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Feedback Of February Challenge 2018

Hi Fellow Programmers,

The Feb long challenge has just concluded. We hope that you enjoyed participating in the contest. Please feel free to pitch in your views about anything that you would like to mention about the contest.

Regards,
CodeChef Admin

asked 12 Feb, 15:12

admin's gravatar image

0★admin ♦♦
18.4k348492529
accept rate: 36%

3

The difficulty level was quite balanced and good. Finally a nice long contest after long.

(12 Feb, 18:06) vijju123 ♦4★
2

@vijju123 @admin The accounts tanujyadav997 and tanujyadav97 look suspicious. The latter account has deliberately badly indented code and in CLANDLABL only variables are changed.

(14 Feb, 11:14) avi2245★

I think codechef should start providing the test cases once the contest is finished. It will help us more in finding our mistakes.

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answered 12 Feb, 18:14

tony7's gravatar image

3★tony7
412
accept rate: 0%

@admin contest was very good .. enjoyed solving it. but please provide test cases after the contest.

(13 Feb, 00:32) worldunique3★
1

Hi, this was discussed many times particularly in this thread: https://discuss.codechef.com/questions/491/will-codechef-provide-test-cases . Codechef does not provide test cases since it spoils the real fun. :)

(13 Feb, 00:59) ayan_nitd3★

can anyone tell how the bitset soln passd. Not only it has memory complexity N^2 but also time complexity of N^2. On the other hand i dont think testcases were very weak as my https://www.codechef.com/viewsolution/17342876 O(Nsqrt(N)log(sqrt(N))) algo failed. Or did i miss something?

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answered 13 Feb, 11:02

soham1234's gravatar image

6★soham1234
839310
accept rate: 10%

Yeah, I thought of the solution using bitsets too, but disregarded it thinking the time complexity of the solution is $O(N^2/\text{WORD_SIZE})$ ~ $6.25*10^8$ operations which should give TLE. But I guess codechef judge is quite fast.

(13 Feb, 18:30) nilesh31055★

I think overall complexity of using bitset was around 3.5 * 10^8 (including test cases) as the optimization factor is of 64 (N*N / 64) , correct me if i am wrong ?

(14 Feb, 15:04) apptica5★

Regarding the gradient of the contest , I feel the first 5 problems were good as per their difficulty level but 6th,8th,9th and 10th problems should have been a little harder than they were, especially the 9th problem which require only a greedy approach.

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answered 13 Feb, 11:44

abx_2109's gravatar image

5★abx_2109
275111
accept rate: 0%

I too agree with @abx_2109,probably that was because the alternative solution was much simpler than the desired solution(like bitmask for CHANOQ,greedy for 9th sum),if the memory limit were strict,then it could have been a more balanced contest.

But overall,liked it.

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answered 13 Feb, 17:44

vivek_1998299's gravatar image

5★vivek_1998299
9888
accept rate: 21%

I really didnt't think of an alternate solution for CHANOQ and that too by bitsets and a lot of people did it that way. Overall I too liked the contest.

(14 Feb, 01:21) abx_21095★

How come this solution to the question Chef and odd queries: https://www.codechef.com/viewsolution/17408003 that uses a bitset of total size 10^10 bits (i.e. 1250 MBs) passed, where normally memory limit is 256 MB or 512 MB in long challenge questions. Also, the memory limit is not mentioned at all in the problem! This solution solves the problem without any segment tree or sqrt decomposition but just using bitsets. I think this kind of solutions having O(N^2) memory are not supposed to pass for chef and odd queries. @admin @markysha

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answered 12 Feb, 16:07

ravibitsgoa's gravatar image

4★ravibitsgoa
412
accept rate: 0%

codechef's memory limit is 1.5gb

(12 Feb, 17:42) mathecodician5★

Another one (Mine)

https://www.codechef.com/viewsolution/17376783

Allocate M bitsets of size N, but still passes.

(12 Feb, 17:51) taran_14074★
1

We have the issue in our mind. We are working on to fix it.

(12 Feb, 18:01) admin ♦♦0★

One of the best long challenges I've ever taken. Well balanced and nice problems.

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answered 12 Feb, 18:03

orlon's gravatar image

4★orlon
382
accept rate: 0%

Hello Community! I wanted to contribute to the community but since I don't have enough karma points, I want you to upvote only if you like my unofficial editorial on PERMPAL : Contest link:https://www.codechef.com/FEB18/problems/PERMPAL

My approach: 1.The answer will be -1 only when the count of any of 26 characters is odd and there exists more than one such character's example : abbc, here we can see both 'a' and 'c' have odd count in the given string. 2.In any other case, we will always find a answer, that is pallindrome. 3.Based on above point, we will deal with two cases : ---(A)When only one character comes with odd count and rest other characters with even count. ---(B)When all the character in the given string are of even count.

Now the question comes How to solve this?

So it is much clear for case 3(A) that the odd count characters should be in the middle of string in order to make it pallindrome, eg abbba n(b)=3 i.e odd

Since the ordering of characters won't matter in case of even count, we will print index one by one from vector<vector<int>> v(26), I have used this for storing the index of each character(a-z) in the string.In the output I need to just print the even count characters index ,simultaneously from beginning and end. Keep in mind that the odd count character should be in the middle.

If you give up, you can see my solution here:https://www.codechef.com/viewsolution/17309552

All suggestions are welcome!

Thanks Community, Happy Coding!

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answered 13 Feb, 12:20

ayushgupta1997's gravatar image

3★ayushgupta1997
15217
accept rate: 0%

Nice contest!

It would be really great if the search for Submissions for a particular problem allowed you to filter by "pts" as well as Language and Result (All/AC/WA/...). If you want to find the fastest 100pts solution, you may have to check through 100 pages of 10pts solutions first.

Also, it would be nice if the Submissions search showed you the size of the program. So you could look for short programs, which could be helpful if you wanted to find the cleanest / most instructive answer. (I suppose this would be controversial, since it would give a little extra information during a contest, but it could be interesting/encouraging to know if there is a short solution.)

If the Submissions search allowed you to download the results as a csv file or something, then people could make their own views of the solutions, e.g., a scattergram of (speed,size).

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answered 13 Feb, 19:57

alexthelemon's gravatar image

4★alexthelemon
524
accept rate: 0%

The Competition was very well balanced!! One of the best long challenges till date!!

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answered 14 Feb, 00:48

yashkapoor112's gravatar image

3★yashkapoor112
111
accept rate: 0%

Thanks for posting quick editorial :)

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answered 12 Feb, 15:52

vivek_shah98's gravatar image

4★vivek_shah98
203
accept rate: 0%

edited 12 Feb, 15:53

Very nice.Enjoyed it a lot!

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answered 12 Feb, 17:43

devansh1497's gravatar image

3★devansh1497
11
accept rate: 0%

plz provide official editorial .... quite good contest!!

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answered 13 Feb, 00:26

abhi007rider's gravatar image

2★abhi007rider
122
accept rate: 0%

They are already out. Please check the forum.

(13 Feb, 00:40) vijju123 ♦4★

@admin @vijju123

Check my soln https://www.codechef.com/viewsolution/17395501 for (Challenge) Biased Committee https://www.codechef.com/FEB18/problems/BIAS

I have been awarded ~20 pts for sinply printing what is given as input. Indeed weak test cases. Such an unoptimized soln got 20 pts and pass subtask 1 of Chef and odd queries awarded 10 pts. Plz don.t allow this type of soln to take away 20 pts on challenge question. As it reduces the moral of other participants when they get to know this.

And they are trying hard to get 100 pts. Or not attempting just because they cannot think of procedure to solve Q.

On the other hand people are getting pts for simply printing input. And that too 20 pts.

This type of soln deserves less than 5 pts. So plz make sure from next time that score should be such that this does not happen again. Because this affects rating and ranking of many deserved ones.

There are many this type of soln. https://www.codechef.com/viewsolution/17425539 https://www.codechef.com/viewsolution/17425340

P.S. Most soln with time ~.012 sec on c/c++. Have either done this or took average.

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answered 13 Feb, 00:43

aryanc403's gravatar image

5★aryanc403
4228
accept rate: 13%

I asked @admin to look into your feedback.

(13 Feb, 13:18) vijju123 ♦4★
4

In an optimization problem, it's really hard to control score on particular submissions. We focus more on how hard it is to optimize the function and test various solutions to check what's the max score we can get, i.e. is the function too bad that it's very difficult to optimize it beyond some easy to get solution.

(13 Feb, 14:22) admin ♦♦0★

while other thinks the contest was well balanced. i think the opposite. see the accuracy fell frm 20% to 5% between 4th and 5th ques. difficulty of prob shld increase gradually (as discussed in @triveni's post). ,

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answered 13 Feb, 13:38

ganesh35's gravatar image

4★ganesh35
211
accept rate: 0%

1

The number of successful submissions matter. I am not sure, but what if accuracy takes into your multiple failed solution as well? What matters is its ultimately solvable, irrespective of number of attempts needed.

(13 Feb, 16:58) vijju123 ♦4★

@admin This is another "Weak testcases in ... " kind of post :p. The testcases of the problem POINPOLY were weak this time. What I did is first, I found the minimum and the maximum y co-ordinate of the vertices of the polygon.Lets say they are miny and maxy. Now, I found all points inside the polygon having a particular y co-ordinate, say d. So I iterated for d from (miny+1) to (maxy-1), and found two intersection points of the line y=d with the polygon.(This is guaranteed that there will be two intersection points, since the polygon is convex and miny < d < maxy.) This I did in O(n) for each d. Once I get the two intersection points at y=d, say (l,d) and (r,d) (l and r will not always be integers, but I manipulated accordingly so that they are inside the polygon, by either taking ceil for left and floor for right.). Now I output all points (x,d) such that l<=x<=r. When I get (N/10) points, I break the loop. This solution has a complexity of O((maxy-miny)*n), but still it passed the 30 pts subtask and got TLE in just one file of the large task. Then I reversed the loop and iterated d from (maxy-1) to (miny+1). This solution got 100 pts. :p . This is slow enough even for the small subtask, only when there are counter-cases for such a solution, which wasn't. I can make it work even faster by choosing random d in [miny+1,maxy-1]. I think making counter-cases for this solution is hard too. But I will be happier to see a TLE verdict for such solution, so that I come up with a better solution. Here are the two solutions :- https://www.codechef.com/viewsolution/17305882 // 30 points, TLE in just one file of large input. https://www.codechef.com/viewsolution/17305969 //100 points

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answered 13 Feb, 19:42

amandeep224's gravatar image

5★amandeep224
01
accept rate: 0%

i did the same thing .. but couldn't find my mistake ..

https://www.codechef.com/viewsolution/17432565

(13 Feb, 19:50) worldunique3★

@alexthelemon can u explain ur method how u solve poinpoly

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answered 13 Feb, 20:05

albert_012's gravatar image

2★albert_012
-15
accept rate: 0%

The ranklist has totally changed ,and that too after updation of ratings.

Here is the ranklist: https://www.codechef.com/rankings/FEB18

Is this a bug or rating would be updated as per the new ranklist?

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answered 13 Feb, 23:33

vivek_1998299's gravatar image

5★vivek_1998299
9888
accept rate: 21%

The admin is checking for any issues(if any).It is because of the change in scores of the challenge problems. Probably the testcases or the scoring function of the testcases after the contest is inconsistent with the pre-cases which were provided before the contest ended because the rankings have changed drastically.

(14 Feb, 01:25) abx_21095★

So will the ratings be updated?

Thanx for ur reply!!

(14 Feb, 01:31) vivek_19982995★
1

The ratings will be updated again if some error or inconsistency is found either in the testcases or the scoring function.

(14 Feb, 01:36) abx_21095★

Isnt it unfair? I mean people would obviously try to code in order to get high score in pre tests. Changing the scoring function,or some totally different test cases doesnt seem fair to me.

(14 Feb, 01:53) vivek_19982995★

They didn't change anything intentionally, probably some fault has occured in the main tests which will be resolved and the ratings will be updated accordingly.

(14 Feb, 01:59) abx_21095★

Ooh sorry,i think i misread ur comment.

Thanx!!

(14 Feb, 02:10) vivek_19982995★
showing 5 of 6 show all

It was an amazing contest. Well balanced and each question was interesting. 4th question was tricky and based on observations . I love this type of questions. Enjoyed a lot:) Thanks

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answered 14 Feb, 02:34

specialak's gravatar image

4★specialak
0
accept rate: 0%

Hi, I am new to CodeChef so I don't know where to find the editorials. Has the editorial been released? If so can someone tell me where to find it?

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answered 14 Feb, 14:46

sreyan32's gravatar image

0★sreyan32
112
accept rate: 0%

1

Yes, they are released. You can find the editorial for any problem with this link: discuss.codechef.com/problems/problem_name

Eg: discuss.codechef.com/problems/POINPOLY

(14 Feb, 15:00) avi2245★

I felt there was a significant number of problems on mathematics and geometry than dataStructures and algorithms based when compared to other Codechef long challenges. If that's how the mixup should be, then I feel it was a good problem set.

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answered 14 Feb, 23:54

tejkiran_v's gravatar image

4★tejkiran_v
2
accept rate: 0%

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question asked: 12 Feb, 15:12

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last updated: 15 Feb, 03:39