Hi everyone! I wanted to write such a blog for a long time, motivated by similar blogs by adamant and by tibinyte; I finally decided to do it after my Universal Cup contest. This is not a super-comprehensive list, I also set some problems for some local contests, but that's most of it.
I want to encourage other setters to write such blogs. For me, it's very interesting to read about the backstories of some problems and also to see all the problems by some author gathered in one place (as most authors give problems to several platforms).
One important point. As you will see from the comments, many of my problems were improved by other people, and I myself improved some problems by other people. I think that it's crucial to have someone to discuss your problems with (apart from the coordinator).
So, here we go.
| # | Date | Problem | Contest | Comment |
|---|---|---|---|---|
| 1 | June 2019 | Vus the Cossack and Strings | Codeforces Round #571 (Div. 2) | |
| 2 | June 2019 | Vus the Cossack and a Graph | Codeforces Round #571 (Div. 2) | My first problem |
| 3 | July 2019 | Keanu Reeves | Codeforces Round #572 (Div. 2) | Split binary string into the smallest number of substrings, in each of which # of 0s is not equal to # of 1s. Initially, we wanted to ask to split into any number of substrings (and the model solution was to split into n characters), but we were told that this is not a real problem |
| 4 | July 2019 | Number Circle | Codeforces Round #572 (Div. 2) | |
| 5 | July 2019 | Candies! | Codeforces Round #572 (Div. 2) | |
| 6 | July 2019 | Add on a Tree | Codeforces Round #572 (Div. 1) | |
| 7 | July 2019 | Add on a Tree: Revolution | Codeforces Round #572 (Div. 1) | We were told to add this problem so that the contest would have at least some implementation |
| 8 | July 2019 | Count Pairs | Codeforces Round #572 (Div. 1) | One of my best problems. Pinnacle of problemsetting... |
| 9 | July 2019 | Make Equal | Codeforces Round #572 (Div. 1) | I came up with the statement, 244mhq with solution |
| 10 | July 2019 | Problem from Red Panda | Codeforces Round #572 (Div. 1) | 244mhq came up with the statement, I with solution |
| 11 | August 2019 | Choose Two Numbers | Codeforces Round #580 (Div. 2) | |
| 12 | August 2019 | Make Product Equal One | Codeforces Round #580 (Div. 2) | |
| 13 | August 2019 | Almost Equal | Codeforces Round #580 (Div. 1) | |
| 14 | August 2019 | Shortest Cycle | Codeforces Round #580 (Div. 1) | |
| 15 | August 2019 | Palindromic Paths | Codeforces Round #580 (Div. 1) | |
| 16 | August 2019 | Almost All | Codeforces Round #580 (Div. 1) | One of my best problems. Unfortunately, a similar idea was used in a problem from IOI 2019 |
| 17 | August 2019 | Expected Value Again | Codeforces Round #580 (Div. 1) | |
| 18 | August 2019 | Beauty of a Permutation | Codeforces Round #580 (Div. 1) | Sadly, I didn't know about permutation tree back then |
| 19 | October 2019 | Find the Array | SEERC 2019 | My best interactive problem |
| 20 | October 2019 | Cycle String? | SEERC 2019 | |
| 21 | October 2019 | Game on a Tree | SEERC 2019 | This is not a good problem, as it indeed is well known for general graphs... Don't do this |
| 22 | October 2019 | Tree Permutations | SEERC 2019 | |
| 23 | October 2019 | Graph and Cycles | SEERC 2019 | |
| 24 | December 2019 | Make Good | Good Bye 2019 | I initially had solution only with adding $$$3$$$ elements. In testing, some testers discovered a solution with $$$2$$$ elements, and after the contest, we realized that it can be solved with only $$$1$$$ element! |
| 25 | December 2019 | Strange Device | Good Bye 2019 | |
| 26 | December 2019 | Divide Points | Good Bye 2019 | One of my best problems |
| 27 | December 2019 | Awesome Substrings | Good Bye 2019 | There is a very nice $$$n\log^2{n}$$$ solution, but we were not able to find it :( |
| 28 | December 2019 | Subset with Zero Sum | Good Bye 2019 | One of my best problems. Initially, I was going to send it to IMO, and there was another problem at this position, but this problem by a recent round by hugopm turned out to be too similar to it :(, so we had to replace it. |
| 29 | December 2019 | Xor on Figures | Good Bye 2019 | Initially, I sent it to IOI 2020, but then decided that it doesn't have meaningful subtasks and that it would be better to use it on CF. I wrote to the PSC and told them that I am retracting the proposal, apologizing. It seems that they missed my email, as in a few months, I received an email from PSC saying: The ISC would like to remind you that this strongly violates the rules of the IOI Call for Tasks, which requires all submitted tasks to be kept strictly confidential until the end of IOI 2020. In the future, if you would like to submit the task elsewhere and not have your task be considered for the IOI, you should email the host directly to let them know of this decision. |
| 30 | February 2020 | So Mean | Codeforces Round #618 (Div. 1) | I just became a coordinator on Codeforces, and wanted to help to conduct the round as soon as possible, so I donated this problem. Now I don't think that it's a good problem... Mostly casework |
| 31 | March 2020 | Kuroni and Impossible Calculation | Ozon Tech Challenge 2020 | |
| 32 | March 2020 | Kuroni and the Score Distribution | Ozon Tech Challenge 2020 | |
| 33 | March 2020 | Kuroni and Antihype | Ozon Tech Challenge 2020 | One of my best problems. Initially, the problem was about the general graph. Unfortunately, minimum arborescence solves the problem, so we had to adjust it this way. |
| 34 | July 2020 | Integer Game | Codeforces Global Round 9 | 244mhq suggested the setup, and I solved it. I will just leave this comment here |
| 35 | July 2020 | Tree Modification | Codeforces Global Round 9 | This is one of the last problems which I created when I still thought that solution->statement is better than statement->solution is a much better way of problemsetting. This problem is a good example of why I switched... making statements not artificial with solution->statement is very hard. |
| 36 | September 2020 | Permutation Forgery | Codeforces Round #668 (Div. 2) | |
| 37 | September 2020 | Watermelon | September Lunchtime 2020 (Div. 2) | |
| 38 | September 2020 | GCD operations | September Lunchtime 2020 (Div. 2) | |
| 39 | September 2020 | Root the Tree | September Lunchtime 2020 (Div. 1) | |
| 40 | September 2020 | Robot Detector | September Lunchtime 2020 (Div. 1) | |
| 41 | September 2020 | Permutation Split | September Lunchtime 2020 (Div. 1) | I was thinking about this problem for quite some time in the background, and then at some point 244mhq brought up the following (old) problem: "Given a graph, check if we can split it into two graphs with same numbers of edges." I realized this problem was a partial case, but the statement was so natural, and the original problem wasn't too well-known from what I've searched, so I decided to still use this problem. Generally, using partial cases of some other problems is a very bad practice... |
| 42 | September 2020 | Few Different Elements | September Lunchtime 2020 (Div. 1) | |
| 43 | September 2020 | Adding on Segments | September Lunchtime 2020 (Div. 1) | My first problem that wasn't solved in the contest |
| 44 | November 2020 | XOR-gun | Technocup 2021 — Elimination Round 2 | |
| 45 | December 2020 | String Operations | Codechef December Challenge 2020 (Div. 1) | |
| 46 | March 2021 | Long Common Subsequence | AtCoder Grand Contest 052 | |
| 47 | March 2021 | Tree Edges XOR | AtCoder Grand Contest 052 | One of my best problems |
| 48 | March 2021 | Nondivisible Prefix Sums | AtCoder Grand Contest 052 | The idea of the setup is by 244mhq in 2020, then I solved it and improved to the counting version |
| 49 | March 2021 | Equal LIS | AtCoder Grand Contest 052 | |
| 50 | March 2021 | 3 Letters | AtCoder Grand Contest 052 | One of my best problems. Thanks to maroonrk for coming up with this nice trick, my initial solution was much more complicated |
| 51 | March 2021 | Tree Vertices XOR | AtCoder Grand Contest 052 | My second problem that wasn't solved in the contest |
| 52 | May 2021 | Reverse Game | SEERC 2020 | |
| 53 | May 2021 | 3-colorings | SEERC 2020 | Initially this was going to be used as AGC52F, but we were afraid of not intended solutions. Sadly, all AC solutions on the mirror were indeed not intended :( In the latter half of $$$2020$$$, I almost stopped coming up with problems; solution->statement didn't work well for me anymore, and I didn't believe I could come up with hard problems in the statement->solution way. This was the first hard problem that I came up with in statement->solution way, and it was the reason why I believed that I can make an AGC set; I messaged maroonrk after I came up with it. |
| 54 | May 2021 | Divisible by 3 | SEERC 2020 | |
| 55 | May 2021 | Fence Job | SEERC 2020 | Initially, I wanted to solve the problem "Choose segment, replace all its elements by their OR, find the number of possible arrays", but my solutions turned out to be wrong. Then bicsi helped to revise the problem into this version |
| 56 | May 2021 | AND = OR | SEERC 2020 | One of my few good data structure problems |
| 57 | May 2021 | Modulo Permutations | SEERC 2020 | |
| 58 | August 2021 | Eulerian? | GP of IMO | |
| 59 | August 2021 | Fancy Formulas | GP of IMO | |
| 60 | August 2021 | Glory Graph | GP of IMO | I was inspired by the fact that for a graph, knowing all degrees, we can find the number of triangles plus antitriangles, so I was looking for some sort of relation for graphs on four nodes. |
| 61 | August 2021 | Hamiltonian | GP of IMO | |
| 62 | August 2021 | Intellectual Implementation | GP of IMO | I was once again inspired by this problem above, and thought that I can try to use this idea for counting something awful. Then, I came up with this statement, and 244mhq helped to solve it |
| 63 | August 2021 | Joke | GP of IMO | |
| 64 | August 2021 | K-onstruction | GP of IMO | Initially 244mhq proposed this problem, asking to construct a set of size at most $$$3\log{K}$$$ for a given $$$K$$$. We were then able to improve it to the bound in the statement. |
| 65 | August 2021 | Little LCS | GP of IMO | Sorry for this problem... but if it fits anywhere, it's a camp training contest |
| 66 | September 2021 | One Pile | September Cook-Off 2021 (Div. 1) | Utkarsh.25dec initially proposed version without counting (just determine who wins). I helped to make it harder, as we needed some hard problems for Cook-Off |
| 67 | October 2021 | Permutating Inversions | October Cook-Off 2021 (Div. 1) | My third problem that wasn't solved in the contest. I wanted to add a pre-last problem to a Cook-Off, as I thought that the contest would be too easy otherwise. Apparently, this problem turned out to be the hardest, with 0 solves, with the next hardest problem having 1 solve... |
| 68 | October 2021 | ABC Identity | AtCoder Grand Contest 055 | Jason Bourne 1 |
| 69 | October 2021 | ABC Supremacy | AtCoder Grand Contest 055 | Jason Bourne 2 |
| 70 | October 2021 | Weird LIS | AtCoder Grand Contest 055 | |
| 71 | October 2021 | ABC Ultimatum | AtCoder Grand Contest 055 | Jason Bourne 3. Initially, the problem was just to check whether a string is good ($$$N \le 2\cdot 10^5$$$), but then AmShZ submitted an unproven greedy in testing, and we were able to improve it to the current version |
| 72 | October 2021 | Set Merging | AtCoder Grand Contest 055 | Certainly my best problem ever, and my fourth problem that wasn't solved in the contest. Read the comment for problem 55 from this list... I decided to solve this problem for OR when $$$a_i = 2^i$$$ initially, and bruteforce + OEIS showed that the answer is the number of sequences with LDS<=3. I was shocked and was stuck on proving this for three days until I finally realized the solution. |
| 73 | October 2021 | Creative Splitting | AtCoder Grand Contest 055 | My initial proposal was: Let's call an array $$$a$$$ of length $$$2n$$$, consisting of integers from $$$1$$$ to $$$n$$$, good, if it is possible to split it into two subsequences of length $$$n$$$, such that in each subsequence $$$i$$$-th element doesn't exceed $$$i$$$ for every $$$i$$$ from $$$1$$$ to $$$n$$$. You are given array $$$a$$$ of length $$$2n$$$, where some numbers are given, and some are $$$−1$$$ (unknown). Find the number of ways to replace $$$−1$$$s with integers from $$$1$$$ to $$$n$$$ so that $$$a$$$ becomes good. With brute force, I knew that the number of "splittable" arrays is $$$\frac{(2n)!}{2^n}$$$ for several months, but I couldn't prove this. Thanks a lot to maroonrk for providing the proof and then for improving my initial proposal to this version. |
| 74 | November 2021 | Many LCS | SEERC 2021 | Initially I needed length of order $$$\sqrt{K}$$$, thanks to Um_nik for improving the problem to the current constraints |
| 75 | November 2021 | Max Pair Matching | SEERC 2021 | |
| 76 | November 2021 | ABC Legacy | SEERC 2021 | Jason Bourne 4 |
| 77 | November 2021 | Counting Phenomenal Arrays | SEERC 2021 | |
| 78 | November 2021 | LIS Counting | GP based on SEERC 2021 | I was really into LIS problems at the time... |
| 79 | December 2021 | Sorting Segments | SnackDown 2021 Online Elimination Round | Yes, this is very similar to Stooge Sort, but I haven't heard about it before setting the problem |
| 80 | December 2021 | Sorter Prodigy | SnackDown 2021 Online Elimination Round | Unfortunately, coincided with CCO '18 P6 :(. This happens |
| 81 | December 2021 | Prefix Suffix LIS Constructing | SnackDown 2021 Online Elimination Round | |
| 82 | January 2022 | Equal Number of Prefix Maximums | SnackDown 2021 Final Round | Unfortunately, this is an easier version of AGC028E :( |
| 83 | January 2022 | Circular Permutation Recovery | January Lunchtime 2022 (Div. 1) | Unfortunately, this turned out to be an easier version of IOI 2020 plants... |
| 84 | January 2022 | Divisible Distances | January Lunchtime 2022 (Div. 1) | |
| 85 | April 2022 | Modular Circular Permutations | April Lunchtime 2022 (Div. 1) | |
| 86 | April 2022 | Odd Split | April Lunchtime 2022 (Div. 1) | |
| 87 | May 2022 | Doubled Distances | May Cook-Off 2022 (Div. 2) | |
| 88 | May 2022 | Triple Inversions | May Cook-Off 2022 (Div. 1) | |
| 89 | May 2022 | Different Subarrays Rearrange | May Cook-Off 2022 (Div. 1) | |
| 90 | May 2022 | Split Powers Of 2 | May Cook-Off 2022 (Div. 1) | |
| 90 | May 2022 | Max Min Circle Difference | May Cook-Off 2022 (Div. 1) | |
| 91 | May 2022 | Plus Xor Increase | May Cook-Off 2022 (Div. 1) | Another rare good algorithmic problem... |
| 92 | May 2022 | Make Grid Comparable | May Cook-Off 2022 (Div. 1) | |
| 93 | May 2022 | Catch Me If You Can | May Cook-Off 2022 (Div. 1) | My fifth problem that wasn't solved in the contest |
| 94 | May 2022 | Everything Everywhere All But One | Codeforces Round #794 (Div. 2) | Watch Everything Everywhere All At Once! |
| 95 | May 2022 | Odd Subarrays | Codeforces Round #794 (Div. 2) | |
| 96 | May 2022 | Circular Local MiniMax | Codeforces Round #794 (Div. 1) | |
| 97 | May 2022 | Bring Balance | Codeforces Round #794 (Div. 1) | |
| 98 | May 2022 | Permutation Weight (Easy Version) | Codeforces Round #794 (Div. 1) | |
| 99 | May 2022 | Permutation Weight (Hard Version) | Codeforces Round #794 (Div. 1) | |
| 100 | May 2022 | The Ultimate LIS Problem | Codeforces Round #794 (Div. 1) | One of my best problems |
| 101 | June 2022 | Split AND Sum | June Cook-Off 2022 (Div. 1) | |
| 102 | June 2022 | Prefix Suffix Distinct | June Cook-Off 2022 (Div. 1) | |
| 103 | June 2022 | Max Minus Min | June Cook-Off 2022 (Div. 1) | This problem was initially proposed by jainmilind for $$$A, B, C \le 10^6$$$, I was able to improve it to the current constraints |
| 104 | June 2022 | XOR, The Detective | June Lunchtime 2022 (Div. 1) | |
| 105 | June 2022 | SPLIT With XOR Not X | June Lunchtime 2022 (Div. 1) | Filler problem, made just for all XOR contest |
| 106 | September 2022 | Longest Unfriendly Subsequence | EJOI 2022 | |
| 107 | September 2022 | Permutations LCS | EJOI 2022 | Dear EJOI 2022 participants. I am deeply sorry for your destroyed mental health |
| 108 | September 2022 | Anti-Increasing Addicts | Codeforces Global Round 22 | |
| 109 | November 2022 | Anti-median (Easy Version) | Pinely Round 1 (Div. 1 + Div. 2) | |
| 110 | December 2022 | My Last ABC Problem | AtCoder Grand Contest 059 | From now on, only problems about 123 |
| 111 | December 2022 | Arrange Your Balls | AtCoder Grand Contest 059 | |
| 112 | December 2022 | Guessing Permutation for as Long as Possible | AtCoder Grand Contest 059 | When I first came up with a statement and solved it, I thought that the problem is standard and garbage. It turned out I was extremely wrong... |
| 113 | December 2022 | Distinct Elements on Subsegments | AtCoder Grand Contest 059 | I came up with the statement in May of $$$2022$$$ and couldn't solve for three months. |
| 114 | December 2022 | Grid 3-coloring | AtCoder Grand Contest 059 | Initially, I proposed a version with construction for $$$n<=100$$$, and my proof was extremely hard (and greedy). A week before the contest maroonrk realized that we can use the same idea as in AGC052E, and the problem became much nicer. Unfortunately, this idea turned out to be present in some papers... |
| 115 | December 2022 | LIDS | AtCoder Grand Contest 059 | My sixth problem that wasn't solved in the contest, and, probably, the hardest problem that I ever made. In March 2022, I noticed that the number of permutations of length $$$n$$$ with $$$LIS = x, LDS = n+1-x$$$ is $$$C(n-1, x-1)^2$$$. (Yes, this is easy with the Robinson–Schensted correspondence, but I didn't know it). I was able to prove this only in August, and after finding a nice bijection in the process, I decided that I can try to write a third AGC. |
| 116 | December 2022 | Divisible by 4 Spanning Tree | SEERC 2022 | |
| 117 | December 2022 | Exercise | SEERC 2022 | |
| 118 | December 2022 | Inadequate Operation | SEERC 2022 | |
| 119 | February 2023 | Adjacent Product Sum | Universal Cup 1 Stage 4: Ukraine | |
| 120 | February 2023 | Binary Arrays and Sliding Sums | Universal Cup 1 Stage 4: Ukraine | |
| 121 | February 2023 | Count Hamiltonian Cycles | Universal Cup 1 Stage 4: Ukraine | My initial solution was in $$$O(n^2)$$$, thanks to Um_nik for improving it to $$$O(n)$$$ |
| 122 | February 2023 | Distance Parities | Universal Cup 1 Stage 4: Ukraine | |
| 123 | February 2023 | Excellent XOR Problem | Universal Cup 1 Stage 4: Ukraine | |
| 124 | February 2023 | F*** 3-Colorable Graphs | Universal Cup 1 Stage 4: Ukraine | Like... who tf likes 3-colorable graphs? |
| 125 | February 2023 | Graph Problem With Small $n$ | Universal Cup 1 Stage 4: Ukraine | This problem was born when me and 244mhq were discussing how to implement checker to Hamiltonian from our Ptz contest... We were surprised that we haven't seen it before, and it seems that almost no one has seen it. Apparently, Golovanov399 showed that this is standard exercise :( |
| 126 | February 2023 | Help Me to Get This Published | Universal Cup 1 Stage 4: Ukraine | Please, help me to get this published |
| 127 | February 2023 | Increasing Grid | Universal Cup 1 Stage 4: Ukraine | One of my most standard problems, but I decided that having such problem in an ICPC contest is good |
| 128 | February 2023 | Jewel of Data Structure Problems | Universal Cup 1 Stage 4: Ukraine | |
| 129 | February 2023 | King of Swapping | Universal Cup 1 Stage 4: Ukraine | |
| 130 | February 2023 | Least Annoying Constructive Problem | Universal Cup 1 Stage 4: Ukraine | My best constructive problem |
| 131 | February 2023 | Most Annoying Constructive Problem | Universal Cup 1 Stage 4: Ukraine | My seventh problem that wasn't solved in a contest, and the first one that wasn't solved in ICPC format |
| 132 | February 2023 | No Zero-Sum Subsegment | Universal Cup 1 Stage 4: Ukraine |
