By awoo, history, 6 years ago, translation, In English

Hello Codeforces!

On Feb/12/2020 17:35 (Moscow time) Educational Codeforces Round 82 (Rated for Div. 2) will start.

Series of Educational Rounds continue being held as Harbour.Space University initiative! You can read the details about the cooperation between Harbour.Space University and Codeforces in the blog post.

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 7 problems and 2 hours to solve them.

The problems were invented and prepared by Roman Roms Glazov, Adilbek adedalic Dalabaev, Vladimir vovuh Petrov, Ivan BledDest Androsov, Maksim Neon Mescheryakov and me. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all the participants!

Our friends at Harbour.Space also have a message for you:

Hello Muscat

Hi Codeforces!

This is not the last call for participation, but we’re almost there. We have extended the early bird discount deadline of Hello Muscat ICPC Programming Bootcamp until this Sunday, February 16th. Remember you can request a support letter to present to your university, employer or local companies to sponsor your participation and trip to the bootcamp.

Register now

We would also like to remind you that if your team is going to the ICPC World Finals in Moscow this June, you can fill out the form below to see if you are eligible for a full scholarship for our Bootcamp (flights not included), and we will contact you within three days to let you know about the results.

Fill out form

UPD: There will be 7 problems in the round

Congratulations to the winners:

Rank Competitor Problems Solved Penalty
1 tmwilliamlin168 7 294
2 Egor 6 173
3 ivan100sic 6 174
4 neal 6 175
5 244mhq 6 179

39 successful hacks and 147 unsuccessful hacks were made in total!

And finally people who were the first to solve each problem:

Problem Competitor Penalty
A sm1lee 0:01
B icecuber 0:04
C MylnikovNikolay 0:09
D waynetuinfor 0:11
E Mehrdad_Sohrabi 0:17
F wucstdio 1:10
G arknave 0:49

UPD: Editorial is out

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6 years ago, hide # |
 
Vote: I like it +3 Vote: I do not like it

second

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OK, is it possible to achieve 1900 for me?

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Auto comment: topic has been updated by awoo (previous revision, new revision, compare).

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6 years ago, hide # |
 
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It's time for me to become pupil. Goodbye, my specialist.

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Vote: I like it +25 Vote: I do not like it

Codeforces Rounds are very good recently. All the best everyone.

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6 years ago, hide # |
 
Vote: I like it +98 Vote: I do not like it

Dear author of statement of problem B. Please learn what's the difference between weather and climate.

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Vote: I like it +49 Vote: I do not like it

Would be better if D statement said "Your goal is to fill the bag with boxes completely."

It took me some time to understand how the bag and the boxes are related at all.

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How to solve D ...?

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    6 years ago, hide # ^ |
     
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    1. Process the exponent (i) from 0 to 62 (actually 60, but it doesn't matter)

    2. If the i-th bit of n is set, then find the smallest existing box that is at least 2**i, then divide it by 2 until it is 2**i, creating two other boxes in each division. Then use a box of size 2**i.

    3. Then use all k boxes of size 2**i to create k/2 boxes of size 2**(i+1)

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    6 years ago, hide # ^ |
     
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    SUM = sum of all Ai. So if SUM < n answer is 0. else SUM = n + reminder. now go from big powers to small. a = 2^k. if reminder>=a so reminder-=a else if n>=a so n-=a else cnt[k-1]+=2 and start iterate on k-1.

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    6 years ago, hide # ^ |
     
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    Greedy. First for each power of 2 count how many boxes with this number we have. Then start looking at the bits of n, starting from the lowest one. If this bit is 0, we just go to the next bit, and in the meanwhile try to merge the boxes with corresponding size (2^i) to "create" boxes with a higher power of 2. If current bit is 1, we use the box of size 2^i, if possible. If it is not possible, we take the "nearest" box with bigger size and divide it until we get a box of the right size.

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    6 years ago, hide # ^ |
    Rev. 4  
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    You just need to construct all of the bits of number 'n' . Start from constructing lowest bit .for constructing ith bit first check if it can be constructed by numbers smaller than or equal to 2^i . Else you need to divide some number 2^j , j>i and j should be closest to i , see if this is possible .if no for some bit then -1 else answer is sum of number of divisions .

    of course above is just rough idea but you can go through the submission and ask if you have some doubt .

    submission 70903270

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What's pretest 2 of prob/C

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The memory limit on problem F is unnecessarily tight. There's no reason to MLE a solution that uses $$$\mathcal{O}(nmc)$$$ memory.

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    6 years ago, hide # ^ |
     
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    Unfortunately, we had to put tight memory limits to disallow solutions with dynamic connectivity offline approach.

    By the way, which approach uses $$$O(nmc)$$$ memory and cannot be modified to use $$$O(nm)$$$ memory?

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      6 years ago, hide # ^ |
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      It is possible to optimize the memory, but depending on the implementation it can be quite tricky. Currently I'm repeatedly running into either MLE from initializing too many hash tables or TLE from using map.

      I'm surprised offline dynamic connectivity is a concern. Shouldn't that just TLE with 2e6 since it's a $$$\log^2$$$ algorithm?

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      6 years ago, hide # ^ |
       
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      Why did you want to disallow dynamic connectivity? It's harder to implement than the solution that uses the constraints on $$$c_i$$$ and it's nicer imo.

      I tried implementing dynamic connectivity during the contest and indeed it got MLE. I'm not sure if it would have passed time limit since it was really close tho.

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Can Anyone Please Tell me what the mistake in my ....code... of problem D..

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How to solve problem E?

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Edu Round 81: copy ApIO 16
Edu Round 82: copy ApIO 17
Lets revise ApIO 18 for Edu Round 83.

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How to solve G? I tried something like centroid decomposition + convex hull trick, but I guess it's too slow.

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What is the dp state of problem E?

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F is so cool, thanks!)

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Can anyone tell me error in my soln of B. 70896681

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    6 years ago, hide # ^ |
    Rev. 2  
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            if(g>=b)
                cout<<n<<"\n";
    

    i cant understands this, you can see my code

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    else if((temp%g)!=0)
    {  
                    cout<<(tp*b)+temp<<"\n";
    }
    

    You need to check if the answer is no less than n, as in the following "else" statements. (Input: 10 3 4, Output 9, Answer 10)

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Finally!!! i AC 4 problems on Div2!!! (LOL it's 1:00AM here, im going to sleep...

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    Imagine waking up and half of them had been hacked, just kidding :P
    Good night.

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my submission for c

please help why this is not working

conditions checked

if size =1 ans is abcd.....

if anyone has more than 2 neighbours answer doesn't exist

if there is no element with 1 neighbours then also answer doesn't exist

please help

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Vote: I like it +3 Vote: I do not like it

Hack my D solution. 70893301 Please!!!

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WA on pretest 5 for B

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can someone find a bug in my code for C 70902882 what im trying to do is create a graph of the given password where to nodes are connected if they are adjacent in the password. then i search for a position to start from(a node that has only one edge going out of it) and do a dfs from there to find if there are loops in the graph. If there aren't I have a found a possible keyboard, if there are their are no possible keyboards

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Can anyone explain how to approach problem C?

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    6 years ago, hide # ^ |
     
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    Represent the keyboard as a graph: letters are vertices and edge between vertices 'a' and 'b' means that 'a' and 'b' should stand next to each other in the keyboard. You can create this graph simply by iterating through the input string and adding an edge between each pair of adjacent letters.

    The trick here is that the keyboard can satisfy the conditions iff it is a linear graph (maybe with some vertices with degree 0). The only thing you have to do is to check if the graph you have built is linear. The simplest way to do so is to make sure that, if all isolated vertices are removed, it is either empty or contains exactly two vertices with degree 1 (these will be the first and the last letters of the keyboard), and the rest of the vertices have degree two. If the graph is indeed such, you can output the keyboard by doing a dfs from one of the ends of this graph and then append all isolated vertices in the end.

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I have a $$$O(\frac{448}{bitset}\sum n^3)$$$ way to solve E. First if we split $$$t$$$ into to parts, there is an easy way (DP) to solve the problem in $$$O(n^3)$$$ for each test case with a fixed midlle position $$$k$$$ in $$$t$$$. As there are $$$O(n)$$$ middle positoins, the approach above is $$$O(n^4)$$$.

To optimise, let std::bitset<448> be the type of $$$dp[i][j]$$$. If $$$dp[i][j][k]=1$$$, then it means that it is possible to get two subsequences from $$$s$$$ where $$$s_1=t[0-i]$$$ and $$$s_2=t[k-j]$$$. We iterate all characters in string $$$s$$$. If $$$s_i=t_i$$$, then we can update $$$dp[i+1][j]$$$ by $$$dp[i][j]$$$. If $$$s_j=t_j$$$, then we can update $$$dp[i][j+1]$$$ by $$$dp[i][j]$$$. To check the answer, just check if there exist a $$$i$$$ such that $$$dp[i][n][i]=1$$$.

code here

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6 years ago, hide # |
 
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I don't understand the calculation for the 3rd example in problem B test case 1. Each cycle of weather takes 1000001 days, of which 1 day is good. As such one can repair 2 units on each cycle. The last two units can be repaired in 2 days, since we don't have to wait for the last cycle of weather to finish. This gives: 1000001 * ((1000000-2)/2) + 2 = 1000001 * 499999 + 2 = 499999499999 + 2 = 499999500001

The answer given in the problem statement is 499999500000 which is 1 less. How can one save a day without ending up with more bad pavement than good pavement?

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    You can have more bad pavements than good pavements during the days, it's only at the end that you can't.

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      In other words, one can lay an extra section of bad pavement in some earlier cycle, so avoiding having to lay it at the end. That makes sense, thanks.

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    6 years ago, hide # ^ |
    Rev. 2  
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    it takes 500000 good days and 499999 period of bad days 1*500000 + 499999*1000000=499999500000

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What is the binary search approach of problem B?

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Previous contests of codeforces used to be much better. But these days contests have just become a waste of time. Most of the people are unable to reach the questions which are really interesting and actually need to be solved.Easy dp and graph problems need to be included in the contests.

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Can anyone please tell me the mistake in my solution for D? 70923522

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Please help me on problem C, I don't know why my code runs into error in the test 2 ,I think the logic of the code is right. I use a pointer to record the position, and put ajacent keys into an array called res. Then I print the res and the other keys. Can anyone help me figure out the reason? Thank you very much! https://mirror.codeforces.com/contest/1303/submission/70923842

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I am unable to understand the solution to problem D, can someone point me to some article(tutorial/video) where i can understand how this bit wise solution solves?. Thanks a lot in advance.

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    Let $$$C[2^k]$$$ be the number of boxes with a size $$$2^k$$$. Let $$$Small[2^k]$$$ be the sum of box sizes smaller than or same to $$$2^k$$$ (i.e., $$$\sum_{i=1}^k 2^i \cdot C[2^i]$$$.) Iterate $$$i = 1$$$ to $$$maxbit$$$ of long long signed integer. If the $$$i$$$th bit of $$$N$$$(the bag size) is $$$1$$$, then we have to fill bags with boxes using their sizes are smaller than or same to $$$2^i$$$. It is possible when $$$Small[i] \geq 2^i$$$. But if not(i.e., $$$Small[i] \lt 2^i$$$), we have to divide a bigger box into the size $$$2^i$$$. Dividing procedure is quite implementive. I hope my submission would be helpful. 70933987

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Problem G is exactly the same as a problem I set months ago,TSUM2Is it a coincidence?

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The fastest system test ever

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So is there a tutorial?

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So...is there a tutorial?

My friends told me how to solve F and G but I can't understand them :C,I think I need the tutorial to help me :C

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Can someone please explain to me why my code runs differently on codeforces from all other ides that I've tried. My code fails the first test case but if you run the code on any ide,It gives the correct output. Here's the link:https://mirror.codeforces.com/contest/1303/submission/70891793

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    It gets wrong answer on my computer, too. :(

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    The problem said that you shouldn't minus one on d.size(), the data type of d.size() is unsigned integer, which means you can't get a negative number by doing any calculation. You'd better put it as pt+1 == d.size(), which is equal to your original expression.

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Attention!

Your solution 70857539 for the problem 1303A significantly coincides with solutions hinatahentai69/70857303, 1412neerujjain/70857539. Such a coincidence is a clear rules violation. Note that unintentional leakage is also a violation. For example, do not use ideone.com with the default settings (public access to your code). If you have conclusive evidence that a coincidence has occurred due to the use of a common source published before the competition, write a comment to post about the round with all the details. More information can be found at http://mirror.codeforces.com/blog/entry/8790. Such violation of the rules may be the reason for blocking your account or other penalties. In case of repeated violations, your account may be blocked.

I received this message 3hr ago. I want to apologize about the submission as both of them were mine. I used 2 accounts to submit it. I wasn't aware of the consequences it could hold. I am really very sorry about the wrong doing and wont repeat the same thing again. Please don't block my account :(

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    6 years ago, hide # ^ |
     
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    If it's the first time you used two account to check your answer, I think it won't get your account banned . As it says in statement, 'In case of repeated violations, your account may be blocked.'.

    However, you should trust the cheat detection system and don't do that again.

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Give us the tutorial, plz!

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waiting for the tutorial

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Please Release the editorial.

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Auto comment: topic has been updated by awoo (previous revision, new revision, compare).

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