Avito Code Challenge 2018 Hi! I'm glad to invite you to take part in [contest:981] which starts on [contest_time:981]. Any participant can join the round and it will be rated for each participant. Hope to see you among the participants! Problems are prepared by me — Ildar Gainullin. This round is conducted on the initiative and support of [Avito](https://www.avito.ru/). Avito.ru is a Russian classified advertisements website with sections devoted to general good for sale, jobs, real estate, personals, cars for sale, and services. Avito.ru is the most popular classifieds site in Russia and is the third biggest classifieds site in the world after Craigslist and the Chinese website 58.com. Many thanks to Vladislav [user:winger,2018-05-24] Isenbaev, Grigory [user:gritukan,2018-05-24] Reznikov, Ivan [user:isaf27,2018-05-24] Safonov，Alexander [user:AlexFetisov,2018-05-24] Fetisov and Shiqing [user:cyand1317,2018-05-24] Lyu for the round testing, Nikolay [user:KAN,2018-05-24] Kalinin for helping me to...

Full text and comments »

Codeforces Round #304 (Div.2) editorial A. Soldier and Bananas ---------------------- We can easily calculate the sum of money that we need to buy all the bananas that we want, let's name it x. If n >  = x the answer is 0, because we don't need to borrow anything. Otherwise the answer is x - n. B. Soldier and Badges --------------------- Let's count the number of badges with coolness factor 1, 2 and so on. Then, let's look at the number of badges with value equal to 1. If it's greater than 1, we have to increase a value of every of them except for one. Then, we look at number of badges with value 2, 3 and so on up to 2n - 2 (because maximum value of badge which we can achieve is 2n - 1). It is easy to see that this is the correct solution. We can implement it in O(n), but solutions that work in complexity O(n^2) also passed. C. Soldier and Cards -------------------- It's easy to count who wins and after how many "fights", but it's harder to say, that game won't end. How to do it? Firstly le...

Full text and comments »

Educational Codeforces Round 106 [Rated for Div. 2] Hello Codeforces! On [contest_time:1499] [contest:1499] will start. Series of Educational Rounds continue being held as [Harbour.Space University](https://harbour.space/) initiative! You can read the details about the cooperation between [Harbour.Space University](https://harbour.space/) and Codeforces in the <a href="http://codeforces.me/blog/entry/51208">blog post</a>. This round will be **rated for the participants with rating lower than 2100**. It will be held on extended ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally. You will be given **6 or 7 problems** and **2 hours** to solve them. The problems were invented and prepared by Roman [user:Roms,2021-03-17] Glazov, Adilbek [user:adedalic,2021-03-17] Dalabaev, Vladimir [user:vovuh,2021-03-17] Petrov, Ivan [us...

Full text and comments »

Educational Codeforces Round 106 Editorial [problem:1499A] Idea: [user:adedalic,2021-03-19] <spoiler summary="Tutorial"> [tutorial:1499A] </spoiler> <spoiler summary="Solution (adedalic)"> ~~~~~ fun main() { repeat(readLine()!!.toInt()) { val (n, k1, k2) = readLine()!!.split(' ').map { it.toInt() } val (w, b) = readLine()!!.split(' ').map { it.toInt() } if (k1 + k2 >= 2 * w && (n - k1) + (n - k2) >= 2 * b) println("YES") else println("NO") } } ~~~~~ </spoiler> [problem:1499B] Idea: [user:BledDest,2021-03-19] <spoiler summary="Tutorial"> [tutorial:1499B] </spoiler> <spoiler summary="Solution (Neon)"> ~~~~~ #include <bits/stdc++.h> using namespace std; int main() { int t; cin >> t; while (t--) { string s; cin >> s; int i = s.find("11"); int j = s.rfind("00"); cout << (i != -1 && j != -1 && i < j ? "NO" : "YES") << endl; } } ~~~~~ </spoiler> [problem:1499C] Idea: [use...

Full text and comments »

Unofficial editorial for Codeforces Round #760 (Div.3) <spoiler summary="A small confession"> Hi, this is the first time I write such a blog like this. All is because of my excitement that for the first time I can solve all problems, not because I am worshiping myself or something. I know this is just a Div-3 contest, so there are many people who can solve it. But if you are stuck with some problems, feel free to read my solutions. This is not an official editorial, so the solutions might not be the best of all solutions out there, so if you have something to discuss, feel free to leave something below. Thanks for all. </spoiler> [A. Polycarp and Sums of Subsequences](https://codeforces.cc/contest/1618/problem/A) ------------------------------------------------------------------------------------- The first two numbers cannot be produced by a sum operation, so we have $2$ of $3$ numbers we must find. How to find the last one? Subtract these two from the largest one. <spoiler summary="Implentation"> ~~~~~ vector<ll> a(...

Full text and comments »

Problems that I authored so far **UPD 1 [May 17 2025]:** I added my problems from Ukrainian Olympiad in Informatics (among others). They are available here: [contest:105820] Hi everyone! I wanted to write such a blog for a long time, motivated by similar blogs [by](https://codeforces.me/blog/entry/108940) [user:adamant,2023-02-20] and [by](https://codeforces.me/blog/entry/108595) [user:tibinyte,2023-02-20]; 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 cruc...

Full text and comments »

ALL CodeForces Rounds Tutorial <html> <body> <br /><br /> >>>>>>>**Hi I just Say Really Sorry For Delay in UPD This Blog...!**<br /><br /><br /> <u>This is not Complete</u> , but i try to find <b>all Tutorial codeforces round</b> and posted here , thanks in advance :D <br /><br />This post will <b> <span class="Apple-style-span" style="border-collapse: collapse;color: rgb(255,0,0);font-family: verdana , arial , sans-serif;font-size: 12.0px;"> UPD</span></span></b> after each contest . <span>If anyone have or writes any editorial that's</span> <b> <span> <span class="Apple-style-span" style="border-collapse: collapse;color: rgb(255,0,0);font-family: verdana , arial , sans-serif;font-size: 12.0px;"> not in here</span></span></b> , Plz post here for ALL . <br /><br /><b>UPD </b> &nbsp;: Table1 updated : in Future!!! <br /><br /><b>UPD </b>&nbsp;: Table2 updated : CF # 118 ...to... CF # 169 added .<br /><br /> </p><hr /><br /></p> <div align="justify"> <br /><br /><b><s...

Full text and comments »

UOJ Easy Round #13 Announcement **UPD: We have changed contest duration from 3 hours to 3.5 hours.** We will hold [UOJ Easy Round #13](https://uoj.ac/contest/106) contest! You will be given **4** problems to be solved in **3.5** hours. - Contest link: <https://uoj.ac/contest/106> - Start Time: <https://www.timeanddate.com/worldclock/fixedtime.html?iso=20260404T1900&p1=33&ah=3> - Writers: [user:Comentropy,2026-04-03], [user:pp_orange,2026-04-03], [user:jiaosiyuan,2026-04-03], [user:Gellyfish,2026-04-03] - Testers: [user:Crysfly,2026-04-03], [user:hos.lyric,2026-04-03] - Duration: 3.5 hours - Difficulty: CF Div 1.5/ARC++ - The point values: 100-100-100-100 The contest will follow the **IOI** rules: Each problem can be submitted up to 50 times, and scores can be viewed in real-time. All the problems will have subtasks, the final score will be based on the highest-scoring submission. You can see the scoreboard during the contest. The contest is rated for everyone, you will be count as rated only if ...

Full text and comments »

Educational Codeforces Round 78 [Rated for Div. 2] Hello Codeforces! On [contest_time:1278] [contest:1278] will start. Series of Educational Rounds continue being held as [Harbour.Space University](https://harbour.space/) initiative! You can read the details about the cooperation between [Harbour.Space University](https://harbour.space/) and Codeforces in the <a href="http://codeforces.me/blog/entry/51208">blog post</a>. This round will be **rated for the participants with rating lower than 2100**. It will be held on extended ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally. You will be given **6 problems** and **2 hours** to solve them. The problems were invented and prepared by Roman [user:Roms,2019-12-18] Glazov, Adilbek [user:adedalic,2019-12-18] Dalabaev, Vladimir [user:Vovuh,2019-12-18] Petrov, Ivan [user:Bl...

Full text and comments »

Разбор задач Codeforces Beta Round #82 (Div. 2) <p><b>A.</b> (<a href="http://codeforces.me/contest/106/problem/A">ссылка</a>) Решение этой задачи описано в четвертом абзаце условия. Его надо было внимательно прочитать и реализовать. Единственная сложность которая могла возникнуть - как опередить какое достоинство старше. Для этого можно было двумя проходами по массиву [ '6', '7', '8', '9', 'T', 'J', 'Q', 'K', 'A' ] определить номера достоинств карт в массиве, а полученные числа сравнить.<br>[cut]<br><b>B.</b> (<a href="http://codeforces.me/contest/106/problem/B">ссылка</a>) Можно было использовать дополнительный массив, в котором true означает, что ноутбук устаревший, а false - что нет. Значение в каждой ячейке этого массива определяется проходом по всем ноутбукам и сравнения его параметров с параметрами текущего ноутбука. За еще один проход среди всех не устаревших ноутбуков нужно было выбрать самый дешевый.<br><br><b>C.</b> (<a href="http://codeforces.me/contest/106/problem/C">ссылка</a>) Создадим массив dp размера n на m. dp[i][...

Full text and comments »

Good Bye 2015! (review, pt. 1) Hey there, I couldn't sleep, so I spend almost an hour to play with some Python and answering some questions about "who was... in 2015 on Codeforces" (also checking if I still remember anything :)). So &mdash; have you ever wondered who was the best in this year on Codeforces? I will try to answer to your (and mostly mine) questions. Here is the first part (and maybe the last :)) of this year review. [cut] In this year, there were 105 contests on Codeforces, 12 of them were not on CF rules, so there were removed from the rest of review (i,e, Educational Rounds, some ACM contests and so on). ### Most problems solved | Handle | Contests | Sum of tasks solved | Average | | :---: | :---: | :---: | :--- : | | [user:xauoyero,2015-12-31] | 52 | 112 | 2.153846 | | [user:alex_bucevschi,2015-12-31] | 53 | 106 | 2.000000 | | [user:Shavkat_Aminov,2015-12-31] | 54 | 105 | 1.944444 | | [user:ershov.stanislav,2015-12-31] | 31 | 100 | 3.225806 | | [user:Endagorion,2015-12-31] |...

Full text and comments »

Educational 106 problem D time limit and hacks Hi, Codeforces! I have 13 successful hacks of problem D on last round, and also my solution is TLed(probably on one of my tests :)), and i am going to show you, how it works. So we have two parts of solution. The first is to precalculate lowest divisor of each number. It can be done with simple Eratosthenes sieve, which has time complexity $O(N \log N)$ if you do sieve loop for all numbers, and $O(N \log \log N)$ if you do loop only when number is prime. It can also be done with $O(N)$ time and memory using linear sieve. Second part is the answer calculation, you need to add $2^k$, where $k$ -- number of different prime divisors. So if you haven't precalucated it on sieve step, it will be $O(\log x)$ per divisor, otherwise it will be $O(1)$. Now we will try to adjust numbers $c, d, x$ to hack $O(t\sqrt{x} \log x + N \log \log N)$ solution. Because we are counting distinct prime factors of numbers like $\frac{\frac{x}{d_x} + d}{c}$, we will use $c = 1$. And we are facto...

Full text and comments »

Codeforces Round #106 (Div. 2) Tutorial [problem:149A] --------------- First, it is clear that if the sum of all numbers $a_i$ is less than $k$, then Peter in any case will not be able to grow a flower to the desired height, and you should output <<-1>>. Secondly, it is easy to see that if we want to choose a one month of two, in which we watered the flower, it is better to choose one where the number of $a_i$ is more. Thus, the solution is very simple: let's take months in descending order of numbers $a_i$ and in these months water flowers. As soon as the sum of the accumulated $a_i$ becomes greater than or equal to $k$ --- should stop the process, the answer is found. [problem:149B] ------------------- In this task required only the ability to work with different numeral systems. Let's try to go through numeral bases, each base to check whether it is permissible, as well as convert hours and minutes to the decimal system and compared with 24 and 60, respectively. What is maximal base, that we need to check? In...

Full text and comments »

[COPS Editorial] Codeforces Educational Round 106 Video Editorials Hello Codeforces! Me and my friends [user:CoderAnshu,2021-03-19], [user:XORring-Samurai,2021-03-19] and [user:CodeForChef,2021-03-19] from Competitive Programming Group, COPS IIT-BHU, attempted to make a video editorial for problems A to E of Educational Codeforces Round 106. Here are the links:<br> [Problem A — Domino on Windowsill](https://www.youtube.com/watch?v=pEEod4CFjr0&t=5s)<br> [Problem B — Binary Removals](https://www.youtube.com/watch?v=dbOxu7yi90M&t=2s)<br> [Problem C — Minimum Grid Path](https://www.youtube.com/watch?v=Fms1iQmZqrU&t=6s)<br> [Problem D — The Number of Pairs](https://www.youtube.com/watch?v=1WhYPCXIu-c&t=3s)<br> [Problem E — Chaotic Merge](https://www.youtube.com/watch?v=3ZdBR1cbd14&t=1107s)<br> We are a bunch of college students, who have been making Video editorials and doing Screencasts of recent contests. We also have a curated list of topic-wise concept videos, which has been divided into 3 sections, Beginner, Intermediate and Advanced. Y...

Full text and comments »

More and More Suspicious Participants in AtCoder Beginner Contests! Hello, CodeForces. As I can't find any discussion thread in AtCoder, and the announcements of AtCoder contests are also posted here, I decided to post this blog in CodeForces. Last time (ABC355), I [reported](https://codeforces.me/blog/entry/129765?#comment-1151921) a suspicious participant [toyuzuko](https://atcoder.jp/users/toyuzuko) who solved problem ABCD in 51 seconds. They also did the similar thing in ABC354. This time (ABC356), there are much more suspicious participants. The average first-AC time (in seconds) of ABC347~ABC353 is in the following table: |Contest|A|B|C|D|E|F|G| |:-|:-|:-|:-|:-|:-|:-|:-| |ABC347|21|45|93|244|177|300|824| |ABC348|15|37|72|302|236|495|430| |ABC349|23|67|54|116|598|139|634| |ABC350|26|47|102|74|240|489|335| |ABC351|22|23|106|352|199|122|902| |ABC352|27|43|49|90|196|989|188| |ABC353|24|79|172|88|206|1161|313| |(minimum)|15|23|49|74|177|122|188| |(prefix sum)|15|38|87|161|338|460|648| Note that to get first-AC in a problem...

Full text and comments »

Code League — National Competitive Programming Contest — Round 2 — Editorial Here is the link to the contest: [Link](https://codeforces.me/contestInvitation/f528fcc29732183e36f82a0adfdc6dbac0743f3f) ### [A. Free Coupon](https://codeforces.me/gym/690685/problem/A) <spoiler summary="Rate the Problem"> - **How good is this problem?** - Very Good - Good - Bad - Very Bad - **How hard is this problem?** - Very Easy - Easy - Hard - Very Hard </spoiler> <spoiler summary="Hint"> Which items should be taken using coupons? <spoiler summary="Answer"> Since a coupon can be used on any item regardless of its price, it is always best to use coupons on the most expensive items. This allows us to spend our coins only on cheaper items. </spoiler> </spoiler> <spoiler summary="Solution"> The problem asks us to find the maximum number of items Abenezer can obtain given $k$ coins, with the offer that every $b$ items bought with coins yiel...

Full text and comments »

Editorial Round #106 This was my first time to participated in Codeforces (<a href="http://www.codeforces.com/">http://www.codeforces.com/</a>) &nbsp;, an online programming contest site.<br /> <br /> This is a link explaining rules about the contest :&nbsp;<a href="http://codeforces.me/blog/entry/456">http://codeforces.me/blog/entry/456</a><br /> <br /> During the contest, I made a newbie mistake of resubmitting the same solution twice because I didn't understand what "judgement failed." meant. I thought it was something like "compilation error." &nbsp;Only after resubmitting my solution did I learn that "judgement failed" meant "there is something wrong with our server. Please wait a while we fix this issue." I lost about 100 points for that mistake.<br /> <br /> <br /> But other than that, things went okay.<br /> <br /> I solved three problems out of five &nbsp;- A , B, and C.<br /> <br /> Here is the link to problem statement :&nbsp;<a href="http://codeforces.me/contest/149">http://codefo...

Full text and comments »