### mohammedehab2002's blog

By mohammedehab2002, 10 months ago, ,

Hi!

I'm back with a new contest, a new color, and a new batch of xor problems.

Codeforces round #525, rated for the second division, is taking place on Dec/04/2018 17:35 (Moscow time). As usual, first division participants can take part out of competition.

I'm the problemsetter of the round. I'd like to thank 300iq for the great effort coordinating the round, isaf27, cdkrot, BudAlNik, and gritukan for testing the round, scanhex for translating the statements to Russian, mahmoudbadawy for giving his opinions about the problems, and MikeMirzayanov for the great codeforces and polygon platforms.

Like my previous round, you'll be given 6 problems and 2 hours to solve them.

After the contest, I'll be on the community Discord server to discuss the problems.

UPD: the scoring distribution will be 500-1000-1500-2000-2500-3000.

UPD: something wrong happened and the editorial was deleted. I'll post it as soon as possible :(

UPD: the editorial has been re-written.

Good luck & Have fun!

UPD: congratulations to the winners!

Div.1+Div.2:-

Div.2:-

• +476

 » 10 months ago, # |   +79 xor problems
•  » » 10 months ago, # ^ |   -43 will fuck these problems..... XD.
 » 10 months ago, # | ← Rev. 2 →   +31 Egyptian problem setters , great ^^
 » 10 months ago, # |   +38 Generally xor problems are really nice problems, hope this contest will be nice too.
•  » » 10 months ago, # ^ |   +33 Really? This might be an unpopular opinion, but I feel like xor problems often involve the same few tricks (trie, solving the problem for each bit independently, making use of the involution property, segment tree of xor's, etc.).
•  » » » 10 months ago, # ^ |   0 What is the involution property?
•  » » » » 10 months ago, # ^ |   +6 An involution is a function f: x -> x that, when applied twice, brings one back to the starting point. -Wikipedia
 » 10 months ago, # |   0 Good!
 » 10 months ago, # | ← Rev. 2 →   0 Thanks :)
 » 10 months ago, # |   +29 your previous round was one of the best rounds i've seen since i join codeforceshope to see such a great contest again
 » 10 months ago, # |   +2 my favourite colour is green
•  » » 10 months ago, # ^ |   +10 And my favorite color is red with a little black; so what? :)
•  » » » 10 months ago, # ^ |   -12 so stfu because no one cares
 » 10 months ago, # |   +32 i will try my best to solve some problem , and get higher rating :')
 » 10 months ago, # | ← Rev. 2 →   +10 Mahmoud and Ehab and the Xor, Mahmoud and Ehab and Xor-MST. These problems are freaking difficult and awesome. Expecting more difficult and great problems ;)
 » 10 months ago, # |   0 good
•  » » 10 months ago, # ^ | ← Rev. 2 →   +5 wow , my friend :D , hope you'll become green after this contest :D
•  » » » 10 months ago, # ^ |   +4 i will become black after this contest, this contest is difficult
•  » » » » 10 months ago, # ^ |   -22 no it’s very easy it’s that you are really bad
 » 10 months ago, # |   +6 Let the strongest be lucky at the contest. All the will of Allah. You can't keep an eagle in a cage.Inshallah!
 » 10 months ago, # |   +4 Whats up with long submission queues!
•  » » 10 months ago, # ^ |   +62 It disappeared.
•  » » » 10 months ago, # ^ |   +17 I can't help, but remember:https://www.youtube.com/watch?v=gOIscbt41KA
 » 10 months ago, # |   +3 Good Luck for us all
 » 10 months ago, # |   +29 GO gritukan GO for newbie this time :P.
•  » » 10 months ago, # ^ |   0 He is the tester of this round.
•  » » 10 months ago, # ^ |   0 i am here to beat him....
 » 10 months ago, # | ← Rev. 2 →   0 I was a bit scared at the first place seeing green color in the thanksgiving section.Then I realized the joke!
 » 10 months ago, # |   +2 With the kind of determination gritukan is showing in making his rating fall, he'll definitely beat the lowest rating of worse one day.
•  » » 10 months ago, # ^ |   0 nobody can beat worse,even tourist
•  » » » 10 months ago, # ^ | ← Rev. 2 →   0 Maybe that's why he wants to achieve this feat. :)
•  » » » 10 months ago, # ^ |   -36 dont get too carried away so soon, mighty pakistan beats everyone in every direction. In east we beat china, in south we beat india and in north we beat afghanistan, and in west we beat europe. hail us! we breath in war
 » 10 months ago, # |   -20 i DO NOT understand what this is about. can someone explain please?
 » 10 months ago, # |   -36 Is it rated?
•  » » 10 months ago, # ^ |   0 rated for the second division
•  » » » 10 months ago, # ^ |   -44 you did not need to use bold letters.
•  » » » » 10 months ago, # ^ | ← Rev. 2 →   -7 ok.
•  » » » » » 10 months ago, # ^ | ← Rev. 2 →   -55 So, change it to normal
 » 10 months ago, # |   -30 sorry i cant read
•  » » 10 months ago, # ^ |   -18 it says it is rated?
•  » » » 10 months ago, # ^ | ← Rev. 2 →   -30 im not understend!!!!
•  » » » » 10 months ago, # ^ |   -14 i not
 » 10 months ago, # |   0 I can't submit my code. Please help me.I tried to send some codes with several languages, but the system always says "You have submitted exactly the same code before", even though "My submission" is still empty.
 » 10 months ago, # |   0 The scoring distribution provided here seems not matching the ones in standings page.Please have a look.
 » 10 months ago, # |   -8 Problem C seems buggy
•  » » 10 months ago, # ^ |   +8 Your code.
•  » » » 10 months ago, # ^ | ← Rev. 3 →   0 input:3x x xoutput:42 3 11 1 11 2 11 3 1what's wrong here?
•  » » » » 10 months ago, # ^ |   +9 You received this array:3 2 1In short, you got things wrong quite a bit.
•  » » » » » 10 months ago, # ^ | ← Rev. 2 →   0 I get it
 » 10 months ago, # |   -30 MAN what is not ok with problem C????!?! it compilation erorr VERBOTEN why post a broken contest i don’t understand??? i’m very sorry if i offended anyone with this matter. dumb russian apes
 » 10 months ago, # |   +23 I have a dream... that one day... problems will go from easy to hard GRADUALLY and not like this
 » 10 months ago, # |   0 Someone else having problems with the interactor of problem D???
•  » » 10 months ago, # ^ |   +1 yesssss
•  » » » 10 months ago, # ^ |   0 It's printing things I am not asking it to print !!! This is crazy, I wrote a solution in the 34 minute and it still didn't pass the first test !!! I don't know what is it printing :(
•  » » » » 10 months ago, # ^ |   0 for me it says my answer is "60" , it's not even possible to mycode print it ;
•  » » » » » 10 months ago, # ^ |   0 Same here, printing 30 -_-
•  » » » » » 10 months ago, # ^ |   +6 I think 60 is the number of questions you made not the real output of your program
•  » » » » » » 10 months ago, # ^ |   0 after any print i have endl .
•  » » 10 months ago, # ^ |   0 Guys, interactive tasks are technically different in preparation :/ The output shown to you is the number of questions. Sorry if it made an inconvenience but some people asked in the clarifications and we cleared it.
•  » » » 10 months ago, # ^ | ← Rev. 2 →   +1 It is possible to write an interactor in a different way — printing the participant's answer, not a number of questions. Interactor should exit with WA if query limit is exceeded. And checker should check the answer if the interactor finishes successfully. For me, it is the most convenient way.And yes, I consider MikeMirzayanov's approach with printing number of queries as a bad practice :)
 » 10 months ago, # | ← Rev. 2 →   +2 what's wrong with problem D ??? i test it in my system and in custom test and i get correct answer in test 1 and when i submit that i gave wrong :///////
•  » » 10 months ago, # ^ |   0 same here
•  » » 10 months ago, # ^ |   0 I think it's wrong.
•  » » 10 months ago, # ^ |   +5 You might print more than 62 questions
•  » » » 10 months ago, # ^ |   0 my code print at most 61 question , it's not possible ;
•  » » » » 10 months ago, # ^ |   0 Can you show your code please?
•  » » » » » 10 months ago, # ^ |   0 hear ??
•  » » » » » » 10 months ago, # ^ |   0 man, i just want to see how you implemented D
•  » » » » » » » 10 months ago, # ^ |   0 he didn't he's a liar
•  » » » » » 10 months ago, # ^ |   0 It's open to see other peaples' code now.
 » 10 months ago, # |   +8 Is the idea for F that output tree must be a strict min heap. And now first value is hoax. For second value we need what is min value in 2^i distance tree from every vertex? How to do find min value in 2^i distance tree for all vertices ??
•  » » 10 months ago, # ^ | ← Rev. 2 →   0 F is only about extracting information from the statement. Let's interpret w as sum of cost of edges. Now, what we need is to find the minimum spanning tree. In the first tree, root at minimum value vertex. For each vertex, we only concern about edges connect it with 2i-th ancestor or root.
 » 10 months ago, # |   +127 me.
 » 10 months ago, # |   +6 Oh God I forgot question marks in D
 » 10 months ago, # |   0 In any universe will this code print 30 for problem D???link
•  » » 10 months ago, # ^ |   0 Because you wrote "for(int i=0;i<30;i++)" instead of "for(int i=29;i>=0;i--)", thus your algorithm is wrong?! x)
•  » » » 10 months ago, # ^ |   0 i wrote for(int i=29;i>=0;i--) :P
•  » » 10 months ago, # ^ |   0 My upvote is for the rick in the profile pic and multiverse reference in the comment.
 » 10 months ago, # |   0 i'm so stupid today. I don't think that a and b in problem A can be equal :(
•  » » 10 months ago, # ^ |   0 I tried the trivial solution with brute force and after I run these test cases, I figured out that they can be equal :D
•  » » 10 months ago, # ^ |   0 i think one of the pretest needed a and b to be equal the first one i sent(thinking they couldn't be equal) got WA on test 10 i changed the one line and got accepted
 » 10 months ago, # | ← Rev. 2 →   +3 -
•  » » 10 months ago, # ^ | ← Rev. 3 →   +16 Solution for E: Observation: the biggest fraction is always equal to the answer for k = 1 : the subtree with maximum sum. How to choose more components now? They all must have sum equal to maximum sum in a subtree, otherwise the fraction won't be the biggestSo first we find answer for k = 1 dp on trees. dp[x] = the best subtree rooted at x dp[x] = val[x] + sum(max(0, dp[y]) | y is son of x); We calculate this dp using dfs.Let us denote maximum sum of a subtree with M.Now we can do a greedy to find maximum number of subtrees. We reapply the first dp algorithm again, but now when we obtain that dp[x] = M, we pick this subtree in answer, and disregard node x so it won't influence answer for father of x.
 » 10 months ago, # |   +8 How to solve D?
•  » » 10 months ago, # ^ |   -18 XOR with powers of 2
•  » » » 10 months ago, # ^ |   0 Could you elaborate more? What do we achieve by this?
•  » » » » 10 months ago, # ^ |   0 first you ask it ? 0 0 and know which one is bigger ( name it first comp ) , then you ask all power of 2 between 0 to 29 , if the comparison was the same with first comp then both have it or both don't have it so you ask c = 2^i and d = 0 and know they both have it or not , and if it wasn't same with first comp then the biggest one has it .
•  » » » 10 months ago, # ^ |   +4
•  » » 10 months ago, # ^ | ← Rev. 2 →   +3 Just find non-equal bits, then you can easily restore equal bits. To find first one if answer for (a, b) differs from answer for (a+1^i, b+1^i) change a and b.
•  » » » 10 months ago, # ^ |   +3 I got stuck restoring equal bits, how do you achieve that?
•  » » » » 10 months ago, # ^ |   +4 ask (a+2^i, b) if it's -1 then both are 1, else both are 0
•  » » » 10 months ago, # ^ |   +10 How do you restore the equal bits?
•  » » 10 months ago, # ^ |   +4 Let a = a1a2... a30 and similarly define b. We will find bits from MSB to LSB. Initially find the result with c = 0, d = 0. This will give you a comparison between a and b. Let us call this r.Now suppose we are discovering (i)th bit. We will always keep c = a1a2... ai - 1 and similarly d. Now query for c + 2i, d + 2i. If the result is opposite of r then you know the values of ai, bi and ai ≠ bi. Here you will need to update r once again for unknown bits.Else we have ai = bi. Here you do not need to update r :). So, to find the value of ai, bi just query c, d + 2i.
 » 10 months ago, # |   +3 interesting problems, thank you
 » 10 months ago, # |   +21 xor + interactive
•  » » 10 months ago, # ^ |   0 you should quit using bold letters. thats annoying
 » 10 months ago, # |   +3 Can anyone explain the second sample of problem F？ I can't get 44 from the optimal tree.
 » 10 months ago, # |   +3 I felt like tourist when I solved A in 1 min xD
•  » » 10 months ago, # ^ |   0 I feel like tourist when I travel around the world.
 » 10 months ago, # |   -23 i do not care
•  » » 10 months ago, # ^ |   -10 Yet you still made a comment.Say goodnight.
•  » » » 10 months ago, # ^ |   -26 you have brain damage
•  » » » » 10 months ago, # ^ |   0 Thanks for pointing out. Not many people knows about my poor brain health, yet you do precisely.We can make good friends actually.
•  » » » » » 10 months ago, # ^ |   -18 that is just fucking great you know
 » 10 months ago, # |   -45 make this platform hacker free..... my solution just got hacked.. use good antivirus
•  » » 10 months ago, # ^ |   -6 Pakistan, apni akal kabar me khod ke aaya hai kya...inshallah, boys played well karte karte dimaag bhi sooj gaya hoga...lol
•  » » » 10 months ago, # ^ |   -21
 » 10 months ago, # |   +20 This is not cool. C — 1500 and D — 1750. And D is way more difficult than C.
•  » » 10 months ago, # ^ |   0 For me C was harder than D. I do agree that the problems were harder than on an average div2 round though.
•  » » » 10 months ago, # ^ | ← Rev. 2 →   0 mango_lassi I just checked out your submissions. How are you able to get wrong pretests at a moment and submit the correct code in just 1-2 seconds after that?
•  » » » » 10 months ago, # ^ |   +6 What do you mean? I resubmitted B 3 minutes after failing pretests, and C a minute after.In C I outputted the numbers for the operations in wrong order, so it was very fast to fix
•  » » » » » 10 months ago, # ^ |   +8 Oh my bad!!! Actually since you out of competition contestant, your submissions were shown in hh:mm format, while others(in-competition contestants' submissions) were in mm:ss format. NVM, I misunderstood minutes as seconds and out of curiosity asked immediately. So sorry.
•  » » » 10 months ago, # ^ | ← Rev. 2 →   +2 Same. I feel so stupid. How do you solve C? Edit: Never mind. I got it. So fucking stupid.
•  » » » » 10 months ago, # ^ |   +8 In first step make all elements equal to 0 by "2 n 1".Make all elements equal to a large number say 1000000 by "1 n 1000000".Now, run a loop from 1 to n-1 (not n) with iterator i and take modulus by 1000000 — i by "1 i 1000000-i".Total n+1 steps and your task is done.Note that all indexes less than 'i' will not be affected by taking mod at any step, as they will become smaller in the previous steps itself.Thus, your final series will look like 1,2,3....,n-2,n-1,1000000 (independent of the array, just depending on the array size).
•  » » » » » 10 months ago, # ^ |   0 cool!!
•  » » 10 months ago, # ^ |   -26 yeah what the hell C was rigged to not accept my problem even if it was completely correct just so we get lower ratings WTF
 » 10 months ago, # |   +13 Why system testing is late today? Any specific reason?
•  » » 10 months ago, # ^ |   -20 yeah they don't actually know how to code so they can't say whether the problems are correct or not so they have to manually call a friend who knows how to code and make him verify every single problem
 » 10 months ago, # |   +1 Thanks for an interesting content! And, I have a question on prob. D, which I've been getting on it for an hour. It annoys me.... My question is why I always get a WA on case 1. I don't think I have printed anything larger than 2^30.. Could anyone help me? Thanks a lot! here is my solution.46617886
•  » » 10 months ago, # ^ |   +14 I don't really understand why people don't test their solutions for interactive problems.Just set the number of bits to 2 instead of 30 (so that you have less questions) and interact with your program. Just don't forget (like me) to set the number of bits back to 30 when submitting.Had you checked, you would've seen that your program outputs some weird numbers. The problem seems to be you used uninitialized variables, i.e. you wrote int a, b; instead of int a = 0, b = 0;.
•  » » » 10 months ago, # ^ |   0 Yeah, you are right! It works. Thanks. I'm so frustrated that I should have made such a mistake. Actually, I tested my solution. That'y why I posted this. Maybe my complier optimized such a mistake so that I couldn't notice. Thank you for your help. It's a stupid mistake.
 » 10 months ago, # | ← Rev. 2 →   +8 Cheaters: 46603335 / 46603474 and 46602505 / 46603800 and 46606199 / 46606182
•  » » 10 months ago, # ^ |   0 Each pair of submissions seems identical. They will be caught by cheating detector for sure.
•  » » 10 months ago, # ^ |   0 umm this is normal in india :)
 » 10 months ago, # |   0 How to solve E?
 » 10 months ago, # |   0 Umm was this round not rated?
•  » » 10 months ago, # ^ |   +3 It usually takes a while after the contest ends for ratings to change
 » 10 months ago, # |   +6 Why is he doing it? 46594591, 46591499
 » 10 months ago, # |   +1 Happened to be my last Div2 contest, and it was a good one. Thank you!
•  » » 10 months ago, # ^ |   +3 Why do the top 2 rated coders from your country have 420 in their nicks? Do you guys smoke a lot of pot?
•  » » » 10 months ago, # ^ |   +5 high there, what
•  » » » » 10 months ago, # ^ |   +1 Just wondering how to score like you
•  » » » » » 10 months ago, # ^ |   0 yh last contest it was too much
 » 10 months ago, # |   +15 paulica ANIMALLLLLLL
 » 10 months ago, # |   0 Got stuck on D for 1.5h during contest without reading E. After contest I read E and solve it in 25 mins :D.
»
10 months ago, # |
0

ok. guys. i am new in competetive programming. please help me.

i have a source code on mergesort whenever i need to use that on problem, i just reform that.

# include<stdio.h>

int temp[100000];

void mergesort(int low, int high, int num[]){

if(low == high){

return;
}
int mid = (low + high)/2;

mergesort(low, mid,num);

mergesort(mid + 1, high,num);

int i,j,k;

for(i = low,j = mid + 1, k = low; k <= high; k++){

if(i == mid + 1){

temp[k] = num[j++];
}
else if(j == high + 1){

temp[k] = num[i++];
}
else if(num[i] < num[j]){

temp[k] = num[i++];
}
else{

temp[k] = num[j++];
}
}
for(k = low; k <= high; k++){

num[k] = temp[k];
}

} int main(){

int n,i,num[100003];

scanf("%d",&n);

for(i = 0; i < n; i++){

scanf("%d",&num[i]);
}
mergesort(0,n - 1,num);

for(i = 0; i < n; i++){

printf("%d ",num[i]);
}
return 0;

} i used the code with just a little change of a book here

at B, i just got my solution coincides. why.

•  » » 10 months ago, # ^ |   0 how to upload photo here please
•  » » » 10 months ago, # ^ |   0 I want to upload a photo
•  » » » » 10 months ago, # ^ |   0 my submission is here.46600999
•  » » » » » 10 months ago, # ^ |   0 It is better (faster most of the times) and easier to use std::sort instead: http://www.cplusplus.com/reference/algorithm/sort/ . I recommend you study the STL of C++.
 » 10 months ago, # | ← Rev. 2 →   0 For C, I'm unsure why my submission fails at pretest 2. I traced out the moves and the array should be strictly increasing.I think I am overlooking something obvious. Can someone take a quick glance?Edit: I mixed up the operations
 » 10 months ago, # |   -21 people who watch anime are braindead
 » 10 months ago, # |   0 Can someone explain problem D Solution?
•  » » 10 months ago, # ^ | ← Rev. 2 →   -18 yes so basically for(int i=0; i<9999; i++)std::cout<<“noob”<
•  » » » 10 months ago, # ^ |   0 lol
 » 10 months ago, # |   0 How would one approach problem C if we were to determine the minimum number of steps to make the array strictly increasing??
 » 10 months ago, # |   +1 problem D was a really nice up and down dp tnx codeforces and authors for these problems ;)