Hello Codeforces!

On June 29, 18:05 MSK Educational Codeforces Round 24 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.

The round will be **unrated** for all users and will be held on extented ACM ICPC rules. After the end of the contest you will have one day 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 15 minutes** to solve them.

The problems were prepared by Mikhail awoo Piklyaev, Alexey Perforator Ripinen and me.

Good luck to all participants!

I also have a message from Harbour.Space University about the ML course held at the university:

Nowadays machine learning technologies are widely used in practice in various applied fields such as retail, mass media, PR and marketing, banking, telecommunications, manufacturing, science and many other areas.

We are pleased to announce our Industrial Machine Learning course, aimed to teach the structure and the life cycle of the machine learning process and covers topics ranging from the problem statement to final quality assessment as well as estimation of the economic effect.

This course is taught by two Data Scientists from Yandex, Emeli Dral and Victor Kantor. Emeli Dral is the Head of Predictive Analysis, whereas Victor Kantor is the Head of User Behaviour Analysis Group at Yandex.

http://in.harbour.space/data-science/industrial-machine-learning/

They share with us their knowledge on everything from the most promising applications of data science today to applying the latest advancements in machine learning to your work.

UPD: The editorial can be found here.

Okay, CF on a hat-trick.

Is it not rated!

tkhkhkhkhkhkhkkhkh :|

If they find a bug in the jury's solution does it mean that the contest becomes rated?

I would honestly not be surprised if at the end of this round they say it's

rated.I love educational rounds because they are announced unrated prior to the contest.

now we will presume all the rounds as unrated.....:D

BledDest's Educational Round from

18 to 23are awesome. I learned something new on every round. Congratulation for your contribution to consecutive7Rounds in a row .Less Enthusiasm to finish all 7 problems since it is unrated :|

.

Read Statements of Problem 1,2 and 4. Most Unclear Problem Statements and Test Case Explanation.

Indeed.

Exactly. In first question, first of all they should mention that students who get diplomas cannot get certificates and vice versa. Also it should be diplomas OR certificates instead of AND. 2nd and 4th are even more unclear than the first

Idk, for me it seems totally fine.

Why am i getting TLE on 7th testcase on D. I think time complexity of this code is O(nlogn)

code:Your Code complexity is not nlogn.

I implemented the same idea as yours but efficiently without the while loop, you can view my solution here : 28150986

UPD : The explanation is wrong, the cause of TLE is something else. See this

well i think this soln is similar to mine and got AC 28144801

The while loop will erase elements atmost n times, coz i am always erasing previously inserted elements, so the time complexity is nlongn for insertion + nlogn for erasing n elements! Total time complexity: nlogn

I am really sorry.

You are right your code complexity is

`nlogn`

.I think what you are missing is to reinitialise flag = 0. Once your flag is set to it never becomes 0 again causing the while loop to run in every loop after that, hence causing TLE.

Actually, i don't think reinitialise of flag is required. Once flag is is set while should run for every iteration. The point is while will never run more than n times, so won't cause any problem, even there is no need of flag in while's condition. 28156733 (submission without flag in while's condition) .

Actually flag is not causing the TLE, the problem is after erasing the color, whe should update its hash in h[] array, coz it can't be part of ans anymore.

AC code: 28157043

WTF, I got AC in C and D, but didn't manage to do it in B...

How to solve B? :/

a[l[i]]=(l[i + 1] — l[i] + n) % n; check if everything is ok.

Yeah, i did the same

So i think there's a bug in my solution

What if a[i] contains two values?

For example l[]={3, 5, 3, 6}

maybe the final permutation contains a 0???..that happened to me

I used a map to ensure that each index gets one value only. If two index try to get the same value I return -1

Finally, we try to assign each unassigned index an unassigned value. We can do this fast enough.

Thanks, just got AC

a[l[i]] should be the distance between l[i] and l[i + 1], and a should form permutation.

How to solve D?

For every color contain if it can be B and its current cnt. When increasing cnt sompare it with cnt[A].

I guess complexity will be

O(n^{2}) then.ignore.

Am I right, then?

I just realised that my solution is different.

I mean considering that I have an array of say 10 elements and out of them 5 have value equal to A.

The other 5 are different (unique) elements.

So I will have to keep on comparing them with A`s indexes.

This will become * =

Am I correct?

Yes, if for each B, you iterate through the whole array.

So, is there any way to do in

O(nlog(n))Yes, put the counts of all the possible Bs in priority queue and keep invalidating them as soon as possible. See my code.

Here's my code: It's O(n + #colors).

How to solve E?

Two pointers, to check if segment is divisible, factor the k, and keep count of each prime.

The most simple solution is binary search with segment tree of multiplication modulo k

submit: http://codeforces.com/contest/818/submission/28143128

E is kind a to easy for E,rather level of Div 1 B.It's just two pointer,and you could easlly hash with prime factors of numbers because number of prime factors of

nis ,so baisicly I did nothing smart for this accepted,unlike some other E's where there is some beautiful observation.My 28153516Can D be done by 2 pointer? I couldn't implement it.

What is you 2 pointer approach? I tried it solving by always keeping set of possible answer for all positions, if at any position set is empty, print -1.

For every color available store their indices and then compare them with indices of Alice's car find who will win how many times and if Alice's car looses then Bob can win by choosing that color.

I tried solving E using 2-pointers but was getting TLE on 16.

D can be done in O(N).

Code-28146537

Here is the Problem E on Hackerearth created by MazzForces .

.

This problem's name is a very, very big irony.

Oh, wow, I guess we were not that creative. Sorry, none of authors and testers heard about this one and we weren't able to google anything.

I got WA at test 4 in both B and D I tried all possible test cases especially in D and I did't figure out what is wrong with my code ?

How to solve C? I used Cumulative sum trick for each direction.

That's exactly what I did too. Plus you also need to consider that the same sofa would also be counted in two directions. For example, in cumulative sum,for say a sofa with coordinates 1 1 1 2 , the cumulative sum in up and down direction will also account for the same sofa you are querying for, but not in the left or right direction.

Good round but I couldn't solve problem C...any help??Thanks in advance

C is just careful case-work if I'm not wrong.Use four arrays to store for every sofa its min x in 1st,max x in 2nd,min y in 3rd and max y in 4th.Sort all of them using counting sort,i.e you could count number of those whose value is z.Now use prefix sum,i.e count how many sofa s are there with min x less then some value r for every r<n.Now for every sofa use opposite value to count number of sofas in each direction,when we want to know number of sofas on left for sofa i,we use max x_{i} and yust use prefix sum we calculated.That's

O(n).Problem E was very nice, can be done with simple algorithms. Thanks for such problems :)

problem A can be solvable using binary search . Equation is k*x+x<=n/2. Now search on x. Here x is number of diplomas. Here is my AC code : http://codeforces.com/contest/818/submission/28144078

Or with division.

like this xd 28143158

There is a simple O(1) solution: 28142651

Hack for A?

Why your solution doesn't get overflow in big cases? 28143158

I just tried to hack but failed

Who hacked me? I try it with several cases :c

me :)

any

nandksuch that should work.When you find a hack in your solution!

When you think you've used the same (maybe sub-optimal) algorithm as everyone, but the only solution you're able to hack is your own. :(

Is really F so easy ?

I think problem D is so easy.

And problem C is much harder than problem D.

can someone tell what is wrong in 28167345

Integer overflow. You are storing values in

longdata types but they can exceed the maximum permissible limit.eg. If you have an array of length 100 in which all elements are 2, s[99] will supposedly store 2^100 which will overflow.

thanks got it

i am not able to understand the question B permutation game how many ever times i try to read it.i cant understand the test case explanation also..can some1 please help me regarding this??..thannk you

Can I get All links to Educational Round?

Good that's informative blog ! Songspk

I got judgement failed in problem G constantly.

How to solve it(?

Here is my submission -> 28719413

muhammedezzat

use this to solve problem A

## include <bits/stdc++.h>

int n, x ; int a[330]; set < int > myset; int main() { cin >> n >> x; for(int i=0; i<n; i++) cin >> a[i]; for(int i=0; i<n; i++) myset.insert(a[i]); int ans = 0; for(int i=0; i<x; i++) { if (!myset.count(i)) { ans++; } } cout << ans << endl; }