By PikMike, history, 3 weeks ago, translation, In English,

Hello Codeforces!

On Jun/30/2019 17:45 (Moscow time) Educational Codeforces Round 67 (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 Ne0n25 Mescheryakov and me. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all participants!

Congratulations to the winners:

Rank Competitor Problems Solved Penalty
1 Radewoosh 7 220
2 kefaa2 7 346
3 wxhtxdy 6 120
4 square1001 6 136
5 HIR180 6 153

Congratulations to the best hackers:

Rank Competitor Hack Count
1 2014CAIS01 76:-1
2 prateek2112 50:-1
3 takumi152 32:-2
4 Holland_Pig 28
5 sysjuruo 37:-24
1268 successful hacks and 1683 unsuccessful hacks were made in total!

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

Problem Competitor Penalty
A Geothermal 0:01
B arknave 0:02
C HIR180 0:06
D RUSH_D_CAT 0:22
E Noam527 0:06
F zhoutb 0:28
G Luma 0:46

UPD: Editorial is out

 
 
 
 
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3 weeks ago, # |
Rev. 3   Vote: I like it +16 Vote: I do not like it

The problems gonna be high-level because it's @PikeMike

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3 weeks ago, # |
  Vote: I like it +36 Vote: I do not like it

This group of people have held many educational rounds.Thanks a lot!

And I hope that the problems will be as nice as the past educational rounds'.

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3 weeks ago, # |
  Vote: I like it +82 Vote: I do not like it

this time it is going to better because they thanked MikeMirzayanov

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3 weeks ago, # |
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I'm looking forward to this round, cause I hope to turn blue:) Good luck to you all!

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3 weeks ago, # |
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Just please prove your solutions :))))

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3 weeks ago, # |
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I'm hoping to learn something new again in this contest. Because of the last educational round, I learnt the sparse table and binary lifting. Thank you, problem setters for nice educational rounds.

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3 weeks ago, # |
  Vote: I like it -17 Vote: I do not like it

Is there any proof why my contribution is negative ? (without your upvote / downvote to this comment)

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    3 weeks ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    If a comment has rating between 0 and -5, it's shown that its rating is 0. Your previous comments had rating 0, but maybe they were like -1 or -2 so, after some time, your contribution became -1. I guess that's the reason.

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      3 weeks ago, # ^ |
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      I think this is may be curse of last contest..

      Spoiler
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3 weeks ago, # |
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Do successful hacks add upto your total score?

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    2 weeks ago, # ^ |
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    No,in educational rounds,you can only hack after the round ends for 12 hours.This will not effect your score.

    But if you hack others' solution successfully,and their ranks are higher than yours,your rank may become higher.

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      2 weeks ago, # ^ |
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      But the possibility of that is quite low .. the hack must include a case the testers didnt notice ..
      I think it's for educational purpose only

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3 weeks ago, # |
Rev. 2   Vote: I like it -34 Vote: I do not like it

12hrs System test will be like

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2 weeks ago, # |
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hope to have a contest !!

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    2 weeks ago, # ^ |
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    hope to have a rated contest.

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      2 weeks ago, # ^ |
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      I hope, too. Moreover, it is important that this contest become rated even to prevent the contest from being unrated twice in a row.

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2 weeks ago, # |
  Vote: I like it +9 Vote: I do not like it

All the misery was necessary

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2 weeks ago, # |
  Vote: I like it +39 Vote: I do not like it

Educational Rounds and PikMike.

Still a better love story than Twilight.
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2 weeks ago, # |
Rev. 3   Vote: I like it +16 Vote: I do not like it

I hope everyone gets what he deserves.

Good luck everyone!

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2 weeks ago, # |
  Vote: I like it +29 Vote: I do not like it

When it clashes with India's batting.

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2 weeks ago, # |
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if( Educational Round == PikMike || Div 3 == Vovuh ){ print("Contest gonna be lit."); }

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

Sure about timings ? :p

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2 weeks ago, # |
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Hope this one's rated :)

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2 weeks ago, # |
Rev. 3   Vote: I like it +6 Vote: I do not like it

The contest start has been delayed for 10 minutes. I think these days so many contests are delayed.

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2 weeks ago, # |
  Vote: I like it -6 Vote: I do not like it

Why did the time suddenly extend?

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2 weeks ago, # |
  Vote: I like it +71 Vote: I do not like it

What's with all the delays nowadays?

Not to say that I don't appreciate this site, but why not just schedule it 10 minutes later beforehand so we can work on other stuff? Is something unexpected coming up?

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    2 weeks ago, # ^ |
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    Exactly I am yet to take my dinner today. And I delayed it for two hours for contest. If i knew it was 10 mins later then I could have managed taking my dinner as well before contest :(.

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    2 weeks ago, # ^ |
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    They are waiting for 10k registrations.

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    2 weeks ago, # ^ |
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    This 10 minutes is the most boring time of day :3

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2 weeks ago, # |
  Vote: I like it -7 Vote: I do not like it

again timming has be changed!!! :((

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2 weeks ago, # |
  Vote: I like it +12 Vote: I do not like it

again a delay ? ! great !

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

Finally codeforces scheduled delay takes its place

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2 weeks ago, # |
  Vote: I like it -6 Vote: I do not like it

expecting nice problems!!

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2 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

Every contest delayed for 10-15 minute . So , why contest time fixed 10-15 minute earlier rather than starting time ?

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    2 weeks ago, # ^ |
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    They don't do that intentionally, I think.

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    2 weeks ago, # ^ |
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    they want to wait until the number of participants increases over 10k

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      2 weeks ago, # ^ |
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      not going to happen even if they delay it by one hour

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2 weeks ago, # |
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Delay by 10 minutes and CF rounds! better love story than Twilight :P

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2 weeks ago, # |
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I wanted to say thanks for not having a delay, after contest :\

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2 weeks ago, # |
  Vote: I like it +51 Vote: I do not like it

Delayforces

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2 weeks ago, # |
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I hope this round is perfect.:)

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2 weeks ago, # |
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Fantastic!Looking forward to it

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2 weeks ago, # |
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arisjo hacked pikmike's account and made this contest

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    2 weeks ago, # ^ |
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    Good explanation!

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    2 weeks ago, # ^ |
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    First of all it's arsijo,, and this kind of aggression towards him !!!! , even aggressive cows didn't have this much aggression towards each other

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2 weeks ago, # |
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The contest "codeforced" again.

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2 weeks ago, # |
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Test case 3

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2 weeks ago, # |
Rev. 3   Vote: I like it +1 Vote: I do not like it

If anyone needs help in E question.You can refer to F. Tree with Maximum Cost question. They both are quite similar :)

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2 weeks ago, # |
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In my opinion, C harder than D and E.

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    2 weeks ago, # ^ |
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    D was definitely harder, it was hacked left and right.

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    2 weeks ago, # ^ |
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    I agree. In my opinion hard ad-hoc problems are always harder than problems that need a general algorithm or paradigm to be solved.

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2 weeks ago, # |
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What is the 6th testcase for problem C?

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    2 weeks ago, # ^ |
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    Probably something like
    5 2
    1 4 5
    0 2 5

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      2 weeks ago, # ^ |
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      I get correct answer for this case but still got wrong on testcase 6

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    2 weeks ago, # ^ |
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    try this 10 3 1 3 6 1 6 7 0 2 8

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      2 weeks ago, # ^ |
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      I'm getting the correct answer on this still my solution is failing on test case 6 . Submission no 56381811.

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2 weeks ago, # |
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A and B super easy. C, D, E, F need some "trick". I do not see the educational point.

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    2 weeks ago, # ^ |
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    C, D and E needed some trick? I didn't use a single data structure, well known algorithm or "trick" in them and in fact I think that C and D were quite interesting.

    Edit: fuck my life.

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      2 weeks ago, # ^ |
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      I got an AC with an horrible brute force, I expect this to be hacked. Worked about two hours on it and I have no real idea of how the solution should look like. Thats what I call "you need a trick".

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    2 weeks ago, # ^ |
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    E did not need a "trick" in my opinion, it was an interesting DP on Trees problem.

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      2 weeks ago, # ^ |
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      Maybe E was to hard for me. But, it is obvious that one need to find the "correct" first node. If you see how to find it, you found the trick.

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        2 weeks ago, # ^ |
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        no you can find the answer for all vertices on o(n)

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2 weeks ago, # |
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What is the approach of Problem E?

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2 weeks ago, # |
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what the fuck was test case 6 of que 3

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2 weeks ago, # |
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E is very similar to 1092F - Tree with Maximum Cost. (I know being original is not a priority, I'm just saying in case it benefits some people).

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    2 weeks ago, # ^ |
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    I solved that F problem without any help,but couldn't solve E now...

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    2 weeks ago, # ^ |
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    I did E using sliding through nodes only(Sliding Window??)

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      2 weeks ago, # ^ |
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      I read your solution could you explain in bit detail please.How are you managing window on tree, heard it first time,seems interesting!!

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        2 weeks ago, # ^ |
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        Not actually windowing but if I find answer for any node say, 1 then for its children it will be just addition of a quantity a-b

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          2 weeks ago, # ^ |
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          So you are using the concept of re-rooting basically and finding the effect of traversing one node below the given node.

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    2 weeks ago, # ^ |
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    Oh,I solved it,They are REALLY similar problem... interesting,of course...

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2 weeks ago, # |
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What is "Unexpected verdict" for hacks?

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2 weeks ago, # |
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How to solve D?

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    2 weeks ago, # ^ |
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    int n; cin >> n;

    int a[n];
        int b[n];
    
        vsi dp;
        si tmp;
        dp.assign(n, tmp);
    
        for (int i = 0; i < n; i++) {
            cin >> a[i];
            a[i]--;
            dp[a[i]].insert(i);
        }
        for (int i = 0; i < n; i++) {
            cin >> b[i];
            b[i]--;
        }
    
        SegTree st(a);
        bool fl = true;
        for (int i = 0; i < n && fl; i++) {
            if (dp[b[i]].size() != 0) {
                int ind = *(dp[b[i]].begin());
                dp[b[i]].erase(dp[b[i]].begin());
                if(ind == 0){
                    a[ind]=(1<<30);// is not necessary
                    st.update(ind , (1<<30));
                    continue;
                }
                int min = st.rmq(0 , ind-1);
                if(a[min] < a[ind])
                    fl = false;
                else{
                    //update segment tree and with INF value instead of delete
                    a[ind]=(1<<30); // is not necessary
                    st.update(ind , (1<<30));
                }
            } else fl = false;
    
        }
    
        if(fl) cout << "YES\n";
        else cout << "NO\n";
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2 weeks ago, # |
  Vote: I like it +7 Vote: I do not like it

Hacks for C and D?

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2 weeks ago, # |
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Testcase #6 for C. My approach : all segment of type 1 is increasing order. initial value = 1e7.(value greater than 1000) fill rest in decreasing order staring from 1000. Check if it violates type 0 segment rule. what is the problem?

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2 weeks ago, # |
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idk how but this code gives me correct output on terminal for test case 1, but I get wrong answer on test case 1 on codeforces, https://codeforces.com/contest/1187/submission/56341353. Can someone please tell me why this is happening, it happened to me so many times before.

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

    You forgot to return true in your chk() function.

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2 weeks ago, # |
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Hackforces?

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2 weeks ago, # |
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Problem E, in first test sapmle, why answer is 36? I think i even didn't understand the problem(

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2 weeks ago, # |
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How to solve C?? what I did was 1st checked whether if there exists an unsorted array b/w some sorted one then answer would be no otherwise yes. To print the array I replaced the non-sorted last index with the min(value int the given range) -1 .. got WA

can anyone provide me the test case(smaller one) where it will be wrong and the correct approach to do it.

code

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2 weeks ago, # |
Rev. 3   Vote: I like it 0 Vote: I do not like it

C -> whats wrong with my approach ? My approach : sort all pairs of type 1; take a initial value greater than 1000. for first segment of type 1 fill the segment with increasing value starting from initial value. for each segment except first. if starting point<=ending point of previous segment then do not reset previous value. continue increasing from there.

else if starting point>previous ending point reset to initial value.

fill rest in decreasing order staring from 1000. Check if it violates type 0 segment rule. what is the problem?

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    2 weeks ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    Overlapping ranges... I think you need to check every single index, if index and index+1 must be sorted, or must be unsorted. Then you can build the array easyly.

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      2 weeks ago, # ^ |
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      that is great thinking but I thought I handled overlapping segments. For type 1 segments I merged the segments . and for type 0 I checked is they are ok after building the array: 56343410 Can You give me a small hack..please?

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        2 weeks ago, # ^ |
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        You might have ranges like

        1 1 10
        1 2 4
        1 6 12
        

        If you sort for 1, 2, 6, you will loose the 10 while checking if 6 is gt 4. So you decrease the counter on segment (6, 12), which violates rule for segment (1, 10).

        ... But I am not really sure about that... just a guess ;)

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

Today Problem A was a night mare to me . Got AC on 6th attempt.

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2 weeks ago, # |
Rev. 2   Vote: I like it 0 Vote: I do not like it

There was DP on tree in E, isn't it?

P.S. Great contest!!!! Wish every contest gonna be like that.

UPD: C dropped

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2 weeks ago, # |
  Vote: I like it +39 Vote: I do not like it

Lol what are all the hacks in D?

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2 weeks ago, # |
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Hack case for D:
1
5
5 4 1 2 3
1 2 3 5 4

answer must be YES

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2 weeks ago, # |
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HACKFORCES at its best.

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2 weeks ago, # |
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C. Maybe someone could help me? I don't understand, why my solution didn't work. What criterium to say "NO"? (In my sol I sort all conditions on left bound, connect them(conditions {1 2 5}, {1 3 6} will be {1 2 6}) and build for O(n) answer). Solution.

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    2 weeks ago, # ^ |
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    After you build the array using the "1" facts, loop through all of the "0" facts to see if any of them are sorted.

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Can anyone tell how to solve B? Tried for like whole contest but Everytime time limit exceeded on test case 4.

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    2 weeks ago, # ^ |
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    Build a pref matrix: matrix[letters][index], then use it to find the least indice that for each letter in the name, matrix[letter][index] >= frequency[letter] in the given string.

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    2 weeks ago, # ^ |
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    First pre-compute the given string by taking a vector of vector of size 26 and insert there indexes . Then for each query make a frequency array for all 26 letters and iterate frequency array and if frequency is not 0 check for the minimum index needed for given frequency in pre-computed array . Here is my solution

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    2 weeks ago, # ^ |
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    Your solution is probably $$$O(N\sum{t_i})$$$. You need to pre-process the string in order to quickly answer the farthest position needed for each name. You can do this by storing the $$$k$$$th occurrence of each letter. Then you just have to use this lookup table to find the answer for each name. Then the runtime is $$$O(N+\sum{t_i})$$$ (plus a constant factor for the size of the alphabet).

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2 weeks ago, # |
Rev. 2   Vote: I like it 0 Vote: I do not like it

I am generating a large Test case for hacking for Problem D. I am generating Test cases in which n is of the order of $$${300000}$$$ and $$${t = 1}$$$. But when I try to hack someone it says Test Cases cannot be longer than 256KB. Why it is happening, In question it is mentioned that $$${n}$$$ can go upto $$${300000}$$$. Can anyone Tell me the reason.

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    2 weeks ago, # ^ |
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    You should generate big tests with "generated input", in case you were just copying your test cases. I may be wrong

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    2 weeks ago, # ^ |
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    For testcases larger than 256KB you need to upload a generator i.e. a program which prints that testcase.

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    2 weeks ago, # ^ |
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    use generator for large test case

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2 weeks ago, # |
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How to solve D? Some people suggested the answer was to count inversions of each individual number and check those, but then they got hacked.

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    2 weeks ago, # ^ |
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    The key idea is the following: you need to sort subsegments of length 2, i.e. swap pair of consecutive elements if the first one is larger.

    Let's suggest that the element $$$b[1]$$$ is in the position $$$a[pos]$$$. If $$$min(a[1..pos]) < a[pos]$$$ then there is no way to move $$$a[pos]$$$ to $$$a[1]$$$. Otherwise, we can move it and, moreover, relative order of $$$a[1..(pos - 1)]$$$ doesn't change. So we "delete" $$$a[pos]$$$ and $$$b[1]$$$ and solve recursively.

    Of course, you need to write it in an efficient way.

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      2 weeks ago, # ^ |
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      but how can i get index "pos"?? will u pls share yur solution !!

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        2 weeks ago, # ^ |
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        Considering that solution as correct, you can make an array of vector of size 3*(1e5)+1 and store the indices of the input elements inside the vector. That way you can always obtain pos in O(1) time

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      2 weeks ago, # ^ |
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      Consider this : 1 5 3 1 2 4 1 1 2 1 3 4 According to your solution, first element 3 can move to a maximum of 3rd index (consider 1-based indexing). But it can go till 4th index if we swap indexes 4 and 5. Also in that case relative order changes. Answer to this test shall be "YES". If I didn't get your code correctly please elaborate

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        2 weeks ago, # ^ |
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        You should move not $$$a[1] = 3$$$ to the right, but $$$a[2] = b[1] = 1$$$ to the left. Then you get array $$$a = [1, 3, 2, 4, 1]$$$ and $$$b = [1, 2, 1, 3, 4]$$$. Next step is to move $$$a[3] = b[2] = 2$$$ to the position $$$2$$$, $$$a[5] = b[3] = 1$$$ to the position $$$3$$$ and that's all.

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    2 weeks ago, # ^ |
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    Match indices of a to corresponding indices of b (this matching is unique if equal elements aren't reordered). Create an array pos where pos[i] = j means that a[i] is matched to b[j]. Then, if there is some pair (i, j) such that i < j, a[i] < a[j] and pos[i] > pos[j], then the answer is NO. So this gives a necessary condition that no such pairs exist. I didn't prove it's sufficient though :(

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    2 weeks ago, # ^ |
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    Keffa2's solution this might help...

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    2 weeks ago, # ^ |
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    First check that arrays are permutations, and get the map fuction $$$p$$$ such that if the $$$k$$$-th occurrence of value $$$v$$$ in $$$A$$$ is in position $$$i$$$, and the $$$k$$$-th occurrence of value $$$v$$$ in $$$B$$$ is in position $$$j$$$, then $$$p(i) = j$$$.

    Then, instead of "sorting a range", think of your operation as "swapping consecutive elements $$$A_i$$$ and $$$A_{i+1}$$$ if $$$A_i > A_{i+1}$$$", if you can get to an array by sorting ranges, you can also get to it by swapping consecutive elements, like that.

    You can notice that your operation allows you to "remove" already existing inversions, but you definitely can't create new ones. Then, finding a single "new" inversion present in $$$B$$$ is enough to answer NO, and, I haven't proved this yet, but I think that if you don't have any "new" inversion, then you can safely answer YES.

    You can check that you are not creating new inversions by going through the array $$$A$$$, and maintaining a segment tree that answers the maximum in a range.

    The idea is that $$$ST[v]$$$ is the maximum index (in $$$B$$$) in which an already seen value of $$$v$$$ (in $$$A$$$) ends up. When you are at position $$$i$$$, check if there was a previous smaller value $$$v$$$ ($$$v \leq A_i$$$) that ends up after $$$A_i$$$, if this happens, you can already say NO. Otherwise, just update the $$$ST$$$ accordingly.

    The code snippet might be easier to understand:

    for(int i = 0; i < n; ++i){
      int mx = ST.query(0, A[i]);
      if(mx > p[i]){
        ok = false;
        break;
      }
      ST.upd(A[i], p[i]);
    }
    

    You can also check my code: 56332559.

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

      Your claim about inversions can be proven by induction on $$$n$$$, the length of the given arrays. In the step you just have to notice that first $$$n-1$$$ values of the first array, on which we swap adjacent elements, can be sorted in the same way they are sorted in the second array, if we neglect $$$a_n$$$ (the arrays are indexed from 1). That is true by the hypothesis. And now it just remains to somehow deal with the $$$a_n$$$. But it is not hard to prove that it can go to its appropriate position. All you need to do is to notice that if there was an inversion that consists of $$$a_n$$$ in the first array before the change of the order of its first $$$n-1$$$ elements, then the same inversion exists after that change and vice versa.

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    2 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    int n; cin >> n;

    int a[n];
        int b[n];
    
        vsi dp;
        si tmp;
        dp.assign(n, tmp);
    
        for (int i = 0; i < n; i++) {
            cin >> a[i];
            a[i]--;
            dp[a[i]].insert(i);
        }
        for (int i = 0; i < n; i++) {
            cin >> b[i];
            b[i]--;
        }
    
        SegTree st(a);
        bool fl = true;
        for (int i = 0; i < n && fl; i++) {
            if (dp[b[i]].size() != 0) {
                int ind = *(dp[b[i]].begin());
                dp[b[i]].erase(dp[b[i]].begin());
                if(ind == 0){
                    a[ind]=(1<<30);// is not necessary
                    st.update(ind , (1<<30));
                    continue;
                }
                int min = st.rmq(0 , ind-1);
                if(a[min] < a[ind])
                    fl = false;
                else{
                    //update segment tree and with INF value instead of delete
                    a[ind]=(1<<30); // is not necessary
                    st.update(ind , (1<<30));
                }
            } else fl = false;
    
        }
    
        if(fl) cout << "YES\n";
        else cout << "NO\n";
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2 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

56341660 56338689

These submissions are very similar

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    2 weeks ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    That's what happen when someone shares his solution with one friend, and don't share it with the other lol

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2 weeks ago, # |
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please anyone explain B ?

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2 weeks ago, # |
  Vote: I like it +126 Vote: I do not like it

So this is what happened today:

  • I solved A, B, C, D, E and was finally about to be master *_*
  • Plot twist 1: 5 mins after the contest, my D got hacked. No master for me :(
  • Plot twist 2: A lot of people had their D hacked. I was going to be a master after all *_*
  • Plot twist 3: My C got hacked. Oops, not going to be a master :(

Requesting all hackers to add another plot twist here (By hacking other people's solutions, not mine :P)

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    2 weeks ago, # ^ |
    Rev. 2   Vote: I like it +26 Vote: I do not like it

    at the end, the editorial get hacked and the contest becomes unrated...

    happy ending for every hacked person :-)

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    2 weeks ago, # ^ |
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    How was the answer for E =36? for 1st testcase?

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2 weeks ago, # |
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The people who end hacking D starts hacking C. XD

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    2 weeks ago, # ^ |
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    What should be the condition for printing NO in problem C?

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      2 weeks ago, # ^ |
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      An unsorted fact is fully contained in a range that has to be sorted which is the union of overlapping sorted facts. You make the all the sorted ranges were non-decreasing and everything in between decreasing and check if it satisfies the unsorted facts.

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        2 weeks ago, # ^ |
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        Thanks , earlier I was considering that even a partial overlapping will lead to answering NO, Also I have another question. The Problem statement mentions that some range will be sorted and some won't, but if the ranges does not cover the entire array , what can be said about the range which is not covered in the queries. Should that be sorted or unsorted. eg:- if array size is 6 and the ranges are (1-indexed) 3-4 4-6 what about 1-2 ?

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          2 weeks ago, # ^ |
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          Make them anything you want, it doesn't matter. You could just make anything outside of a sorted range unsorted so that you satisfy as many unsorted facts as possible, and then check if all the unsatisfied facts are true. If one of the facts aren't true, there's no solution since you're already making as much of the array unsorted as possible.

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2 weeks ago, # |
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simple test hack for D -_-:

1 3 3 2 1 1 3 2

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

Before start hacking, the number of a test case of Problem D was 18, and there are 74 now. :O

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    2 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Pretty much means some solutions might be filtered out during the system test so don't be too happy if your solution hasn't been hacked (notes to myself... ahem).

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    2 weeks ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    Lol, there are 152 test cases right now :)

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      2 weeks ago, # ^ |
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      How do you check that?

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        2 weeks ago, # ^ |
          Vote: I like it +11 Vote: I do not like it

        You can resubmit your solution and it will be tested on the full set of test cases

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2 weeks ago, # |
Rev. 2   Vote: I like it +1 Vote: I do not like it

Many people have been hacked on D, including me.. We should use data structure to solve.

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2 weeks ago, # |
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Any hint for F?

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    2 weeks ago, # ^ |
    Rev. 2   Vote: I like it +26 Vote: I do not like it

    Answer is $$$E[(1+\sum_{i = 1}^{i = n-1}(x_{i} != x_{i+1}))^2]$$$ = $$$E[(1 + \sum_{i=1}^{i=n-1}(x_{i} != x_{i+1})^{2} + 2\times \sum_{i=1}^{i=n-1}(x_{i} != x_{i+1}) + 2 \times \sum_{i!=j}(x_{i} != x_{i+1})(x_{j} != x_{j+1})]$$$.
    Now use linearity of expectations to calculate it.

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2 weeks ago, # |
  Vote: I like it +124 Vote: I do not like it

shrpy

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2 weeks ago, # |
  Vote: I like it +37 Vote: I do not like it

Can anyone explain the idea of Problem F ?
Thanks in advance.

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2 weeks ago, # |
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C and D — R.I.P. It's gonna be very nice system testing))0)

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2 weeks ago, # |
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Do upvotes increase contribution points? If yes, can I get some upvotes just to get some contribution points? I currently have none :(

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2 weeks ago, # |
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After system testing no one has solved problem D ^^

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2 weeks ago, # |
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what is the hack in C? so many purples have been hacked

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2 weeks ago, # |
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Wow, more than 600 solutions on problem D passed on pretests and now it's only slightly more than a 100. What's so wrong with this problem, did everyone try to do something greedy? My solution is stupid, what could possibly go wrong.

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    2 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    even C is turning out like that

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      2 weeks ago, # ^ |
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      C is a constructive task, so no doubts with a small amount of tests you can't really filter the small mistakes. But in D they even had multitests! It blows my mind.

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    2 weeks ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    there was not strong tests for D , unlike E.

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2 weeks ago, # |
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I was rank bout 200 at the end of contest.But now,I am rank74. That's hackforces.

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    2 weeks ago, # ^ |
      Vote: I like it +16 Vote: I do not like it

    Happiness comes to you quickly like a storm LOL :D

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

This round's D will like this problem,which has 588 tests

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2 weeks ago, # |
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7 7 2 3 6 5 4 1 1 2 3 6 4 5 7

problem D, i excute some ac codes, result is YES, and unsuccessful hack

anyone can help explain? i think it is NO.

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    2 weeks ago, # ^ |
    Rev. 2   Vote: I like it 0 Vote: I do not like it

    Apply the operation to [5, 6] to swap them, [1, 2], [2, 3], [3, 4]... to move 7 to the end, and [5, 6], [4, 5], [3, 4]... to move 1 to the beginning. (numbers in brackets are indices, 1 indexed)

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    2 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    7 7 2 3 6 5 4 1 1 2 3 6 4 5 7

    first sort [5,4,1] then it'll be 7 2 3 6 1 4 5 then [7,2,3,6,1] it'll be 1 2 3 6 7 4 5 then [7,4,5] it'll be 1 2 3 6 4 5 7 which is equal to B

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2 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

Weak test cases for C and D

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2 weeks ago, # |
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How to solve B? I think I have overcomplicated using the precomputation and binary search.

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2 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

Okay, so I think you gave such weak tests on purpose.

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2 weeks ago, # |
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Another 2 solve contest lol im gonnna drink some beer

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2 weeks ago, # |
Rev. 2   Vote: I like it +57 Vote: I do not like it

When you go from -65 to -13

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2 weeks ago, # |
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What is the idea behind C?

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    2 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Greedy. Lets make unsorted all the subarrays that don't required to be sorted. Note that if we have to intervals $$$[i_1,i_2]$$$ and $$$[j_1,j_2]$$$ that needs to be sorted, and if $$$i_2 >= j_1$$$ then the interval $$$[i_1,j_2]$$$ needs to be sorted as well. So lets find all actual intervals that needs to be sorted using the rule above. Suppose that those intervals are $$$[l_1, r_1],[l_2, r_2],...,[l_k, r_k]$$$. We can fill in them starting from the first one by assigning the $$$n$$$ to all the positions within the $$$[l_1, r_1]$$$, $$$n-1$$$ within the $$$[l_2, r_2]$$$ and so on. Finally check wherther all condition with $$$t==0$$$ holds. If not, there is no answer, otherwise you have already found one.
    It may seem that we need to fill in the segmends between sorted intervals somehow but actually they can be handled as a sorted intervals with length 1.

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      2 weeks ago, # ^ |
        Vote: I like it 0 Vote: I do not like it
      • Thank You But How we fill other that sorted intervals?
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        2 weeks ago, # ^ |
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        Imagine that we need to produce an unsorted array of length 4. Its clear that intervals $$$[1;1],[2;2],[3;3],[4;4]$$$ needs to be sorted (yep, array of length 1 is allways sorted, but the goal is to see how the algorithm above will handle them). If we run an algo on this intervals we will get the following array: $$$[4,3,2,1]$$$. As you can see, all the subarrays of this array with $$$length > 1$$$ are unsorted.

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          2 weeks ago, # ^ |
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          Hi Can you provide Me the code?

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            2 weeks ago, # ^ |
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            56403000
            Sorry for the delay in my response, I was at work.

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              2 weeks ago, # ^ |
              Rev. 2   Vote: I like it 0 Vote: I do not like it

              Thank you bro. But i think you code is very complex. I saw some of the red codes and they were just like 20-30 Lines But i couldnt get them :(

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    2 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I use the Disjoint set to make the interval of sorted, then i test the unsorted case in it.

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2 weeks ago, # |
  Vote: I like it +21 Vote: I do not like it

problems are good,but pretests are too weak

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

Hey PikMike, Editorials please !!

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2 weeks ago, # |
  Vote: I like it -51 Vote: I do not like it

whats wrong in my code????? please someone tell me...

include <bits/stdc++.h>

using namespace std;

int main() { int t; cin>>t; string str; getline(cin,str); int n; cin>>n; string p; getline(cin,p); vector a(t); for(int j=0;j<n;j++) {for(int k=0;k<t;k++) { if(p[j] == str[k]) {a.push_back(k); } }} double max = *max_element(a.begin(), a.end()); cout<<"Max value: "<<max<<endl;

return 0;

}

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2 weeks ago, # |
  Vote: I like it +22 Vote: I do not like it

Is there any simple solution for D which does not use segment tree?

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2 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Could someone check out my submission and tell me what do the diagnostics mean. I am not able to interpret it. Submission no — 56381811 Thanks in advance.

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    2 weeks ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    The actual error is:

    Error: attempt to subscript container with out-of-bounds index 161, but container only holds 161 elements

    This is saying that you are trying to access element number 161 in a container (a vector, I think) that contains only 161 elements. This is an error because, in such a vector, the elements are numbered from 0 to 160.

    Looking at your code this might be in merge(), where you index with j+1:

                 for(;j<one.size();j++){
                      if(one[j+1].first <= one[j].second)
    
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2 weeks ago, # |
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for(int i=0;i<n;i++) { cout << cur << " \n"[i==n-1]; cur+=arr[i]; } can someone explain [i==n-1]; in above and for (i = 0; i < n; i++) cc[i] = (i % 2 == 0) == (s[i] == '(') ? '0' : '1' what does that == after (i%2==0) does?

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    2 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it
    • " \n"[i==n-1] is a way to print a space when i<n-1 and a newline for i==n-1, because " \n" is a string with two characters : " \n"[0] is ' ' and " \n"[1] is '\n'
    • the "==" tests if both conditions have the same result, so (i % 2 == 0) == (s[i] == '(') is equivalent to ((i % 2 == 0) && (s[i] == '(')) || ((i % 2 != 0) && (s[i] != '('))
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2 weeks ago, # |
  Vote: I like it +14 Vote: I do not like it

So where is the Editorials?

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2 weeks ago, # |
  Vote: I like it +6 Vote: I do not like it

Was not able to solve any question but will try next time Wish me luck

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2 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

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

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2 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

In problem C, for sorted subarrays, most of the solutions are doing +1 for L[i] to R[i] — 1. Why not L[i] to R[i]? Why is R[i] not being taken into consideration?

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2 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

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

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2 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

It would be so helpful if we could get a brief tutorial/solution guide.. :)