Thanks for the participation!

1492A - Three swimmers was authored by meshanya and prepared by ch_egor

1492B - Card Deck was authored by Noam527 and prepared by ch_egor, adedalic

1492C - Maximum width was authored and prepared by Jatana

1492D - Genius's Gambit was authored by voidmax and prepared by rip

1492E - Almost Fault-Tolerant Database was authored by wrg0ababd and prepared by Sehnsucht

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Why was B unusually hard for me?

Actually something greedy works for B and that is quite intuitive from test cases so without knowing the proof many solved the problem.

I kept on overthinking it, for like 1 whole hour, and everything except the simple greedy came to my mind but at last I proved it by taking an input like [3, 2, 1] and [2, 3, 1] and finally got AC.I guess the lesson is to play with the input more instead of just formalizing everything especially for easier problems.

I guess lesson is to use test cases as inspiration or insight.

Second lesson is that you shouldn't spend too much time finding formal proof, although you should have an idea of proof in mind. You need to find a golden middle between theory and practice.

what would be the answer for 7 8 1 lexicographically greatest possible is 8 1 7. but shouldnt the answer be 7 8 1

Why isn't

`8 1 7`

the answer?You can take $$$2$$$ cards from the top of $$$p$$$ and move it to $$$p'$$$: $$$[8,1]$$$

And then take $$$1$$$ card from the top and move it to $$$p'$$$: $$$[8,1,7]$$$.

See, that's it.

Can you provide the proof? I figured it out partially but not completely

I noticed that you cut the input in segments for example the third test case of the first example 6 4 2 5 3 6 1 if you notice here we have 3 major keys{4,5,6} by keys I mean the each of those keys all the elements after it till the next key is smaller than it 4 > 1 && 4 < 5 so the first segment here is {4.2} you then sort these segments according to there keys key 6's segment then key 5's segment and so on...

what would be the answer for 7 8 1 lexicographically greatest possible is 8 1 7. but shouldnt the answer be 7 8 1

The order of the input here is what matters we do not sort lexographically

It's literally just finding the big number in base $$$n$$$ with given digits.

The answer is the largest number in base $$$n$$$, so the left most number should be the maximum. But along with it you need to pick up the entire suffix from that position, because of the stack nature. Repeat the process.

If u have powers of n

n^x greater than summation of n^(1->x-1)

eg. 2^5 (100000 in binary) is larger than 2^0 + 2^1 + 2^2 + 2^3 + 2^4 (11111 in binary)

so giving it largest coefficient will give largest solution

A > B & C > D

A * C > B * D

Given an array $$$[p_1,...p_n]$$$, its order formula is like representing the $$$n$$$-digit "number" $$$p_1...p_n$$$ in base n. This then suggests the strategy of picking the current largest $$$p_i$$$ and the corresponding suffix (from $$$p_i$$$ to the end). Seeing the order as a base-n number also proves the strategy is correct.

My $$$O(n)$$$ solution just in case.

You can use geometric progression

take make element as some x and second max element as x $$$-$$$ 1

take some constant c which the number being used in the polynomial expression

now compare $$$x * c^{n}$$$ and $$$(x - 1) * (c^{n-1} + c^{n-2} ... c^{0})$$$

the gp formula will always yield a lesser value than $$$x * c^{n}$$$

our expression has a lesser value than the gp so I guess the rest is obvious.

you can write order as n^n(P1/n + P2/n^2 + P3/n^3.....Pn/n^n).. now when you choose some element to put into position Pi you will try to maximum element to maximize the sum

PS:- sorry for not using math syntax I don't know how to use that

The important part to note is the elements in the array are less than equal to n (size of array). Thus we should have proved that the MAX*N^(N-1) will be greater than all possible summations, when MAX is not in the first place of new deck (Same way as 900 is greater than all permutations of {2,6,9} creating a three-digit number when 9 is not in MSB, eg — 629, 692, 296 etc.) Basically treating the input as base-n number creation. Thus lexicographical sense is understandable. I missed this fact completely. If values aren't bound by n, not sure if this would work, need to check.

Yes, I noticed that.

it wont work if they are not bound by n. for example 7 8 1

From test data you could see greedy.

What a fst round :(

Test case 73!

64 and 76 too!

And test 21 :(

Test case 1 too xD

Video editorial for problem C: https://youtu.be/7uXOxPAej7Y

fstforces

Why downvoting? The problems were great and the editorial is clear. It just has weak pretest. Even so why not to downvote the announcement instead of editorial?

I know pretests of problem D were weak but otherwise it was a good contest. I don't think it's right to downvote it's editorial.

ch_egor, can you translate the name of the problem E to English in the bottom of the editorial? It is in Russian now.

there is an excellent google translator for this

FSTforces。:(

My answer to A is $$$\min(a-p\bmod a,b-p\bmod b,c-p\bmod c)$$$. Maybe the idea is easier?

UPD: You need to check that if $$$p\bmod a=0$$$ or $$$p\bmod b=0$$$ or $$$p\bmod c=0$$$.We need to check if any of p%{a,b,c} is 0, explicitly in this case.

Yeah I know that. I just forget to write it in my comment. thx for pointing it out.

my solution is

so you can avoid edge cases.

Nice idea :)

can you prove it

Why for 0 1 0 in problem D answer will be No while 1,1 satisfy the given condition

Answer is YES 1 1

Thanks for fast editorial and good problems.

my easy answer E

randomly select elements from arrays and test it is real.×100times

https://codeforces.com/contest/1492/submission/108293277

UDTthis is very easy to hack, i tried some another way to pass, but maybe random can'tIt can be hacked by this.

SpoilerYou wrote special case a = 0 for problem D in editorial but no pretests for this? Seems someone likes to see fst

CDE are nice, B is kinda "guess-by-sample" problem, and A just boring.

Great contest overall

## C is copied

https://leetcode.com/problems/longest-common-subsequence/

It's not, and what if it was? Does it matter to you? Stop shitposting on CF and take a shower buddy, no one thinks you're funny

The system tests of problem E are weak too. Some random solution like https://codeforces.ml/contest/1492/submission/108282677 can be hacked by this test case:

62500 4

1 1 1 1

1 1 1 1

1 1 1 1

...

2 2 2 2

Problems were really interesting and looks like pretest were really strong as many solution got failed during system testing. Superfast editorial as well .....

Although I didn't count how many people failed to pass the system test on some problem, I found that my rank changed from 1238 to 764.

This round was really Fst forces!

only if

a+b-2==ktestcases were included in pretests for D :()Everyone's commenting "Fstforces". What does it mean?

FST is equal to "Failed to pass System Tests".

Can anybody explains to me how B can be implemented in O(N)?

let pr be n(last element), loop this: 1.find the index of the maximum element in the remaining array , say id 2.push elements from id to pr in the ans array 3.update pr as id-1, break when pr==-1 finding index of largest element in the remaining can be done using a prefix maximum array.

Here is my Solution i think it's O(N)

https://codeforces.com/contest/1492/submission/108251677

It is $$$\mathcal O(n)$$$.

here's my submission, i used a stack and each time i reached the max unused card (starting from the right side of the array), i printed and emptied the stack 108442718

This gives me RE on pretest 5

REwhereas this gets AC on pretest

ACI have changed only this much part of the code and nothing else. Someone please help me out in debugging the error.

UPD:I got the mistake I was doing. Thank you all.Just a friendly advice, you should post the code as a spoiler or drop-down box.

I tried that but it kept overflowing out of the box in the preview, so I removed it.

I will see to it why it happens and make sure to put code in spoiler.

UPD:Now it's not doing the weird behavior. Still don't know why though.I don't know what causes this, but with the following input:

your code prints

`576479918390712804`

which is clearly wrong.Thanks for the testcase, I understood where it went wrong.

Solve D in ten minute, WA after two hours.

Editorial of E seems incomplete :(

[Deleted]

My performances during the contest

Quite awful. I opened the problem pages, after a couple of minutes, I finally see the statment of A, I think I spent little time solving it, but I forgot to save the code after I have a few changes, and I submit the code without realizing it. So I got a wrong attempt.

The following story is boring, until I try to solve D, at first I think there is only one way to make the answer have digit

`1`

：`1000-0001=0111`

. Obviously some cases were left behind, so I got WA on pretest #8. I suddenly find the problem, and I deside to recode — that is, to delate the original one and to solve the problem again from zero.That is proved to be a bad idea, for the method when I recoded havn't been design carefully. So I finally WA in main test #64, and I didn't get in the first 1000.

But if I didn't recoded and chose to mend the code I write first, everything may be changed.

So I got a lesson after the contest: Think twice, code once.

Getting FST stimulates me !

SpoilerAnd the contest ended :(

Lots of contestants passed E with a random algorithm like 108293277. There is a hack test:

The answer is

`1 1 1 1 1`

but if you randomly choose the answer from every sequence, the accuracy is $$$O(\frac{1}{n^2})$$$.Problem D discussion thread

Problem E discussion thread

Really thank you for the GREAT pretests with Problem D.

Please make pretests STRONG next time.

FST Round The pretests of D are too weak :(

Let

`s`

be`codeforcescontest704d`

and`t`

be`fst`

, the maximum width of`p`

is 7 XDHow did my comment become a downvote attractor? :( The meaning I wanted to explain was similar to the one the guy upstairs(Chinese:楼上, which means the previous comment or its poster, translated literally) referred to. :( One of my classmates used his three accounts or so to downvote me, and now there're so many:(

LearningfromProblem Aespecially forJAVAusers :Ceil Value Problem... Let's take variable, long max=(long)1e18;

So, The ceil value of max/(max-1) should be 2. But in JAVA, even if I use long, it is giving 1.

This costs me 1 wa .

Learning... So,to calculate the ceil value of A/B, always use this :

`long ceil=A/B;`

`if(ceil*B<A)`

`ceil++;`

It will handle all cases easily.

AC submission with this trick.

Thank You !actually this problem is also in C++ . I submitted by using ceil function and converting denominator to floating point by multiplying with 1.0 but it gave WA mostly due to precision issue.

we can do $$$(num+den-1)/den$$$ for ceil.

Why is this getting downvotes?

I have faced some problems, so I write it so that many users can correct the same mistake.

Because of two reasons:

1) In every single contest that requires ceil, there is a ton of comments describing exactly what you just did, it is clear that most people don't really enjoy repetition in comments thus they downvote.

2) The algorithm you provide to get ceil is not really 'optimal', to be more precise most people use (A+B-1)/B (as the guy above me said) because it requires only one line of code and even is a bit faster because it doesn't use multiplication.

To sum up, you did not do anything wrong by describing your issues, but people get tired of reading this exact comment every time there is a task that requires a ceil operation.

Thanks for the clarification.

i think my solution to the Problem A was correct,but my submission to it didn't pass the test cases. Could you pls hava look over over it and correct me if i am wrong and where did i go wrong???? https://codeforces.com/contest/1492/submission/108247518

This is because of an integer overflow. Try with long long.

I have already defined int to long long

## define int ll

I think, the problem is happening due to the loss of precision.

Check my comment regarding this issue.

Thank You.

How can I optimize my B solution? My solution

Any help would be appreciated.

In your solution, it waste too much time to calc the

`maxi`

. Have a look of my code ? :) let`M[i]`

be the largest p after pi.Thanks a lot for your reply! But I dont understand how you calculate M[i] Could you please explain it again?

Oh, my fault... I meant that

`M[i]`

be the largest among {p[1]..p[cur]} (cur has the same meaning of the var 'end' in your code).In order to get the array M, we can scan from p[1] to p[n], maintain the largest's id, just like this

ooh Thanks a lot! I understood

:)

https://codeforces.com/contest/1492/submission/108260474

For problem A, this was my following code:

It is giving me WA on pretest 2. What am I missing?

See my comment.

It is explaining the same.

Floating-point arithmetic over large numbers doesn't always give you exact results.

Using integer arithmetic or reading input as Decimal would have saved you.

Alternative solution for 1492E - Почти отказоустойчивая база данных is following: find any two vertices of diameter, then try all 2^(diffs) variants. Is it wrong? Can someone prove it or hack it? 108299916

tests are wonderful!!!!

Can somebody please tell why my solution for C failed .. - I have calculated max, min possible indexes for each t[i] keeping my mind p1<p2<p3 .. - where max of every index is smaller than max of i+1 - and min of i is greater than min of i-1. - and calculating the ans as the max of (max[i+1]-min[i]) for valid indexes. - and if we get answer from certain i, we can use max of each indexes >i+1 so that given condition is verified. Here is my submission: https://codeforces.com/contest/1492/submission/108280740

upd: got AC now :)

When Problem D is doable in O(n), why restrict the limits to $$$2\times 10^5$$$? The editorial doesn't even mention DP or any alternative solution that only works below $$$2\times 10^5$$$.

You have to print $$$2 \times 2 \times 10^5$$$ characters. Longer string may cause I/O bottleneck.

Codeforces Round #704 (X)

Hackforces & FSTforces Round #704 (O)

why am I getting runtime error in this soln

In calc() what if q1 is empty? (Which is a possibility.)

then the answer will already be in the vector ans

oh no now I realize. What a stupid mistake.

Thank you

what is wrong with my idea

The solution is not to get increasing subarrays but to have the maximum element at last (otherwise there will be another subarray with higher score). So the first subarray will be the one containing nth element as last element (from the end as seen in testcases). Then recursively solve for n-1, n-2, ..., 1.

ah ok thank you

Good questions and fast editorial.

Personally, leaving some unexpected spaces for hacks is ok.

But it seems like this contest is another lesson for the authors that it's important to keep an eye on the chances for using randoming algorithms (or other tricks like brute force + #pragma)to pass the tests...

How to solve problem E's

bonus 2? I can only come up with $$$O(nm+k^kn)$$$.Hint: $$$\sqrt{nm}$$$ and $$$\min{(n, m)}$$$ have something common.

I know it, but I don't know how to deal with the case of $$$n>m$$$. Can you give more hints?

Simple solution for D:

Make

xasazeroes andbonesMake

raskones and max(a-k, 0) zeroesThen if x-r >=0 and

x-rcontainsazeroes andbones we found answer, otherwise no solution108313076

How did you find it?Could you please provide any proof or intuition behind it?

Bonus1 seems to be the same as the original task.

In Bonus2, intuitively split nm into 2 cases , where n <= 500 or m <= 500 , to reach the sqrt time complexity.... But have no idea how to keep on :(

Hope to get some hints.

Doubt in problem E editorial :

Take following input : n=3 m=3 , 1 2 3 , 4 5 6 , 3 3 7

We can see that 3 5 3 will work .

Now if input was : n=4 m=3 , 1 2 3 , 4 5 6 , 3 3 7 , 1 3 7

Then according to editorial 3 5 3 should work as answer because we ignore the 4th array . But then 3 5 3 differs from fourth array at 3 positions.

"Ignoring" an array doesn't mean that we immediately throw away it from input.

The solution is to try to consider the first array as an answer, therefore we can skip ("ignore") other arrays which doesn't differ in more than 2 positions.

If it doesn't work, then we change the first array and check again. In particular, after changing, "ignored" arrays may be differing in more than 2 positions (and therefore stop being "ignored").

So, although you ignored the 4th array when considering "1 2 3" as a possible answer, you can't ignore the 4th array when considering "3 5 3" as a possible answer.

https://codeforces.com/contest/1203/problem/D2

Same problem C

For all those who are downvoting without knowing the truth

see this

submission of 704C :- https://codeforces.com/problemset/submission/1492/108350993

submission of https://codeforces.com/contest/1203/problem/D2 :-(after adding 2 lines for corner cases) https://codeforces.com/contest/1203/submission/108351056

can anybody tell me why am i getting WA after using same logic for question : https://codeforces.com/contest/1492/problem/C my solution : https://codeforces.com/contest/1492/submission/108314097

Test with this input:

Your code prints

`6`

. It seems that your code assumes the letter`o`

at the beginning/end of the first string can be reused, which is not true.thankyou so much for replying

https://codeforces.com/contest/1492/submission/108277935 This is my solution for problem C.I don't know where I went wrong.I have implemented the same idea as given in tutorial.Please someone check it out.Thanks in advance.

Check this video soln it really helped me to understand it. Easy video solution

test case 64!Why are some editorials in english and some in russian?

some people have used randomized solution for problem E . Could someone please explain how to solve problem E using that.

Question C detailed solution Video Solution Explanation

If anyone is feeling stupid after the contest than let me tell you there are people way more stupid than you.

Failed solution during contest## include <bits/stdc++.h>

using namespace std;

typedef unsigned long long ull; typedef long long ll; typedef long double ld; typedef pair<int, int> pii;

## define pb push_back

## define forn(var, start, end) for (int var = start; var < end; ++var)

## define in(array, end) for (int i = 0; i < end; ++i) {cin >> array[i];}

## define MOD 1000000007

## define ceil(a, b) (a+b-1)/b

## define nl "\n"

void fastio() { ios_base::sync_with_stdio(false); cin.tie(NULL); }

int main() { fastio(); int n, m; cin >> n >> m; string a, b; cin >> a >> b;

}

Passed solution after contest## include <bits/stdc++.h>

using namespace std;

typedef unsigned long long ull; typedef long long ll; typedef long double ld; typedef pair<int, int> pii;

## define pb push_back

## define forn(var, start, end) for (int var = start; var < end; ++var)

## define in(array, end) for (int i = 0; i < end; ++i) {cin >> array[i];}

## define MOD 1000000007

## define ceil(a, b) (a+b-1)/b

## define nl "\n"

void fastio() { ios_base::sync_with_stdio(false); cin.tie(NULL); }

int main() { fastio(); int n, m; cin >> n >> m; string s1, s2; cin >> s1 >> s2;

}

In case it is not clear the only difference between the two is while calculating the first(left) array I have used m as the exit condition in the inner loop instead of n.

I can't understand the meaning of E problem.

Did anyone solve C with a DP approach?? Please post it if possible

Can someone explain how to approach the problem B if we have random numbers in the array. If we have test cases of random numbers like [9,10,1] then the greedy logic fails

Why it's wrong, the answer sequence is [10,9,1] Edit: sorry [10,1,9]

Nope the answer is [9,10,1] as for [9,10,1] the value of the deck is (9*9 + 10*3 + 1*1) = 112 and for [10,1,9] the value of the deck is (10*9 + 1*3 + 9*1) = 102

wait... hold on, I think we neglect a key constrain that 1≤pi≤n. so this sequence is invalid

That's why I asked for an array with random numbers

ok, my fault.

ok.my fault

ok my fault

Thanks in Advance.

I cannot understand why my submission for C got the wrong answer on Pretest 5.Can anyone please provide me a simple test case (for which my program will fail) so that I can understand my error.

Here is my submission : C.Maximum Width

Hi, for problem E, can anyone explain a little bit what does the variable

`cnt`

mean in the`dfs()`

function of this submission?https://codeforces.com/contest/1492/submission/108321457

Thanks for help.

cnt is the no. of changes we can make in the first array;

In first question, it was all about constrains given in problem 10^18

Hi. I would appreciate if anyone tells me why this O(n+m) solution fails test 57 on time. Thanks a lot.

## include

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

## include

using namespace std; int main() {

}

In the editorial of problem C, I think the answer should be $$$\max_{i=1}^{i=m-1}\textit{right}_{i+1}-\textit{left}_{i}$$$ ($$$n$$$ is replaced by $$$m$$$.)

Problem C, 108446832 is getting WA on test 7. Can anyone provide a testcase?

Hello, can someone please explain how the solution to E is not 6^2 * (n*m) ^ 2? The way I understand it is like this: We go trough the list o(n * m), for every single guy, if he has more than 2 differences we go into the maximum of 6 ways of completing him (so times 6). For every one of those n * m * 6, we go trough the list a maximum of one more time, to look for another array that has a difference of more than 2 with our current solution array. So n*m*6*n*m. o((n * m) ^ 2 * 6) so far. Now once we find one we have a maximum of 6 ways to change our solution array so o((n * m) ^ 2 * 6 ^ 2) is our final complexity. Where am I misunderstanding?

I implemented the solution in editorial in following way (I will the prove it's complexity) :

We can notice that we can make atmost two changes to the first array so that it becomes original array and all array differ from it in atmost $$$2$$$ positions.

Now we compare first array to all other arrays and we find another array such that it differs in more than $$$2$$$ positions then we must modify atleast one position in first array . Number of ways we can modify one position in first array is $$$4$$$ in worst case because as editorial says , the case with more than $$$4$$$ difference will not have answer.

After the modification in first array , we again compare it with all other arrays and if we find some array differing in more than $$$2$$$ positions we must again modify it in one position. We will consider all possibility and maximum possibilities here will be also $$$4$$$.

Thus it's like calling compare function recursively after modification , also in particular recursive function , we won't compare further once we found array with difference greater than $$$2$$$.

Number of times we will call this recursive function is atmost $$$3$$$ because we can make atmost $$$2$$$ changes.

So suppose in first function call in worst case we found the last array which differs in greater than $$$2$$$ position then number of operations in first function call would be $$$O(n*m)$$$.

Number of operations related to second function call will be $$$O(4*n*m)$$$ because previous function will call this function atmost $$$4$$$ times.

Number of operations related to third function call will be $$$O(4*(4*n*m))$$$ because number of times previous function will call this function will $$$16$$$.

Thus total operations will be $$$O(21*n*m)$$$. The key point which saves time complexity from being $$$O((n*m)^2)$$$ is that in particular instance of function call , we don't compare with further arrays once we find the array which differs in greater than $$$2$$$ positions with first array.

submission related to above reasoning.

I am unable to find out what wrong am I doing here: https://codeforces.com/contest/1492/submission/108491871

Hi, guys! May u help, please, find mistake in #C. I use binary search to find left/right indecies in the arrays of indecies of each char. I can't pass test 5. https://codeforces.com/contest/1492/submission/108497604