Batrr's blog

By Batrr, history, 2 months ago, In English

First: We apologize for any inconvenience caused. On the second day, we'll try to correct all today's mistakes.

We still hope that you enjoyed the contest.

You can upsolve here

Let's discuss the problems.

Upd: Day 1 unofficial results here

Upd: Day 1+2 unofficial results here

Upd: Day 2 now available to upsolving.

Upd: Editorials.

 
 
 
 
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2 months ago, # |
  Vote: I like it +41 Vote: I do not like it

Can you say how to solve A?

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    2 months ago, # ^ |
      Vote: I like it +63 Vote: I do not like it

    Let's assume that we are at point $$$p$$$ now. We have two options.

    1) Go to $$$t$$$ directly, paying extra $$$|p - t| * a_p$$$ money;

    2) Go to another "optimal" point. There are two choices for that. Either it is minimum $$$r$$$ such that $$$a_r < a_p$$$ and $$$r > p$$$, or maximum $$$l$$$ such that $$$a_l < a_p$$$ and $$$l < p$$$.

    So run Dijkstra, maintaining minimum distance. Update answer by the first option. Or go to the next optimal point paying $$$a_v * dist$$$ ($$$r - p$$$ or $$$p - l$$$) money. We are multiplying $$$a_v$$$ because each time we are going to the index with a lower value.

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      2 months ago, # ^ |
        Vote: I like it +46 Vote: I do not like it

      I also did djikstra, but there is no need for it. The graph you get is a DAG, so simple dp is enough.

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    2 months ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    If I'm not wrong, it can also be solved by the Convex hull trick. The naive solution will be using DP. Let's say $$$dp_i$$$ is the minimum time needed to reach the point $$$i$$$. Recurrent relation will look like $$$dp_i = dp_j + |i - j| \cdot a_j$$$. It is easy to notice that the next point's value will be always less than the current's (i.e $$$a_{cur} > a_{next}$$$). So the $$$a's$$$ will be decreasing.

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      7 weeks ago, # ^ |
      Rev. 2   Vote: I like it +22 Vote: I do not like it

      I also solved it with a convex hull trick. I used two convex hulls. One for queries for the left (i < j) decreasing "ladder" from the start, the other for the right (i > j).

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2 months ago, # |
  Vote: I like it +25 Vote: I do not like it

Can you say how to solve B?

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    2 months ago, # ^ |
    Rev. 5   Vote: I like it +10 Vote: I do not like it

    To solve in n^2logn, we can notice that actually if we reassing our element from L to R, their id to from 1 to R — L + 1, it is easy to notice that actually after each separation their indexes goes left shift. So having M different groups, we can answer the sum with the formula. Then we can notice that 1 to R — L + 1, does not matter and we can just add their initial ID. So with that we can do MO in Nsqrt(N)log(N), but it will also be TL. Then we can notice that it is enough to keep the last Position where such division starts, so just by fixing R we can answer L easily with that information. My realization was with Trie and Fenwich. Nlog^2

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      2 months ago, # ^ |
      Rev. 3   Vote: I like it +29 Vote: I do not like it

      I Don't understand what you mean by "such division starts". I had trie and Mo's algo but I still got wa-s(and also TLE :P). Here's what I did, it's probably the same thing. You are splitting the array into multiple subarrays based on prefixes in base 2. For 2 equal values you are trying to get to the point where the prefixes of the indexes differ. Another thing is that if A and B have a common prefix of some length, A — X and B — X will also have a common prefix of the same length. In my VALMAX trie I added indexes from the original array(thanks to the observation it still works) based on the value of respective index. I counted all the nodes that have at least 2 paths going through them, because that means you have to split up to that point(maybe even further down). I also had an unordered map which included all the points in which I have to split. From multiple values would've collided. It still got WA, so it's probably wrong.

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2 months ago, # |
  Vote: I like it +45 Vote: I do not like it

Can you say how to solve C?

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2 months ago, # |
  Vote: I like it +63 Vote: I do not like it

Can you say how to solve D?

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2 months ago, # |
Rev. 2   Vote: I like it +52 Vote: I do not like it

Is the second problem reference to this post?

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    2 months ago, # ^ |
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    Yes. To meme and author

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      7 weeks ago, # ^ |
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      Wowowowow, it's extremely cool, thanks :D :D :D

      Is Aizhan a real person?

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        7 weeks ago, # ^ |
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        Is that meme a real life situation?

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          7 weeks ago, # ^ |
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          DavitMarg foresaw that he wasn't going to win a medal, so he decided to bring potatoes instead. He didn't bring potatoes either, though :D

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2 months ago, # |
Rev. 2   Vote: I like it +69 Vote: I do not like it

Will be official results published today?

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    2 months ago, # ^ |
      Vote: I like it +42 Vote: I do not like it

    Unofficial results already published.

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2 months ago, # |
  Vote: I like it +15 Vote: I do not like it

Are all the participants official in the standings?

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2 months ago, # |
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Thank you Batrr

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2 months ago, # |
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Lol who was responsible for the 6th subtask of 3rd problem? It genuinely costed me 20 minutes :P

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    2 months ago, # ^ |
      Vote: I like it +10 Vote: I do not like it

    somehow, i had "ai = i; bi = i + 1" from the start of the contest, or i accidentally reloaded statements. im not sure

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2 months ago, # |
  Vote: I like it +19 Vote: I do not like it

How to solve day2 A for n <= 30 and n <= 2000?

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Do you know the cutoff for the medalists?

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    2 months ago, # ^ |
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    Or, at least, what it was (roughly) in the past years?

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      2 months ago, # ^ |
        Vote: I like it +5 Vote: I do not like it

      18 is gold

      43 is silver

      80 is bronze

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        2 months ago, # ^ |
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        Okay but how many participants were there?

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          2 months ago, # ^ |
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          Something around 290

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            2 months ago, # ^ |
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            There were 295 this year, yes. But how many were there in 2020, I heard somewhere that the no. of participants became quite bigger this year, right?

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2 months ago, # |
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Dies from cringe(

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2 months ago, # |
Rev. 2   Vote: I like it -28 Vote: I do not like it

Why its not working in C on n <= 1e3?

Code
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    2 months ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    Put your code in a spoiler, please.

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    2 months ago, # ^ |
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    Explain your idea

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      2 months ago, # ^ |
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      So. Let's for every vertex i and L border find max R, where in F(S[l, r]) no contains i. So clear, that with increasing L, the R monotonically increasing. So let's do this with 2 itterators, and check this. Condition is met, If there is a vertex in the subtree and depth LCA set <= d[i]. And i do this

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      2 months ago, # ^ |
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      Или более человечески по-русски. Для каждой вершины найдем все моменты времени, когда она находится в этом "обтянутом дереве". Для этого для каждой левой границы найдем минимальную правую границу, для которой это выполняется (понятно, что тогда выполняется на всем суффиксе). И что правая граница не убывает. Тогда будем это делать двумя указателями, а проверкой будет следующее условие: в поддереве этой вершины кто-то есть, и LCA этого множества выше, то бишь его глубина меньше, что, как мне кажется, у меня написано

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        2 months ago, # ^ |
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        ты писал стресс? мб поможет

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          2 months ago, # ^ |
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          Мне было лень( И я ручками 1.5 часа дебажил...

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            2 months ago, # ^ |
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            Имхо дебагать полтора часа +25, когда у тебя 0 и при этом по задаче с прочтения набирается 30 баллов — не очень хорошая идея :/

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              2 months ago, # ^ |
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              Ахаххахаха, я условие неправильно понял, тут было без шансов)

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                2 months ago, # ^ |
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                I don't speak russian, but it with google translate it seems to me like you found the mistake yourself. Good job?

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                  2 months ago, # ^ |
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                  Yes, im find mistake, wrong read statement)

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    2 months ago, # ^ |
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    Nice code, everyone in Novosibirsk writes so beautifully?

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2 months ago, # |
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Will there be editorial or author's solutions?

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2 months ago, # |
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Day 2 upsolve?

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7 weeks ago, # |
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How to solve B day 2?

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6 weeks ago, # |
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Isn't there an easier solution for day 2 B (rooms)?