### Pyqe's blog

By Pyqe, history, 4 weeks ago, ## 1725A. Accumulation of Dominoes

Author: Pyqe
Developer: nandonathaniel
Editorialist: Pyqe

Tutorial

## 1725B. Basketball Together

Author: FerdiHS
Editorialist: Pyqe

Tutorial

## 1725C. Circular Mirror

Author: Pyqe
Developer: steven.novaryo
Editorialist: steven.novaryo

Tutorial

## 1725D. Deducing Sortability

Author: Pyqe
Developer: TakeMe, Pyqe
Editorialist: Pyqe

Tutorial

## 1725E. Electrical Efficiency

Author: steven.novaryo
Developer: steven.novaryo
Editorialist: rama_pang

Tutorial

## 1725F. Field Photography

Author: Pyqe
Developer: Pyqe
Editorialist: Pyqe

Tutorial

## 1725G. Garage

Author: Nyse
Developer: Nyse
Editorialist: Pyqe

Tutorial

## 1725H. Hot Black Hot White

Author: Pyqe
Developer: steven.novaryo
Editorialist: steven.novaryo

Tutorial

## 1725I. Imitating the Key Tree

Author: Pyqe
Developer: Pyqe
Editorialist: Pyqe

Tutorial

## 1725J. Journey

Author: gansus
Developer: gansus, steven.novaryo
Editorialist: rama_pang

Tutorial

## 1725K. Kingdom of Criticism

Author: Pyqe
Developer: Pyqe
Editorialist: rama_pang

Tutorial

## 1725L. Lemper Cooking Competition

Author: Pyqe
Developer: steven.novaryo
Editorialist: rama_pang

Tutorial

## 1725M. Moving Both Hands

Author: Pyqe
Developer: Pyqe
Editorialist: rama_pang

Tutorial  Comments (34)
 » 3 weeks ago, # | ← Rev. 2 →   Fast tutorial. Thanks. Btw there is two pointers solution in B. Adding two pointers tag would be great I guess.
•  » » is this solution o(n)?
•  » » » Which solution?
•  » » » » the two pointers one for B
•  » » » No, because that method requires sorting first.
 » How can you get 4 + (n * 4 — 3) / 3 just by 4 + 4a?
 » i did'nt got solution for problem m anyone pls help
•  » » 3 weeks ago, # ^ | ← Rev. 2 →   Run shortest path algorithm.Change the direction of all edges.Run shortest path algorithm.
•  » » » more detailed? pls
•  » » » » You wanna find a point x that has min minway(1, x) + minway(p, x). Also, we change the direction of all edges, so now, minway(1, x) + minway(p, x) = minway(1, x) + minway(x, p'), where p' is a new point for p, that shows, that p' is p in graph with reversed edges. We can see, that every point on the shortest way from 1 to p' has min sum of that 2 ways.
•  » » » » » I don't know where my code is wrong. Can you help me find out the details? thank you 171057638
 » 3 weeks ago, # | ← Rev. 2 →   For E, what's the intended way to build the auxiliary tree? We used small to large merging but that was O(n*log(n)*log(A)*map) which seems sus
•  » » It is actually possible to build all sparse trees simultaneously using small-to-large, but the time complexity is worse. The intended solution uses an algorithm that runs in $O(|S| \log N)$ for each set $S$. The algorithm is as follows: Sort the vertices in $S$ based on their euler tour traversal order. All extra vertices in the sparse tree can found as the $\text{LCA}$ of every pair of vertices in $S$ that are adjacent in the sorted order. Once all required vertices are found, we can find the edges by iterating the vertices (including the extra ones) in euler tour traversal order and maintaining a stack. You can optimise it further to make the total time complexity $O(N(\log N + \log \max(A)))$. But the time limit is not that tight that even the small-to-large solution is able to pass.
•  » » » Ah thanks, that's really cool. Haven't really seen many problems with this "auxiliary tree" idea, so its nice to learn good techniques for it.
 » On the third task O() calculates in wrong way, min(Cntpair, m) = O(n), so O(n logn), or wthether we check case, where min(CntPair, m) = m, so in that way it'll be better if we say that it's O((n + min(n, m)) * logN) or something like that
 » L had weak testcases, we submitted L very late and only realized after the contest we forgot to check whether every element of the prefix sum was non negative and it passed the submission 170883681
 » how M?
 » Problem: 1725B - Basketball TogetherSolution: 170864702In this block of code: int Temp = a[i]; while(Temp <= D) { Temp += a[i]; } when I set n = 1, D = 10^9, and a[i] = 1; in theory it should run in 10^9 steps, which will give a TLE verdict. But when I use "Custom Invocation" to test it, I found that it only ran in 500ms, which is way below the time limit. Why did it happen? Is it because of the codeforces judging machines, or is there something that I'm missing?
•  » » It probably just runs that fast. The hot path only includes an add, a compare, and a conditional jump, which is < 3 cycles with branch prediction. Computers run at a few GHZ, so 500ms sounds right.The $10^8$ things/second heuristic is just a heuristic for usualish groups of operations, you can do better if u have a fast loop body (especially if the compiler avx-ifies, which might be happening here).To see exactly what is happening u can try putting it in https://godbolt.org/ with the correct compiler version/flags.
•  » » » 3 weeks ago, # ^ | ← Rev. 2 →   Alright, I got it. Thanks :D
 » Our team enjoy solving this problemset. Especially for Problem L. We didn't think it could be done using prefix sum. Very nice problem
 » why 1 and 4 cannot be expressed as $(b^2−a^2)$
•  » » 3 weeks ago, # ^ | ← Rev. 2 →   we can write (b^2-a^2) as(b-a)*(b+a) // proof for 1 can not be expressed in terms of b^2-a^2 so the min positive value of a we can take is 1 and for b its 2 as (b>a) as stated in the problem so, (b-a)=1; (b+a)=3; and their multiplication would give us 3 as the min value which can be expressed in terms of b^2-a^2; and one more conclusion can be drawn is that after three all odd numbers can be expressed in the form of b^2-a^2 (because we can express every odd number (lets say a)as 1*a; and a can always be represented as a sum of 2 consecutive numbers which are always odd **** lest take a=4 ,b=5; b-a=1; b+a=9; 9*1=9; that is odd)// proof for 4 cannot be expressed in terms of b^2-a^2 to make equation even we have to make (b-a) even first so in order to make that even min value of a we can take is 1 and for b is 3 so, (b-a)=2; (b+a)=4; and their multiplication will give us 8 that is the minimum even value we can achieve and from here we can draw one more conclusion that all the even values will be multiples of 4 as no matter what we take values of a and b whenever (b-a) is even (b+a) would also be even (because to make the diffrence of 2 numbers even their parity should be same and if we add same parity numbers then result is even) so that would make the result divisble by 4
•  » » » Thank U
 » Problem M solution tight for Java. WOrks in C++ but gives TLE in java. https://codeforces.cc/contest/1725/submission/170984982
•  » » NO Java AC solution for M. Please test the solutions for Java, Python as well before the round to know if the same solution passess the time limit. This is not fair that although both the solutions has the time complexity, it gives TLE in Java and AC in C++
 » 3 weeks ago, # | ← Rev. 3 →   For those curious about the $O(1)$ formula for problem G (Garage): Spoiler$f(N)=\left\{ \begin{array}{ll} 3 & \mbox{if$N = 1$}\\ \left\lfloor \frac{4N}{3}\right\rfloor + 3 & \mbox{if }N > 1 \end{array} \right.$One could verify the $N > 1$ case by noting that values $N = 3k + [ 0, 1, 2 ]$ map to the correct values $f(N) = 4k + [ 3, 4, 5 ]$ respectively.
•  » » noob
•  » » I could come up with this result by building a sequence,I added the difference between b^2 and a^2 in the sequence as follows:4 — 1, 9 — 4, 16 — 9, 9 — 1, 25 — 16 ..by listing the "difference between squares" in nondecreasing order,I got the sequence:3, 5, 7, 8, 9, 11, 12, 13, 15, 16, 17, 19, 20, 21, 23, 24, 25, ,27, 28, 29, 31..I noticed that the first number "3" doesn't follow the pattern, so I assumed it was a special case, but the remaining numbers follow a consistent pattern that I came to figure out as:"3 + 4 * (N // 3) + N % 3"
 » Isn't F's TL too tight?
 » I think problem J has insufficient tests. In particular, I found solutions (including mine) that get AC, but give an incorrect answer to the following simple test: 8 1 2 1 2 3 50 2 4 50 1 5 1 5 6 1000 6 7 1 6 8 1 As far as I understand, the correct answer here should be 106.
 » Could problem C be somehow solved by subtracting the configs with three same colours in the right angled triangle from $m^n$ rather than summing over bin. coefficients?
 » Can anyone tell me whats wrong with my code for Problem C? ~~~~~ def nCr(n, r): return (fact(n) / (fact(r) * fact(n - r)))def fact(n): if n == 0: return 1 res = 1 for i in range(2, n+1): res = res * i return resn,m=map(int,input().split()) d=list(map(int,input().split())) cf=sum(d) t=0 ptr1,ptr2=0,0 val=d[ptr1] while True: if val*2==cf: t+=1 ptr2+=1 if ptr2>=len(d)-1: break val+=d[ptr2] elif val*2=len(d)-1: break val+=d[ptr2] else: val-=d[ptr1] ptr1+=1if t==0: print(m**n) else: c=n-(2*t) count=0 x=min(t,m) for i in range(x): count+=(nCr(t,i)*nCr(m,i)*fact(i)*((m-i)**(t-i+c))*((m-i-1)**(t-i))) print(int(count)%998244353) ~~~~~
 » 173417871 Can anyone tell me what is wrong with my code for problem C?