atcoder_official's blog

AtCoder Beginner Contest 319 Announcement
By atcoder_official, history, 15 months ago, In English

We will hold AtCoder Beginner Contest 319.

The point values will be 100-200-300-400-450-550-575.

Until ABC 318, we used 8 problems per contest because there were many difficult tasks we wanted to use in ABCs. Since we have used most of them, from this ABC, we will use 7 problems per contest.

The difficulty of problems A-F will not be changed. The range of difficulty of new problem Gs will be wider, some of them can be as easy as current problem Gs, while others can be as hard as current problem Exs. Please check the point values of each contest for more accurate estimation of difficulties.

We are looking forward to your participation!

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15 months ago, # |
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Number of Tasks: 8

from this ABC, we will use 7 problems per contest.

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    15 months ago, # ^ |
    ← Rev. 3   Vote: I like it +11 Vote: I do not like it

    Number of Tasks: 8

    The point values will be 100-200-300-400-450-550-575.

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15 months ago, # |
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It says there will be 7 problem in ABC, but in this blog, it says Number of Tasks: 8.

So WHY?

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    15 months ago, # ^ |
      Vote: I like it -9 Vote: I do not like it

    Until ABC 318, we used 8 problems per contest because there were many difficult tasks we wanted to use in ABCs. Since we have used most of them, from this ABC, we will use 7 problems per contest.

    It means ABC319 will be 7 problem.

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15 months ago, # |
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Number of Tasks:8

So where is the 8th problem???

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15 months ago, # |
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This game only has 7 tasks, I don't know if this means the overall difficulty has increased

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    15 months ago, # ^ |
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    The post said:

    The difficulty of problems A-F will not be changed. The range of difficulty of new problem Gs will be wider, some of them can be as easy as current problem Gs, while others can be as hard as current problem Exs.

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15 months ago, # |
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I hope it can be simpler this time.

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

What the hell is problem statement of C?

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

The atcoder crashed...

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15 months ago, # |
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dude can you explain qC

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15 months ago, # |
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I don't have any ideas about C-False Hope.

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    15 months ago, # ^ |
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    You can consider all permutations of the order Takahashi sees the grids, check if it satisfies the condition easily. I agree the statement is not so clear, and that reading is the hardest part in solving the problem xd.

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    15 months ago, # ^ |
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    How we're calculating the probability tough!

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      15 months ago, # ^ |
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      wdym? isn't the probability equal to "total number of permutations that satisfy condition"/(1*2...*9) ?

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15 months ago, # |
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The difficulty of problems A-F will not be changed

But I think it's harder than usual

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15 months ago, # |
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C has the worst problem statement I've ever seen.

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15 months ago, # |
← Rev. 2   Vote: I like it -33 Vote: I do not like it

.

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15 months ago, # |
  Vote: I like it +3 Vote: I do not like it

Literal False hope

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15 months ago, # |
  Vote: I like it +17 Vote: I do not like it

After reading C:


wth-is-c

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15 months ago, # |
  Vote: I like it +1 Vote: I do not like it

The Problem statement and explanation for C is too bad

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15 months ago, # |
  Vote: I like it +6 Vote: I do not like it

Task C was really a disaster.

Never seen such a bad description of a problem ,Even the Explanation of test cases are not clear.

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15 months ago, # |
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Am I the only one who thought E >>> (F and G)

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    15 months ago, # ^ |
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    how F please

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      15 months ago, # ^ |
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      you can do bitmask dp (bitmask denotes consumed medicines), notice that it's always optimal to defeat any enemy as soon as you can defeat it

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        15 months ago, # ^ |
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        Thank you very much! I didn't see the medicine constraint during the contest :(

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    15 months ago, # ^ |
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    Probably because $$$G$$$ is a kinda-standard problem?

    But I think E is easier than F. $$$1\leq P_i\leq 8$$$ gives it away.

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      15 months ago, # ^ |
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      How did you do E?

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        15 months ago, # ^ |
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        Consider lcm(1 ... 8) = 840, so all the starting time that have the same value after mod 840 has the same min time to transport from station 1 to n. Just preprocess the min time of station transportation starting at time x(mod 840) and answer the questions after this.

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          15 months ago, # ^ |
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          Could you provide with a proof for this?

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            15 months ago, # ^ |
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            sort of obvious rly... I am really sorry, but I dont know how to put the explanation into mathematical terms :(

            but maybe you can figure out by examining all the time mod p[i] after visiting station i and you will see.

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            15 months ago, # ^ |
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            Let's say you have $$$2$$$ buses having $$$p$$$ $$$=$$$ $$$[5, 6]$$$ and $$$t$$$ $$$=$$$ $$$[3, 4]$$$

            If you reach the first bus stand at $$$0$$$, you can take the first bus at $$$0$$$, reach the bus stop after $$$3$$$ units of time, start bus two at $$$6$$$, and reach the final bus stop at $$$10$$$.

            You can find the same for different values when you reach the first bus stop. It will be different for starting times (in this case) $$$6, 11, 16, ...$$$

            Now try to find the bus travel time if you reach the first bus stop at time equal to $$$lcm(5, 6) = 30$$$. You'll realize that the position of buses is exactly the same as it was at the time $$$= 0$$$. So the value is same and this pattern will continue.

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        15 months ago, # ^ |
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        Hint 1
        Hint 2
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15 months ago, # |
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Can anybody explain C ?

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    15 months ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    just find all valid orders by brute forcing over all permutations of length 9

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15 months ago, # |
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how to solve D?

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    15 months ago, # ^ |
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    Binary Search

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      15 months ago, # ^ |
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      In D, what if the order of the words didn't matter, so would that problem be solved by dp ? How would we implement the solution in that case ?

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15 months ago, # |
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First five problems without C, how awkward, and bad problem.

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15 months ago, # |
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Laugh at A, get confused about B, and then give up understanding at C.

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15 months ago, # |
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I request Atcoder to take some action against a mass group whose user name starts with "klu". These all students belong from a certain university and have clearly mass copied, this drastically pushed the ranks.

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    15 months ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    Even with mass cheating, these losers could solve only the easiest three.

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    15 months ago, # ^ |
      Vote: I like it +16 Vote: I do not like it

    wtf why do those Indian cheaters start invading AtCoder too?

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15 months ago, # |
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Can someone explain the statement of C?

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15 months ago, # |
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In problem C,Takahashi can see numbers in each cell in 9! different ways and we have to calculate valid permutations.A valid permutation is one in which for each row,column and diagonal which consists of two same values and one different value,he must visit the cell with different value before atleast one of the other two cells with same value. After that , the implementation is pretty much brute force.

But the definition of valid permutation (not disappointed) was not clear at all atleast for me personally, it took me 40 min to understand what question is asking us to do and 7 odd minutes to implement it.

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    15 months ago, # ^ |
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    Same with me.The problem statement should be more clear.

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15 months ago, # |
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G is even easier than F

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15 months ago, # |
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I think this can be called a ABC319C disaster.

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    15 months ago, # ^ |
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    Can't agree more. Many of my classmates give up solving D,E when seeing C.

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

What is C? Why is C?

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15 months ago, # |
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Why there's no English Editorial?

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15 months ago, # |
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When will the English Editorial be released

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15 months ago, # |
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How to solve G? I used simple BFS with o(n^2*log(n)) solution using sets and got TLE.

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15 months ago, # |
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Is there any problem related to G? Very thanks!

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15 months ago, # |
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Can someone help me understand why my code for problem D doesn't pass all the tests? https://atcoder.jp/contests/abc319/submissions/45406059

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15 months ago, # |
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C is the most shit problem

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15 months ago, # |
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I have a strange solution for F:

Enumerate over all possible visiting orders of vertices with $$$t_i=2$$$. For each permutation, consider the following greedy strategy:

  • Find a monster you can reach with the minimum strength. If you can't defeat it, take the reachable medicine with the highest priority (determined by the visiting order).

Repeat this step, until all monsters are cleared or there is no medicine available.

The overall time complexity is $$$\mathcal O(m!\times n\log n)$$$, where $$$m$$$ is the number of medicine vertices. So it can't fit in the time limit, but actually with some simple optimizations it ran in $$$800\mathrm{ms}$$$ (maybe can be hacked?).

However, I got WAx1 (submission). Did I make a mistake in the program or the idea is wrong?

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    15 months ago, # ^ |
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    Oh,I found another one.Using random_shuffle instead of next_permutation can get a higher probability to pass this task.I tried 3 times,and they all passed.

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      15 months ago, # ^ |
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      Thanks. But my code tried all the permutations, without any randomization. I expected TLE, but why does it get WA? Could you please have a look at my code?

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      15 months ago, # ^ |
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      Some optimizations:

      • use __gnu_pbds::priority_queue instead of std::priority_queue for a better constant factor
      • if $$$(\text{the current strength})\times (\text{the product of all the available }g_i)\ge (\text{maximum monster strength})$$$, then print Yes.
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        15 months ago, # ^ |
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        I'am in China now,I can't download the testcase.But you can try it.And I found another thing:I'am wrong,I can reach the subtree of a medicine without taking it.But I passed!

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          15 months ago, # ^ |
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          Maybe the test cases are too weak?

          And the test cases for ABC319 are't published yet (in fact, the latest public test cases from AtCoder is that of ABC311).

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15 months ago, # |
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Can anyone give proof for E, how time taken for t1 and t2 will be same if t1%840==t2%840

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    15 months ago, # ^ |
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    $$$lcm(1,2,3,4,5,6,7,8) = 840$$$

    So the state of the bus stops repeats after every $$$840$$$ unit of time.

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15 months ago, # |
← Rev. 2   Vote: I like it +16 Vote: I do not like it

In the F，the data may be weak

5

1 1 1 1

2 2 0 5

3 1 1 1

4 1 15 1

My AC code can be hacked

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    15 months ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    My submission https://atcoder.jp/contests/abc319/submissions/45439007

    And the reason is that I don't consider the limit of ti=2 must take the medicine

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    15 months ago, # ^ |
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    it was originally came up with DJ2006 :-)

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      15 months ago, # ^ |
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      but it is not important. plz add this data into the problem, thx.

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    5 months ago, # ^ |
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    I was looking at the submission of hitonanode, which seemed to more or less implement the algorithm described in the second editorial, the correctness of which I had tried to understand and prove. Eventually, to my great surprise, I constructed the case

    6
1 2 0 3
1 2 0 2
2 1 3 3
2 1 15 1
3 1 2 2

    for which the answer should be Yes, on which the aforementioned submission outputs No.

    In my drawing of the corresponding tree below, the two multipliers are circled.

    As for why it's Yes:

    1 -> 2 -> 3 results in 1*3+3 = 6, then 6*2 = 12, 12+2 = 14, 15 cannot be defeated. However, if one does 1*2 + 2 = 4, then 4*3 = 12, then 12+3 = 15, then 15 can be defeated.

    Afterwards, I checked that the submissions of some other top contestants also output for this Yes.

    I had thought of private messaging hitonanode about this, but then, I felt a bit too shy to do so to such a highly rated coder for something as relatively unimportant as this. It is also not the first time I've seen solutions with more minor bugs AC. However, I do not think it is impossible that hitonanode or someone else could tell me thru what channel to submit an after contest case. Another thought is that maybe maspy, with whom I once had a minor exchange on GitHub, could point me to the right direction?

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15 months ago, # |
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I'm really delighted to see that I forgot the $$$\pmod {998244353}$$$ in G. That's why I got WA.

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