### flamestorm's blog

By flamestorm, 14 months ago, Thanks for participating!

1742A - Sum

Idea: flamestorm

Tutorial
Solution

1742B - Increasing

Idea: mesanu

Tutorial
Solution

1742C - Stripes

Idea: flamestorm

Tutorial
Bonus
Solution

1742D - Coprime

Tutorial
Solution

1742E - Scuza

Idea: mesanu

Tutorial
Solution

1742F - Smaller

Idea: SlavicG

Tutorial
Solution

1742G - Orray

Idea: SlavicG

Tutorial
Solution Tutorial of Codeforces Round 827 (Div. 4) Comments (60)
| Write comment?
 » B was just a few lines with the help of STL :D (Submission: 176051852)
•  » » Nice!
•  » » 14 months ago, # ^ | ← Rev. 2 →   Or int n; cin >> n; vector a(n); for(auto &x : a) cin >> x; cout << (set(a.begin(), a.end()).size()==a.size()?"YES":"NO") << '\n'; which I find even simpler.
•  » » 14 months ago, # ^ | ← Rev. 3 →   But the complexity of your algorithm being O(n*log(n))?? It can easily be solved using O(n) using maps. The logic is simple if any number is present more than once then it is impossible to rearrange in increasing order Here is mine: Submission 177430404
•  » » » 5 months ago, # ^ | ← Rev. 2 →   complexity of insertion in map is O(log N).While in the worst case, the time complexity of insertion in an unordered_map is O(N). So in worst case your solution is O(N*N) not O(N). On the other hand if you used map it would be O(NlogN)
•  » » 4 months ago, # ^ | ← Rev. 2 →   I know your solution is correct but here he elaborates to us the idea of the problem not just the solution. great respect
 » 14 months ago, # | ← Rev. 2 →   Problem C,When I judge the horizontal and vertical directions of a color,my submission-->175928516, I can't pass the test. But when I only judge the corresponding direction of a color, it passed,my submission-->176020169 Can someone tell me why
•  » » Look at the title,some horizontal rows have been painted red, and some vertical columns have been painted blue.
•  » » » I also didn't read it properly costed me more than an hour to cause it took more than 15 mins to run on pretests :'(
•  » » Consider this caseBBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB RRRRRRRRAnswer : 'R' Output : 'B'
 » for problem C, many of us missed this case :( BBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB BBBBBBBB RRRRRRRRAnswer : 'R' Output : 'B'
•  » » I think the answer should be R, as it has the occurrence 8 consecutive times in a row. Please tell, why are there so many messages related to this?????
•  » » » 14 months ago, # ^ | ← Rev. 2 →   Because many of us just checked whether the entire row is same color rather than checking entire row is Red Or Not. This also follow for column also.
 » why is my 176088793 getting WA on 11 (problem D)
•  » » when the input array is 2 6 10 15 your output is 0 because when you reach 6 the gcd will be 1 but actually there is no coprime number between 6 , 10 , 15 but it is true that their gcd is 1 The correct output should be 5 by taking 2 and 15
 » I got TLE in problem E. My submission is 176039360 . How can I improve this?
•  » » As per your solution I suggest you to have a read on Time Complexity Analysis It would be helpful for you to understand why your code is getting TLE
 » I got TLE on Problem D. Not sure why. How can I improve this solution
•  » » Instead of keeping track of every index , you can consider the values of the array elements , that can go up to only 10^3 . Now , if you run two for loops , then time taken O(10^3*10^3) and for T=10 test cases , O(10*10^3*10^3) = O(10^7) [neglecting time complexity for __gcd(a,b) stl ].
 » Video Editorial for Chinese：Bilibili
•  » » Is that Chinese youtube?
•  » » » YES，and better than YouTube
 » I think there is an easier way to implement the algorithm in G's tutorial. 176054281
 » Weak pretest in problem F.
 » sadly, i misread the "OR" as "XOR" and failed to ak
•  » » oh bad luck
•  » » » it's more like oh bad concentration
 » 14 months ago, # | ← Rev. 2 →   flamestorm is the solution of bonus of problem C like this :we go through each row except the ones with full red then check whether if a column is painted blue whether it is painted red in any of the previous rows and if a column is painted red whether it is painted blue in any of the previous rows and if a column is white whether it is painted blue in any of the previous rows; If any of these three happens then input grid is invalid otherwise validIs it correct ?
 » 14 months ago, # | ← Rev. 2 →   Hey flamestorm,In problem C, the valid input must contain exactly one of theses situations:1- Have one or more rows full of R's2- Have one or more columns full of B'sotherwise, the input is invalid Is it correct ?
•  » » 14 months ago, # ^ | ← Rev. 2 →   Well, those are a part of the conditions, but there seems to be more invalid cases to consider. The following is one example. .B.BB... .B.BB... .B.BB... RBRRBRRR RBRBRRRR .B.BB... .B.BB... .B.BB... 
•  » » » I don't see any reason of downvoting him , he made a correct point.Maybe he has bad record in past in terms of commenting & posting but that doesn't have any relation with this,And why am I saying this? I am saying this because we all make mistakes in life ,doesn't mean we can't move forward
 » I got TLE in F question for Test Case #2, any ideas/hints how can I improve the code? submission
•  » » You need to save the count of every character, as k can be 10^5 every time and there are 10^5 queries the size of the string can exceed 10^10. You can check out my submission : https://codeforces.com/contest/1742/submission/175994173
 » In problem D I have gotten wrong answer on test 24 on an invalid test int the statement they said that n must be greater than 2 but I have had wrong answer on a test with n=1 so please rejudge the problem SlavicG , mesanu , flamestorm please do something 175973439 .
•  » » Help him pleez
•  » » Help him, he deserves achieving pupil :(
• »
»

Agree

# Help_Rida

•  » » it is unfair not to rejudge his submission
•  » » » 14 months ago, # ^ | ← Rev. 2 →   totaly agreeeehiz frendz need mental aid
 » In $C$, why didn't they put $n$ and $m$ $(1$ $\le$ $n,m$ $\le$ $1000)$ or something like that?I think when $n=m=8$ problem is more like Div.4 $B$ problem, this could make problem $C$ little harder, just as hard as a Div.4 $C$ should be.
•  » » I don't think constraints matter at all in C ,n,m<=1000 doesn't make any difference in solution
•  » » » In some solutions it actually does matter.Example: 175929785 from ltunjic
•  » » » » how does it matter? you can just create RRRRR....RRRR string just by appending R m times
•  » » » » » Right, but I still think problem would be little harder for beginners.
 » In Problem B, the checker log is telling me that I gave a wrong answer in test case #2 while my answer is the same as the output. As you can see in the submission, the 25th token is correct. Am I missing something? 176210112
•  » » Your code is dereferencing the value of $a[n]$. This is undefined behaviour, and may have caused a WA any moment.
 » I faced some issue in the coprime problem. can anyone please identify the flaw in the code. Thanks in advance. Your code here... #include using namespace std; int gcd(int a,int b){ if(a==0){ return b; } if(b==0){ return a; } if(a==b){ return a; } if(a>b){ return gcd(a-b,b); } return gcd(a,b-a); } bool isCoPrime(int a, int b){ if(gcd(a,b)==1){ return true; } return false; } int main(){ int t; cin>>t; while(t--){ int n; cin>>n; vector arr(n); for(int i=0;i>arr[i]; } int j=n-1,i=n-1; bool found=false; int ans=0; while(i>=0){ if(isCoPrime(arr[j],arr[i])){ found=true; ans=i+j+2; break; } i--; } if(found){ cout<
•  » » you are not moving j.while(i>=0){ if(isCoPrime(arr[j],arr[i])){ found=true; ans=i+j+2; break; } i--; }In this loop you are not decreasing j.this approach is wrong according to me if you try with two loop then it will show TLE. see Editorial.check for: 1 2 3 6 according to your code ans should be 4 but real ans is 6 because 2 and 3 are coPrime so, 2*3 = 6.
 » Have a problem with G176665365but the test is really huge and I can catch the problem/Maybe someone has the idea?
 » great！！！
 » Can anyone tell me what i'm ddoing wrong for problem G. Here is my submission i'd
 » In problem D (Coprime) , would anyone help me to understand that how its time comlexity becomes O(a2i (logai+n))
 » 11 months ago, # | ← Rev. 2 →   I guess I didn't understand F correctly...The is the 10th test: 1 5 2 1 bb 1 2 b 2 3 b 1 2 c 2 3 bcdbweakpretests Official answer is:YESYESYESYESYES.Why it isn't:YESNOYESNOYES?UPD: solved it
 » trash round
 » 192675897 My code shows wrong in the 10th word. But I can't see the 10th word. It finished after the 6th. How can I find all test examples?
•  » » Take a look at Ticket 16727 from CF Stress for a counter example.
 » Can someone please explain B solution in Java?
 » In problem 1742D, I am getting my 1st test(5th array) wrong which is 31 instead of 10, also i don't know why 10 is the answer for the fifth array, as 15 and 16 are coprime also.
•  » » https://gist.github.com/ryze-7/285ff124c4e164a62e9e945e5a1d1310 link for my code