We will hold AtCoder Beginner Contest 178.

- Contest URL: https://atcoder.jp/contests/abc178
- Start Time: http://www.timeanddate.com/worldclock/fixedtime.html?iso=20200913T2100&p1=248
- Duration: 100 minutes
- Number of Tasks: 6
- Writer: YoshikaMiyafuji
- Rated range: ~ 1999

The point values will be 100-200-300-400-500-600.

We are looking forward to your participation!

Love atcoder beginner contest

Reminder: Contest starts in 10 mins!

Are there any other contest on atcoder for beginner? ABC are very good but aren't that frequeant and i tried AGC and i shouldn't have... so any other recommendation

The ARC's should be doable, they're gonna be frequent again soon.

deleted

Inclusion Exclusion. Number of ways that we can pick any value from 0 to 9 = 10^N . Number of ways that 0 doesnt occur = number of ways to pick any value from 1 to 9 = 9^N. By same logic, number of ways that 9 doesn't occur anywhere = 9^N Number of ways that both 0 or 9 don't occur = 8^N Hence answer is 10^N — 9^N — 9^N + 8^N . Use mods wherever required

mnist why u added 8^N instead of subtracting it from 10^N

its because in both the the 0 and 9 case, the case of both 0 and 9 is considered and thus has been subtracted twice instead of just once.

got it now....thanks bruh :)

when you take [0,8] by missing 9 and [1,9] by missing 0 — you double counted and get minused the value [1,8] twice that's why add back 8^N

I tried (nC2)*2*10^(n-2). nC2 to choose any two places for 0 and 9. 2 because {0,9} has two permutations. 10^(n-2) to fill remaining places with 0-9. I am still not sure why this approach is wrong. Could someone point out?

You are counting duplicates

for example,

when 0,9 is fixed like this 0_9 and like this 09_ in both cases you are counting 099

I also tried this same approach. Don't know what went missing :')

I am not understanding the line "Number of ways that both 0

or9 don't occur = 8^N.".Here we are finding the number of wayes without 0 and 9 so i think there may be 0 and 9. Can you help me to find out my error?Yes it's and, anyways understable from the calculation.

I thought that we could first choose one 0 in the sequence so Nc1 ways to do that and then choose one position for 9 in N-1c1 ways and then for remaining N-2 positions each has ten options so 10^(n-2) ways so my final answer was n*(n-1)*10^(n-2) ..My answer got WA.Can anyone please point out what is wrong in my reasoning.Any help or feedback is appreciated

Why so many downvotes he is just asking the approach didn't say anything wrong or asked anything wrong U can just use Inclusion exclusion

He asked during contest which is not allowed

I was just try to be first so i can get reply so i can get reply quickly without having intention to get solve yes i can give these immediate after contest but i have a online class during the contest for that i gave that during the contest to be honest i have no intention to get a solve from here during contest

The issue is that even if you weren't going to cheat yourself, if someone answered you during contest, anyone else could have seen the answer too, which wouldn't have been fair.

In this question you need to find all sequences so:

(both conditions must be met)

Let's iterate k — number of positions i such that Ai = 0/9

Number of ways to create "good sequences" with 0/9 is

2^k — 2(on every position you have 2 ways to put 0/9) (-2 because you don't need to create sequences 00..000 and 99..999)Let's count

C(n, k)— the number of ways to select a set of i positions from n positions, where we want to put 0/9Finally we want to fill the remaining positions with number 1..8. It is equal to

8^(n — i)And final number of ways for some k equal to

C(n, k) * (2^k — 2) * (8^(n — i))P.S. more details about Binomial Coefficients you can read here Binomial Coefficients

Any ideas on how to do F? If the frequency of any number in first array is greater than n — (frequency of that element in the second array) than answer is "No" else answer exists.

I made a logic that the maximum frequent number in the first array will get the least frequent number from the second array. If there are multiple numbers with the same frequency then the smallest number with the smallest frequency goes to the largest number with the largest frequency. I was not quite able to implement it.

Contest has not finished yet

I understand, I thought there's just 1 minute left which won't really make any difference. One cannot implement it that quick. Sorry though I should have been a bit more patient

Solved E but coudnt solve C and D :(.

approach for e

There are only two cases to consider,

if we only consider results such that Xi <= Xj.By breaking it down into these cases, I have gotten rid of the absolute value function making it much easier to reason about the distances.

So, we simply pick points with minimum and maximum x+y, and compute the distance.

Then pick points with minimum and maximum x-y, and compute the distance.

One of those two distances is your maximum.This can be done in O(n).My submission

Copy-pasted from: stackoverflow Why can't you provide the link directly instead of copy pasting the text XD

So people dont have to visit stackoverflow and look for answer.

its simple as you prob. dont want to jump from one site to another for a simple reason ,so posting a simple basic explanantion is not that big of issue

The manhattan distance of two points is the distance of the parallel diagonals of these points.

The points at a distance x from a given point p form a diamond.

How to do D?

Simple DP. very similar to this

Just coins here are {3, 4, 5 .. N}

But using coin change dp method does not give us the different permutations right? Like for sum = 7. the answer = 3 ( {3,4} , {4,3} , {7} ). But dp gives us only answer = 2 ( {3,4} , {7} ). Please correct me if I'm wrong

Imagine filling DP array for n = 7 by this code:

Spoilerwhy 4+3 gets considered? answer: imagine

`sum = 7, coin = 3`

the number of ways to get sum 7-3=4 will be added to dp[7]Hope you understood

Awesome! I understood now...thanks for the efforts :)

wow !! nice idea..

can you please describe how can we implement it by top-down dp approach?

I have implemented this but it isn't correct.

CodeUPD: https://codeforces.com/blog/entry/82566?#comment-697474

Thanks.

As we cant take 1's and 2's, our number of ways to form i is : dp[i] = dp[i-3] + dp[i-4] + ....

SnipI did it using Combinatorics, Stars and bars. Divide the number into x sums, (1 to s/3) and for each, find all possible combinations, by keeping each of it exactly 3 initially.

I also did the same,this approach is bit intuitive as this revises the P&C concepts :)

Yeah, but dp would be a lot easier.

Could you elaborate more on this approach?

Sure.

Firstly let's notice that we can only have sequences of size from 1 to s/3. (because each one should be at least 3)

Then, let's call these "number of groups" as x,

example: For s=9, we have,

x=1, {3} ; (with 6 still remaining)

x=2, {3,3} (with 3 still remaining)

x=3, {3,3,3} (with 0 remaining)

Now, we can distribute the remaining sum, for each x, among the x groups, in (n+r-1)C(r-1) ways,

This is, distributing n identical objects among r people (stars and bars).

So iterating over each x from 1 to s/3 by calculating the combination for each iteration, it's easy to find the answer.

Submission

thank u a lot

It’s just a simple coin change dp problem.

How to solve E? I tried to find smallest and largest points on x and y axis(total 8 points) and found maximum among them.

https://stackoverflow.com/questions/8006697/finding-maximum-distance-between-x-y-coordinates

Thanks! Completely forgot to break mod, went in the wrong direction. :(

I removed the mod (absolute value) by considering 4 cases.

case 1 : xi > xj and yi > yj

we need to find now max(xi-xj + yi-yj) = max((xi+yi)-(xj+yj)) = max(xi+yi)-min(xj+yj)

for other cases I multiplied coords by -1 and solved by case 1 again

case 2 : multiply all x by -1

case 3 : multiply all y by -1

case 4 : multiply all x and y by -1

Manhattan distance between two points is: |$$$x$$$

_{a}— $$$x$$$_{b}| + |$$$y$$$_{a}— $$$y$$$_{b}|.Now if can be evaluated in 4 ways:

$$$dist$$$ = ($$$x$$$

_{a}— $$$x$$$_{b}) + ($$$y$$$_{a}— $$$y$$$_{b})$$$dist$$$ = ($$$x$$$

_{a}— $$$x$$$_{b}) — ($$$y$$$_{a}— $$$y$$$_{b}).$$$dist$$$ = -($$$x$$$

_{a}— $$$x$$$_{b}) + ($$$y$$$_{a}— $$$y$$$_{b}).$$$dist$$$ = -($$$x$$$

_{a}— $$$x$$$_{b}) — ($$$y$$$_{a}— $$$y$$$_{b}).Now if you simplify them a little then the distance can be one of the two below:

1) $$$dist$$$ = ($$$x$$$

_{a}+ $$$y$$$_{a}) — ($$$x$$$_{b}+ $$$y$$$_{b})2) $$$dist$$$ = (-$$$x$$$

_{a}+ $$$y$$$_{a}) + ($$$x$$$_{b}— $$$y$$$_{b})Now the distance will be maximum from above expressions.

submission link

Wow, nice breif explanation.

Thanks. Here is shorter implementation I did with your help.

Oh, i see. It's really concise. 80% of time i overkill (: problems.

https://leetcode.com/problems/maximum-of-absolute-value-expression/discuss/339968/JavaC++Python-Maximum-Manhattan-Distance

the approach to solve was same as this question.

you were just given distance function instead of f(i, j)

codeAlternatively, you can calculate the convex hull of the given points and brute force the vertices of the polygon.

This will TLE if to much points are on the convex hull, it could be all.

Yes, you're right. However, I got an Accepted verdict. I guess the test cases are not strong enough.

Pure Mathematics Contest!!

A and B was no math ;)

and D was also of dp ..No math

You could do it with stars and bars

Maybe E is just a too well known problem, but F solution was more obvious to me than E solution.

how to solve F ? It seems easy at first.

how to solve F ? Think about 70 mins and more but still fail to figure

What was the approach for F?

Can you please explain your solution to E, I don't know which one test case was failing, I got 17 AC and 1 WA

This might help https://atcoder.jp/contests/abc178/submissions/16705345 I sorted the points considering points to be equal if they lie in the same circle centered at the origin

Put three comparators. one comapring x1-y1 other with x1+y1 one with y1-x1 Take the max of difference of the end terms and max of three would be the answer.

Here is my take on E,

ans=(x2-x1)+abs(y2-y1) ,x2>x1

Now we have two options for answer

ans1=(x2+y2)-(x1+y1) ==== max(x+y) -min(x+y) for optimal answer

ans2=(x2-y2)-(x1-y1) ==== max(x-y) -min(x-y) for optimal answer.

Final answer would be the max of two.

Hey can you please suggest which one test case is failing for this solution

Try this

2

392850397 652442710

430284160 923935256

Correct answer:308926309

Your answer:493651095

Sorry for prvious wrong case.I updated it. Please check it,

Sorry for my bad English.

I have a doubt. The two cases are coming from assumptions, like case 1 comes from y2 >= y1. So how is it that when we calcuate the answer, we don't check for these restrictions and still get the correct answer. Can't it be that for i and j which gives max(xi + yi) — min(xj + yj), yi is actually less than yj which makes that pair invalid?

Actually I have this same doubt and I have used the same assumptions that invalid pairs wont affect the best answer for the problem and got a AC, I was really trying to find a intuitive or mathematical proof for the same but havent got one, I would really appreciate if someone comes up with a proof for the same.

Here we are only concerned about the largest value

Lets get our definitions clear first

option1 = (xj-xi) + (yj-yi) for yj>=yi

option2 = (xj-xi) + (yi-yj) for yi>=yj

Now for yj>=yi we can never have option2>option1 and vice versa.

So though we are considering some invalid pairs our answer is never going to be affected

Nice. Thanks!

Can you check why I am getting WA on F. here is submission

More than being well known, E is pretty easily searchable I think.

MathCoder literally :V

What are the solutions for problems E and F?

e is well-known, u can google it

I googled couldn't find the same problem, can you please provide a link where solution of this problem is explained. like gfg or something?

here

you're welcome :))

Hi, this is my code for F problem

I am failing in 3 test cases and I couldn't figure out those TCs,

you can find my code in this link please help me

https://ideone.com/7PaZ9k

My submission link https://atcoder.jp/contests/abc178/submissions/16710181

can I get any test case for which my code is failing?

there is dropbox link to all atcoder test cases in one of the rng_58's blog

Can someone share his approach for solving F?

F solution: Reverse array B, then A will be in ascending order and B will be in descending order. If we now look at positions where A[i] == B[i], they will form a segment [l, r], and will all have equal element A[i] == B[i] == C. Then you just need to swap all positions i from this segment with such positions j that A[j] != C and B[j] != C. And if there are not enough such positions j then I think it's not hard to prove that reordering is not possible at all.

I think reordering is not possible only in the case where lets say x occurs m times in A and k times in B and both k>=(n+1)/2 and m>=(n+1)/2. By pigeonhole principle!

No consider A = [1 1 1 1 2] B = [1 1 2 3 4], maybe we can say that frequency of some element must obey $$$f_A + f_B <= n$$$ ...

Ya I missed it thanks , Amazing F!

How to prove that answer is always Yes when every f_A + f_B <= n

That is shown from answer of madn. When A is ascending and B is descending, and lets say $$$f_A + f_B = N$$$ for boundary condition, maximum intersection is possible if both A and B have near equal frequencies, intersection will be of length $$$\lfloor N/2 \rfloor$$$. So we have adequate numbers ($$$\lceil N/2\rceil$$$) to put in this positions of overlap, so answer is always possible

oh,actually its very obvious...what am i thinking...Thanks!

My code https://atcoder.jp/contests/abc178/submissions/16703079

great idea man

This is the best solution I have ever read!!

Yeah I find it very simple and easy to see correctness. In my contest I was just wondering why they gave A, B in sorted order, maybe it was hint toward this solution

Yes, key insight is that only a[i] = b[i] = c will be the only value, and form an interval

Can you prove it ?

if there are two different value that a[i] = b[i], let a[i] = b[i] = c, a[i+1] = b[i+1] = d, and c != d

a[i] < a[i+1], so d > c

b[i] > b[i+1], so d < c

contradiction

Amazing!!!!!!

I am not claiming following is correct but I also cannot see why it is wrong. It must be wrong because it gives runtime error. The process was:

I think it runs into dead end where it no longer finds a suitable pair, but why it happens? Thanks!

i think thats the easiest code to read and understand.

I thought of the exact same idea but it gave WA.

check this: https://atcoder.jp/contests/abc178/submissions/16969818

Can you help me to find what is wrong? I think this method isn't right.

Man problem C ruined the round for me!:) Can anyone please explain the approach for it?

10^n-2*9^n+8^n Inclusion exclusion

Wow, thanks! Very interesting!

Can you please help me to understand what wrong with this approach.

Take two numbers and assign 0 and 9 to them , ways=n*(n-1)

remaining (n-2) numbers can be arranged in 10^(n-2) ways making

ans=n*(n-1)*10^(n-2)

Thanks.

The 10 ^(n-2) replicates in many cases when n>=3 see lets say you place 0,9 at 1 2 so possible values are 0 9 1, .....0 9 9

SImilarly suppose in another permutation you fixed 0 9 at position 1 3 and other 10 values accordingly at remaining position .Observe both the vectors carefully.

0 1 9........0 9 9.

You can clearly see 0 9 9 appearing common in both thus WA!

There will be repeation, like in your solution you will count (2 1 9 2) twice.

Intuition behind this:

There are $$$x$$$ ways of choosing which of $$$x$$$ numbers to place in a spot, each choice is independent of each other, so for $$$n$$$ places there are $$$x^{n}$$$ ways.

Number of ways of placing numbers in the range $$$[0, 9]$$$ -> $$$10^{n}$$$.

Number of ways of placing numbers in the range $$$[0, 8]$$$ or $$$[1, 9]$$$, that is the number of ways of generating an array that is guaranteed to be missing either a $$$0$$$ or an $$$8$$$ -> $$$9^{n}$$$ each.

But note that we have double counted the number of ways to place $$$[1, 8]$$$ (both missing) while removing the bad array count, so add back the number of ways of doing that which is $$$8^{n}$$$.

You can use DP for accounting for double counting

Thanks! I tried to do a dp approach but i didn't manage to solve it during the contest! In my opinion, D was easier than C.

Exactly! I was stumped by C as well. Here's my DP solution to C

CodeNice solution!

Please, can you explain your solution? I couldn't get it.

Answer = (total sequence with length n and all element <= 9) — (total sequences such that it contains neither 0 nor 9)

first term above is simply

`pow(10,n)`

for the second term I used inclusion exclusion:`A = Set of sequences that doesn't contain 0`

`B = Set of sequences that doesn't contain 9`

`n(A union B) = n(A) + n(B) - n(A intersection B)`

`n(A)`

=`n(B)`

=`pow(9,n)`

`n(A intersection B)`

=`pow(8,n)`

Nice explanation, thank you!

I am still confused with duplicates. Can anyone please write all the sequences if n = 3 ? Thanks in advance.

answer for 3 is 54 which is too big to enumerate manually. But if you are asking why

`pow(10,n-2)*C(n,2)`

is not right answer, I'll try to show duplicates. Suppose n = 3 and you decide to fix first and third position one of the sequence you get is`009`

which is repeated when you fix second and third position, thus counting same sequence twice.thanks, it helped.

nice explanation!!

deleted

what's wrong in my solution of F? getting wrong answers on 6/50 test cases?

https://atcoder.jp/contests/abc178/submissions/16721049

UPD: I did some changes in my above code but still it's giving wrong answer on 3 test-cases:(https://atcoder.jp/contests/abc178/submissions/16724818Anyways to do C or D without DP ?

C is simple inclusion — exclusion. My one line code :

cout << sub(add(power(10, n), power(8, n)), add(power(9, n), power(9, n)));

10^n-2*9^n+8^n

D could be solve by brute force the number of elements in the sequence and the do a star-and-bar-like counting trick.

CodeAK. Well explanation brother.

I did D using combinatorics. It is the modified version of distributing n balls into r urns such that each urn contains at least 1 ball.

Solution of standard problem: (n-1)C(r-1).

Here problem can be reformulated as if you can use any number of urns (because the length of sequence can vary) and each urn should contain at least 3 balls.

Solution: summation of (n-1-2*i)C(i-1) for all i, where i is the number of urns and n is the sum we want. The term -2*i because we first put 2 balls in every urn so that problem is now transformed into a standard one containing at least 1 balls, for each i.

https://atcoder.jp/contests/abc178/submissions/16705277

For problem C: I'm thinking like this, place 0 and 9 in any two places (nC2) then other n-2 places will have 10 possibilities hence (10^(n-2)). So overall answer is nC2 *(10^(n-2)).

Can anyone say what fact am i assuming wrong/missing. Thanks :)

nC2 * 10^(n-2) will duplicate

can you give correct combination formula for this problem ?

I used dp https://atcoder.jp/contests/abc178/submissions/16709001 Here,

dp[n][0][0] means sequence of length n and 0 zeros and 0 nines,

dp[n][0][1] means sequence of length n and 0 zeros and atleast 1 nine,

dp[n][1][0] means sequence of length n and atleast 1 zero and 0 nines,

dp[n][1][1] means sequence of length n and atleast 1 zero and atleast 1 nines

thanks

so the 0's and 9's are getting duplicated?

In cases when you put 9, 0 like this: ab90 09cd sequences may coincide, when a = 0, b = 9, c = 9, d = 0: 0990 0990

There are repetitions in your formula. For n = 4 the answer is 974. Check with your submission

I also thought of it at first but if you think about it cleary it will have places with repetitive counting.

for example, consider n = 5 and you choose places 0 9 8 0 9 will be a valid sequence but it will be counted twice (as nc2 will include 1&2 and 4&5 ).

in this way you will count some sequences more than once

lets say that n=3

one of possible way is to put 10 as a first element and 9 as a last element and lest say that the element in middle is 10 so the sequence is 10 10 9

another possible way is to put 10 as a second element and 9 as a last element and lest say that the first element is 10 so the sequence is 10 10 9 again and we already count it

atdynamicprogrammer

lose again because of my speed

Was trying to do D , using DP but gave TLE... https://atcoder.jp/contests/abc178/submissions/16721075 Can someone pls help what is wrong

Index is not necessary in the state.

Damn I messed up !!!, after removing the index state got AC....Thanx

Editorial:

Acout<<x^1<<endl;

BYou can see that x must equal to a or b,y must equal to c or d.

CPrinciple of inclusion-exclusion can work.

DSimple dp.

$$$dp_0=1$$$

$$$dp_{i}$$$ means the number of the sequences that sum is i.

EFLet cnt[i] = how many i in b+how many i in a.

If for some i that cnt[i]>2*n,the answer is "NO". Otherwise the answer is "YES".

You can construct it as follow.

We can find all the positions that a[i]=b[i],and store them in one "vector".

We can let count[i] = the number of times that i appear in the vector.

If for all i that count[i]*2<=vector.size(),we can construct them like this probelm:https://codeforces.com/contest/1381/problem/C

Otherwis we can swap the most appearing elements with other positions that are not in the vector,to satisfy the condition above.

All codes:https://atcoder.jp/contests/abc178/submissions?f.User=Gary

In $$$C$$$ constraints are smaller, so if someone has weak combinatorics(like me) then dp is more intuitive imo. here is my sol using dp:

SpoilerYeah, dp can also work!

Hey, in problem D, I think you also need to add 1 to dp[i].

Fixed!

Is there a case to be handled for mod of negative numbers in C ?

if a<0 then use mod-abs(a) will work.

Don't forget to add mod when you are subtracting two numbers under mod.

I just have learnt a very valueable lesson.

we can do this :

`ans = (a-b+MOD)%MOD;`

How does it work?Can you please explain or give me any reference?

I'll leave that for you to figure out :)

And.. AtCoder becomes MathCoder.

Lol

c explanation ?

Total Number: 10^n

Without digit '9': 9^n

Without digit '0': 9^n

Without both: 8^n

Final Result: 10^n — 9^n — 9^n + 8^n

:O i feel dissapointed about myself

Too simple to understand!!

Total numbers you can make using all digits (10^n) — numbers with all digits but 9 (9^n) — all but 0 digits (9^n) + all but 0 and 9 (8^n) (because they were deleted twice);

Total possible sequences = 10^N. Sequences with no 0 = 9^N. Sequences with no 9 = 9^N.

Subtract

sequences with no 0andsequences with no 9from total possible combinations. But observe after doing so you will removesequences with no 0 and no 9 twice. So add sequences with no 0 and no 9Sequences with no 0 and no 9 = 8^N.

Ans = 10^N — 2*9^N + 8^N.

Maybe this is an off-topic.

I think Atcoder should show counts of solve for each problem in dashboard/tasklist(like CF). I know it is in the standings page, but viewing standings itself some number of times is well-disgusting.

I have tried to solve F using bipartite graph matching but it failed just in test 16 :)))

WA or TLE?

TLE, actually I was expecting it! I was checking a random edge to find a match each time.

I don't think it can work , the nodes and edges are too many !

It worked :)) submmision #16726993

I guess test-cases are not that much strong.

of course it will :))))

maybe you can try using a segment tree to build the graph.

But the nodes are too many :)

yeah, it is.

I found my solution for F is realy complex(Though it is right)!!

Can you share your idea for F?

Resulting ordering of B array is some cyclic shift of itself. (If no shifting is possible, no solution).

Thank you for your reply:)

But can you prove it?And how to achieve it?

I did not scratch a full proof yet.

For each element in B, we find what shifts are not possible. This can be done using set/segment tree.

Can you share your code.

Here

my F solution

Whenever there exists index i where A[i] is equal to B[i], shift array B one time. Loop until there are no such indices in one interation. I use two pointers to maintain the shift

Can you prove it?

Sorry, I didn't prove it during contest. I generate all possible cases for small N and it seemed work.

alvinvaja , I really like the quality of your profile picture. May I know which camera was used?

It wasn't mine. Got it from pinterest

I think mine is similar where I take a number, x, in both a and b and if the start of b's interval of x isn't over the end of a's interval of x, I calculate how many shifts it would take to make b's start over a's end of x's interval (if b's start isn't already over a's end) and take the maximum shift for all numbers that are both in a and b. Maybe there's a counterexample but I couldn't find one.

After the greatest shift needed is done, for that number x in b that needed the largest shift, all the numbers < x in b should be assigned to a number bigger than it in a and all the numbers >= x in b are shifted up to match numbers bigger than it in a until n, and then they match to lower numbers that are <= x because the shift circles it to the beginning of a. Unless the array is impossible I don't see how any numbers can conflict yet.

I used the same approach, but I also can’t prove it. But I know how to achieve it. Each position of B can be expressed as "not shifting in range [l, r]" which is equivalent to

~[x >= l and x <= r] = (~(x >= l) or (x >= r + 1)).

Now the requirement is just AND product of all boolean clauses which is nothing, but 2-SAT. I thought that they put some problems to practice Atcoder library, turned out that I overcomplicated things up. lol (They didn’t add library yet, so I got compile error.) code

you can see this solution I just tried to solve F using this and got AC. Logic is simple as well as code. Here is my code for this approach

Thanks:)

Can someone check why I'm getting WA on C . here is my submission

Result of a1 — a2 + b1 may be negative, so just add MOD to it before taking remainder

Thanks, it worked!

"ll ans = (a1 — a2 + b1)%M" should be ll ans = (a1 — a2 + b1 + M)%M;

`ll ans = (a1 - a2 + b1 + MOD)%M;`

Just 2 min short to get AC in problem F. Nice F anyway

What was your idea behind F? Was it greedy?

The idea is tough to explain for me. I try. 1.A is in ascending order and B is in ascending order. So, just try to pick up elements like two pointer from start to end. Then, pick the unpicked elements too. Now, check the answer. If its OK then print.

2. Otherwise, reverse B once again and go like 1. Check the answer. If OK then print it, else there is no answer.

A Combinatorics solution for D — RedistributionRecall that, by the famous Stars and Bars Theorem, the number of

non-negative integralsolutions ofis

Let us denote this value as $$$stars(n, r)$$$. Clearly, the value is 0 if $$$n$$$ is negative.

Moreover, if we add a constraint that $$$x_i \geq t$$$, then, we can replace $$$x_i$$$ by $$$y_i$$$, where

This transforms the problem to finding the non-negative integral solutions of

Clearly, the number of valid solutions is $$$stars(n - r \cdot t, \ r)$$$

So, for each $$$r$$$ from 1 to $$$n$$$, we sum up the values of $$$stars(n - 3 \cdot r, \ r)$$$.

DP Solution for Problem C — Ubiquity

Codecan you Please explain the states and transition.

A great contest.

Unfortunately,I nealy AK(AC 5 problems),but I'm in trouble with the sixth problem.

All of them are Math problems(.(xiao xue ao shu in Chinese)

And there's Chinese in English task:配点。

Hope the abc will be better!

：）

So I did a randomized solution for F. I try three ways:

Reverse $$$B$$$.

Random shuffle $$$B$$$ for $$$T$$$ times.

Rotate $$$B$$$ randomly for $$$T$$$ times.

$$$T = 20$$$ is sufficient to get AC, and it runs in

61 ms. Of course, I have no proof (well not even an intuition) for this. :DSo, I'll be glad if someone can hack this solution or give some proof/intuition for it.

Assume odd n, and n/2 '1' in a[], and n-n/2 '1' in b. Then we need to rotate exactly n/2 positions.

Reversing B works in this case.

Ok, let the sympobls be '2', not '1'. And a '1' in first position of both arrays. Then the reverse of b[] does not work, but still we need to rotate exactly n/2 positions.

Yeah it prints No for this case.

gengive us editorial please..

How to prove that on F, if a solution exists, one of the cyclic shifts of b is also a solution?

I wrote my solution based on it and it passed.

Isnt there a lot of cyclic shifts possible?

My Unofficial tutorial (A-F) for this contest.

And the Chinese version.

What is the codeforces problem similar to F?

1381C - Mastermind

Just a thought: How to solve F if the elements are unsorted in both the arrays?

You can look at my solution, which does not depend on whether the arrays are sorted or not.

You can always sort them and use your solution for the sorted input...

Can somebody please help me find

the only test casethat is failing for my solution of problem E?Changing the initial value of your variable ans1 to LLONG_MIN should solve your problem, because it's not necessary that x+y or x-y is always greater than -1.

Thanks dude, Another stupid mistake

What's wrong with my solution of D?

I used dp, $$$dp_{ij}$$$ means the sequence‘s length is $$$i$$$ and its sum is $$$j$$$.

SpoilerThe elements of the sequence can be arbitrary positive integers rather than digits.

Yes, I get it! Thank you so much!

Can be simplified using a 1D dp array. Starting from

`i=4`

,`dp[i]=dp[i-1]+dp[i-3]`

and then take the mod.Nice solution:D

Why do you have the problem whose solution is available in CPH? The most popular book on CP, talking about problem E.

For atcoder editorial I search always on codeforces.lol.. xD..

Sry for a newbie question, how to we see the test cases at atcoder???

https://codeforces.com/blog/entry/46389

This page might not updated for a while... There are no test cases for last or last-to-last ABC

How to solve E??

Can someone Tell me the approach for E ?

Can someone plz tell me why my code for task C is failing for 5 test cases?

UPD : Error found The error is, it should be replaced with

ll res = (x- 2*y + z + 2*mod ) % mod;I guess you forgot to take mod from the res at the end

I have changed it after you said, but still those same 5 test cases are failing. I'll update the latest code in my comment. Can you plz look into it again. Thanks!

Since it is c++, mod can also give negative result, you need to handle this

Thank you so much, sol accepted!

I got

wrong answerin problem E my approach is : my approach is getting the distance between each point and point(0,0) then sort these distances ascending and the answer will be the distance between the point which has the longest distance and the point which has the smallest distance. May anyone tells me why this approach fails.Consider (1,1),(100,1),(1,2).Here (1,1) is at min from (0,0) and (100,1) is at max from (0,0) so,according to you, answer should be |(100-1)|+|(1-1)|=99 but the point (100,1) and (1,2) gives |(100-1)|+|(1-2)|=100. There are lots of such points.

F can be solved by maintaining an invariant as follows:

Hint1Let $$$count(x)$$$ denote the number of occurrences of $$$x$$$ in $$$B$$$ + that in $$$A$$$.

If there's an $$$x$$$ where $$$count(x) > n$$$, then there is no solution. Else, there is.

We need to construct a solution iteratively while maintaining that invariant, that there's no $$$x$$$ where $$$count(x)$$$ exceeds $$$n$$$. Note that $$$n$$$ is decremented each iteration, and also $$$count(x)$$$ represents the count of $$$x$$$ in the remaining elements in $$$A$$$ and $$$B$$$.

Hint2Let us call $$$x$$$ critical, if $$$count(x) = n$$$.

There can be at most 2 critical numbers. Let us assume there's only one, and it'll be easy to see how the same idea solves the other case.

After an iteration, the invariant will be broken

iffthere's $$$x$$$ s.t. $$$count(x)$$$ exceeds $$$n$$$, hence in the previous iteration $$$count(x)$$$ was $$$n$$$ (i.e. $$$x$$$ was a critical number) and its $$$count$$$ didn't change.Hint3Hence, if the current iteration is $$$i$$$ and there's a critical number, then assign it to $$$b_i$$$, so that its count decrements in the next iteration and the invariant is maintained. But what if that critical number is equal to $$$A_i$$$? Then actually its count will be decremented too.

So either way, you can maintain the invariant. If there's no critical element, just put any number and the invariant will be maintained anyway.

Thanks to Yousef_Salama for showing me this idea, you can check his submission.

I have written unofficial English editorial, you can find it at: editorial

great!!

For F, I am reversing B and then checking if B[i]=A[i]. If so at index=p, I am reversing the array from B[0] to B[p] and from B[p+1] to B[n-1]. This works as for the reversed B, all numbers left of B[p] will be >= B[p] and to right of B[p] will be <= B[p]. Whereas in A, all numbers left of A[p] will be <= A[p] and to right of A[p] will be >= A[p]. Theoretically it should work but I am getting wrong answer in 3 of the 60 test cases and I cant figure out why. Can anyone point out the fault in my algo? Submission: https://atcoder.jp/contests/abc178/submissions/16733148

Check this test case:

Thank you!

Problem F is

EXACTLYthe same with Perm Matrix (just with smaller constraints).I've seen this problem,so I got an unexpected high rank.

My atcoder account: sjcsjcsjc

It's nothing new at atcoder, many time I got the exact questions which I had previously solved or seen somewhere. Problem E was also copied in the same contest.

Problem E is just a standard problem of changing Manhattan distance to Chebyshev distance.

Problems using this technique are everywhere.

How can I solve D using top-down?

https://atcoder.jp/contests/abc178/submissions/16705977

I don't understand why dp[0]= 1 and also why ret+=f(x-i) ?

if S = 0, there is only one sequence i.e. empty sequence which sum to zero, hence dp[0] = 1. ret += f(x-i) is simply there to implement the formula above.

Hi can someone please tell why my submission for problem C is working in atcoder c++ compiler but not in my local compiler.

I have tried recursive dp solution for this problem and it is working well . But in some large input example for n = 869121 it gives segmentation fault in local compiler as well as in online cpp compiler but works well upon submitting can some please tell why this happening ? Can this be due to memory limit or recursion depth ? please help

Can someone point out the error in this solution of F? Basically to print array B after validation, I reverse it (that is, descending order) then there will be an interval where there might be some common elements. I m swapping those with elements on either side since on left, elements will be greater and on right will be smaller. There is just one test case which is giving WA here which I cannot crack.