{x_i})})$$$ using a prime sieve and some additional observations.
37	December 8th 2022	Atat s-a putut	Moisil++ 2022, 10th Grade	1000?	data structures, strings	N/A, 7/10	Filler problem requiring basic prefix sum usage.
38	December 8th 2022	Lex	Moisil++ 2022, 10th Grade	1400?	implementation	N/A, 4/10	Another filler problem. Initially created in 2020 and inspired by really old Romanian OI problems.
39	December 8th 2022	Elcapoc	Moisil++ 2022, 11-12th Grades	900?	graphs, math	N/A, 9/10	This problem was supposed to be the div2A for both rounds #804 and #833, however it was rejected both times.
40	December 8th 2022	Enzero	Moisil++ 2022, 11-12th Grades	1800?	greedy, two pointers	N/A, 9.5/10	The best problem from moisil++ 2022 in my opinion.
41	December 8th 2022	Kentroizi	Moisil++ 2022, 11-12th Grades	1900?	binary search, dp, trees	N/A, 9/10	Kinda standard, but a really good problem otherwise.
42	December 9th 2022	Vraja	Moisil++ 2022, 5th Grade	1300?	math	N/A, 7/10	Challenging math problem for 5th graders (nobody solved it mid-contest).
43	December 9th 2022	Strehaia	Moisil++ 2022, 6th Grade	1400?	math, brute force	N/A, 8.5/10	Nobody solved this one during the contest, which really surprised me. Quite a neat problem.
44	December 9th 2022	Romanul adolescentului miop	Moisil++ 2022, 7-8th Grades	1400?	math, greedy, matrices	N/A, 5.5/10	Filler problem, named after a novel by the Romanian author Mircea Eliade.
45	December 9th 2022	Ion	Moisil++ 2022, 7-8th Grades	1800?	math, greedy	N/A, 8.5/10	Deceivingly difficult, a couple of edge cases, named after a novel by the Romanian author Liviu Rebreanu.
46	January 29th 2023	Divizorus	Moisil++ 2022, 5th Grade	800?	math, constructive algorithms	N/A, 7/10	Still cannot believe that nobody solved this during the contest :(
47	January 29th 2023	Oglinditus	Moisil++ 2022, 5th Grade	1600?	math, implementation	N/A, 2/10	Try this problem at your own risk, it's horrific
48	January 29th 2023	Piscinus	Moisil++ 2022, 6th Grade	1000?	math	N/A, 8/10	Cool problem.
49	January 29th 2023	Fibus	Moisil++ 2022, 6th Grade	1700?	math	N/A, 6.5/10	Filler problem
50	January 29th 2023	Votus	Moisil++ 2022, 7-8th Grades	1300?	implementation	N/A, 7/10	Old problem I came up with in 2020, the statement is the editorial.

FIICode

#	Date	Problem	Contest	Difficulty	Tags	Problem Feedback	Additional comments
20	May 14th 2022	Awesome Atek	FIICode 2022 Round #1	1000?	greedy, constructive algorithms, math	N/A, 7/10	Disclaimer: I did not write the statement. TL;DR: Given $$$n,m$$$ and $$$s$$$, check whether an array $$$a_1,a_2,\ldots,a_n$$$ exists such that $$$1 \le a_i \le m$$$ and $$$\sum_{i=1}^n i \cdot a_i = s$$$.
21	May 14th 2022	Crazy Atek	FIICode 2022 Round #1	2000?	combinatorics, brute force, implementation	N/A, 8/10	The implementation for this problem is really funny, I highly recommend giving it a try.
22	May 15th 2022	Awesome Software	FIICode 2022 Round #2	1000?	brute force, number theory	N/A, 7/10	Decent problem, nothing more to add.
23	May 15th 2022	Boundless Software	FIICode 2022 Round #2	1600?	dp	N/A, 5.5/10	Meh

Codeforces Rounds

#	Date	Problem	Contest	Difficulty	Tags	Problem Feedback	Additional comments
24	July 4th 2022	The Third Three Number Problem	Codeforces Round #804 (Div. 2)	800	greedy, constructive algorithms, math	Participant score: 8.4/10	Cool problem.
25	July 4th 2022	Almost Ternary Matrix	Codeforces Round #804 (Div. 2)	900	bitmasks, constructive algorithms, matrices	Participant score: 8.22/10	antontrygubO_o said he liked this problem.
26	July 4th 2022	The Third Problem	Codeforces Round #804 (Div. 2)	1700	combinatorics, constructive algorithms math	Participant score: 8.59/10	My favourite problem from this contest.
27	July 4th 2022	Almost Triple Deletions	Codeforces Round #804 (Div. 2)	2300	data structures, dp, greedy	Participant score: 9.5/10	Overall a good problem.
28	November 12th 2022	The Ultimate Square	Codeforces Round #833 (Div. 2)	800	math	Participant score: 6.87/10	Truly a 7/10 problem.
29	November 12th 2022	Diverse Substrings	Codeforces Round #833 (Div. 2)	1400	brute force, implementation, strings	Participant score: 8.44/10	Quite the unusual problem.
30	November 12th 2022	Zero Sum Prefixes	Codeforces Round #833 (Div. 2)	1600	brute force, data structures, dp, greedy, implementation	Participant score: 9.05/10	I initially proposed this problem as a div2D since I only had the dp solution. Thanks to IgorI for finding the much more beautiful greedy solution.
31	November 12th 2022	ConstructOR	Codeforces Round #833 (Div. 2)	2100	bitmasks, chinese remainder theorem, combinatorics, constructive algorithms, math, number theory	Participant score: 7.14/10	My favourite problem from this contest and a very controversial one among the participants. The intended solution contains an unusual trick regarding modular inverses.
32	November 12th 2022	Yet Another Array Counting Problem	Codeforces Round #833 (Div. 2)	2300	binary search, data structures, divide and conquer, dp, flows, math, trees	Participant score: 8.72/10	This problem is directly inspired from RMI 2020's floppy and is the first problem featured in tibinyte's blog about cartesian trees :)
33	November 12th 2022	Circular Xor Reversal	Codeforces Round #833 (Div. 2)	3000	bitmasks, constructive algorithms	Participant score: 8.46/10	By far the hardest problem I've ever set. I initially came up with a solution using $$$2.5 \cdot n^2$$$ operations, and IgorI found a better solution using $$$1.5 \cdot n^2$$$ operations
54	May 28th 2023	Twin Permutations	Codeforces Round #875 (Div.1, Div. 2)	800	constructive algorithms	Participant score: 8.77/10	Decent problem.
55	May 28th 2023	The BOSS Can Count Pairs	Codeforces Round #875 (Div.1, Div. 2)	2000	brute force, data structures, math	Participant score: 5.72/10	This problem is unfortunate, nothing more to add.
56	May 28th 2023	Hyperregular Bracket Strings	Codeforces Round #875 (Div.1, Div. 2)	2400	combinatorics, data structures, hashing, math, number theory, sortings, two pointers	Participant score: 9.4/10	Probably the best problem I've ever set, definitely give it a try if you have some time to spare.
57	May 28th 2023	Mex Tree	Codeforces Round #875 (Div.1, Div. 2)	2800	brute force, dp, trees	Participant score: 6.96/10	I initially proposed this problem as a div2. C, thinking that the bipartite coloring approach worked. After we found out that that was not the case, I was barely involved in the preparation of this problem. I absolutely despise the fact that $$$O(N \sqrt{N})$$$ memory doesn't pass, and I hate the forced use of some random tricks from obscure blogs. This is the problem that I hate the most out of everything I've ever written.

ICPC Camps

#	Date	Problem	Contest	Difficulty	Tags	Problem Feedback	Additional comments
58	October 28th 2023	Yet Another Colored Tree Problem	RCPC 2023 Day 1	2100?	trees, dp, dfs and similar	Participant score: 8.7/10	This problem can be solved without any fancy techniques
59	October 28th 2023	Coins	RCPC 2023 Day 1	1400?	games, number theory	Participant score: 8/10	Decent problem I guess
60	October 28th 2023	Anton Would Approve This Problem	RCPC 2023 Day 1	1500?	strings, greedy/dp	Participant score: 10/10	Would Anton approve this problem?
61	October 28th 2023	Difference In Skill	RCPC 2023 Day 1	1600?	data structures	Participant score: 8/10	One of the problems of all time
62	October 28th 2023	Sign Flipping	RCPC 2023 Day 1	1600?	greedy, data structures	Participant score: 9/10	Decent problem I guess
63	October 28th 2023	The Binary Matrix of All Time	RCPC 2023 Day 1	900?	math	Participant score: 4.3/10	Funny problem
64	October 28th 2023	Weird Divisibility	RCPC 2023 Day 1	2200?	number theory, brute force	Participant score: 7/10	Awesome number theory problem
65	October 28th 2023	Triple Reverse Sort	RCPC 2023 Day 1	800?	sortings	Participant score: 8/10	Rejected Codeforces div2A lol
66	October 28th 2023	Distinctness Queries	RCPC 2023 Day 1	2500?	implementation, data structures	Participant score: 8/10	It took me 2 hours to write the model solution for this problem :skull: