Codeforces Round 914 (Div. 2)

11 месяцев назад, # |

+11

C>D

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iiand

11 месяцев назад, # |

+17

Cool problems, especially $$$C$$$(with idea that we must check only $$$K \leq 2$$$) and $$$D$$$.

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phukaoz

11 месяцев назад, # ^ |

I should do D first T-T.

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tgp07

11 месяцев назад, # |

Cool problems and cool starting time

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Something.

11 месяцев назад, # |

A > C > B

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ismoilmirzouz

11 месяцев назад, # ^ |

for me C>A>B

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blaze

11 месяцев назад, # |

Found the error in D1 one second after contest ended. Nice!

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NovusStellachan

11 месяцев назад, # |

Can anyone tell me if this works for F? I ran out of time to implement in contest.

First we build a directed graph $$$G$$$ whose vertices represent paths in the tree as follows:

For each vertex $$$u$$$ and number $$$0 \leq i \leq 20$$$ we construct a vertex corresponding to the path between $$$u$$$ and its $$$2^i-1$$$th ancestor along the path (LCA stuff). We draw directed edges from the "vertex" $$$[u, p^{2^i}(u))$$$ to vertices $$$[u, p^{2^{i-1}}(u))$$$ and $$$[p^{2^{i-1}}(u), p^{2^{i}-1}(u))$$$.

This directed graph $$$G$$$ has $$$\leq 20n$$$ vertices, each of which has at exactly two edges going outward. So the graph has at most $$$40n$$$ edges. Create another copy of such a graph, but now with edge directions reversed, call it $$$G'$$$. Finally consider the $$$1$$$-vertex paths in both graphs $$$G, G'$$$ and merge them (they are one single vertex now).

This new graph $$$G+G'$$$ has $$$\leq 40n$$$ vertices and $$$\leq 80n$$$ edges. Now for each of the $$$m$$$ conditions we do the following. Suppose that the condition is of the first kind, the second kind is analogous. Split the paths from $$$[a, c)$$$ and $$$(c, b]$$$ into at most $$$40$$$ vertices of $$$G$$$. Draw a directed edge from vertex $$$c$$$ to each of these $$$40$$$ vertices of $$$G$$$ (in the opposite direction if $$$G'$$$ were considered).

Now just check if the $$$G+G'$$$ graph has a topological sort. If it does, then the order of $$$1, 2, \dots, n$$$ in that is the answer, otherwise there is no answer at all. Time complexity of above is $$$\mathcal O((n+m)\log n)$$$

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willy108

11 месяцев назад, # ^ |

Yes, this ought to work.

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NovusStellachan

11 месяцев назад, # ^ |

ahghghgh thank you for confirming, after bricking C, D1, D2 I read F in last 30 min and figured this out but hands too slow ugugugughghh

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vitosevskich

11 месяцев назад, # |

← Rev. 2 →

+14

What? D1 is actually EJOI 2020 Day 1 Task "Exam" Subtask 6.

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shenzihan

11 месяцев назад, # |

+15

datastructureforces

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Mayank_Pushpjeet

11 месяцев назад, # |

C was cool, D1, D2 were also good, crossing 1800 barrier for the 1st time after so long from being stuck and lot of demotivation.

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Scyther_07

11 месяцев назад, # ^ |

Congo man!

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Mayank_Pushpjeet

11 месяцев назад, # ^ |

Thanks man !

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eyfmharb

11 месяцев назад, # |

+26

I absolutely totally did not stare at $$$C$$$ for $$$30$$$ minutes with no ideas until I realised you can make it $$$0$$$ in $$$3$$$ trials.

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Red0

11 месяцев назад, # ^ |

← Rev. 2 →

I absolutely did not spend 20 minutes staring at my solution for C without realizing that it works.

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alpha1215

11 месяцев назад, # |

nice contest i enjoyed the problem thanks to authors. First time solved 4 problems

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electric_boogaloo

11 месяцев назад, # |

← Rev. 2 →

C is such a troll problem with $$$K$$$ up to $$$10^9$$$, you could have taken it a step further and not include a case with $$$K = 3$$$ in the samples. Not a bad problem though.

D was okay, just hoping I don't get FSTed.

Looks like I didn't get FSTed after all. Is there an easier way to do D2?

What I did:

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ShivanshJ

11 месяцев назад, # ^ |

Yes there is an easier way, Let $$$R(i,x)$$$ denote the smallest index $$$j$$$ such that $$$j \ge i$$$ and $$$a[j] = x$$$, similarly $$$L(i,x)$$$ denotes the largest index $$$j$$$ such that $$$j \le i $$$ and $$$a[j] = x$$$.

Now, the answer is "YES" iff for every element $$$i$$$,

1) $$$Min(b[i],b[i+1]...b[R(i,b[i])] \ge b[i]$$$

2) $$$Max(a[i],a[i+1]...a[R(i,b[i])] \le b[i]$$$

1) $$$Min(b[i],b[i-1]...b[L(i,b[i])] \ge b[i]$$$

2) $$$Max(a[i],a[i-1]...a[L(i,b[i])] \le b[i]$$$

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electric_boogaloo

11 месяцев назад, # ^ |

Yeah, this looks like a more concise way to do it. Basically, in my solution I'm checking the same conditions, just in a more roundabout way (it was heavily influenced by my brute force solution of D1)

I'm curious what is the authors intended solution (it might as well be the same as the one that you have proposed)

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11 месяцев назад, # ^ |

236555393 Though I'm not sure about the time complexity.

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11 месяцев назад, # ^ |

Hacked. But I think you can optimized it a bit and pass.

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11 месяцев назад, # ^ |

I was thinking it's time complexity to be $$$O(n^{2})$$$ in worst case.

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11 месяцев назад, # ^ |

Your solution solved my test case in about 6 seconds while most of the N^2 solution takes more than 1 minutes.

That's why I think your solution might pass if you can somehow optimized it.

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tgp07

11 месяцев назад, # ^ |

I did something similar for D2. It barely passed in time lol

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vaal

11 месяцев назад, # |

+11

Was that for only me initially the diagram for problem A was incorrect?

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vitosevskich

11 месяцев назад, # ^ |

Yes it was. They could've announced it.

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tickbird

11 месяцев назад, # |

couldn't solve c

even though found case work k >= 3, k == 1, k == 2

too unfortunate

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phsads

11 месяцев назад, # |

A is miles harder than B and C for me lol (and maybe even D1 but i didnt have enough time)

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RunAt

11 месяцев назад, # |

Wow A was unusually difficult to understand and implement.

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fernandezpalomojosue

11 месяцев назад, # |

Weak pretest on A

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bestial-42-centroids

11 месяцев назад, # |

← Rev. 2 →

Congratulations to the authors for this great round :))

As usual, here is my advice about the problems (not correlated to my performance)

I think the only downside of this round is that the gap between D2 and E was quite high but I don't know if that's a really big deal as usually E don't get that many ACs anyway.

Looking forward to compete in another of your rounds :)

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oogerbooger

11 месяцев назад, # |

I couldn't do problem A, what a shame.

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anil9717

11 месяцев назад, # ^ |

‘Think of rising higher. Let it be your only thought. Even if your object be not attained, the thought itself will have raised you.’ — Thirukural

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BF_OF_Priety

11 месяцев назад, # ^ |

in last div-3 I couldn't solve $$$C$$$ sometimes these things happen

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PiscesDream

11 месяцев назад, # |

Not sure you feel the same. But for me, B > A and C > D. lol

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11 месяцев назад, # ^ |

Can you give me an edge case for this solution why it is not working B

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PiscesDream

11 месяцев назад, # ^ |

In general, I will write a data generator and a brutal force baseline solution to test it. I did that sometimes in the contest (like for the C today), not a good way but the last resort to self-debug.

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11 месяцев назад, # |

how to solve c when k = 2?

i tried to take every element of a with the nearest abs(a[i] — a[j]) but it gave wa

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iiand

11 месяцев назад, # ^ |

Just check all pairs. if you have $$$f = abs(a[i] - b[j])$$$, you current answer must be $$$answer = min(answer, f, \text{first_el_greater}(f) - f, f - \text{last_el_less}(f))$$$. The last two can be calculated using binary_search = std::lower_bound. So $$$O(n^2 \log n)$$$

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11 месяцев назад, # ^ |

oh thanks alot i tried to do this in one of my submissions but i was only looking for the adj elements thanks

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amos_aidoo

11 месяцев назад, # ^ |

maybe you can get a zero if any difference already exists

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11 месяцев назад, # ^ |

i already handled this by setting the default answer to be min absolute value

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Suri429

11 месяцев назад, # ^ |

make a binary search back in array for that (a[i]-a[j]) to find best move in next run.

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https://codeforces.me/contest/1904/submission/236573798

11 месяцев назад, # ^ |

ok thanks alot

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fengyss

11 месяцев назад, # ^ |

← Rev. 2 →

before it, sort(a),then try every s=(a[i]-a[j]), after it, look for minimize(a[i]-s). every step should update answer.

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alpha1215

11 месяцев назад, # ^ |

brute force every possible |Ai — Aj| think of bs

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kriepiekrollie

11 месяцев назад, # |

AdamantChicken2 didn't participate :(

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fengyss

11 месяцев назад, # |

I have to say it is a little late for me, but I outdo myself. Unbelievable!!!!

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EN_SA

11 месяцев назад, # |

← Rev. 2 →

+20

You changed the order of picture of problem A and didn't send a notification. Is it because it's not statement?

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EN_SA

11 месяцев назад, # ^ |

+11

Additionally, I feel that the description of the new way of movement of the knight in statement A is inappropriate.

I think we should write statement even someone who doesn't know how the knight in the original chess moves can understand how the knight moves with that statement alone.

I think the current statement is open to misunderstanding.

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RD_TheCoder

11 месяцев назад, # ^ |

I left the contest for this reason.. After some min i refreshed and saw the change (:

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Sokol080808

11 месяцев назад, # |

+24

oursaco https://ejoi2020.ge/static/assets/Day1/Problems/Exam.pdf Problem D is a subgroup of this task

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oursaco

11 месяцев назад, # ^ |

Sorry, we were not aware of this :(

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g2-g4

11 месяцев назад, # |

amazing problems! but E and F is too hard for div. 2 (

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shviv1

11 месяцев назад, # |

← Rev. 2 →

Nice problemset

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helpmeimprpve

11 месяцев назад, # |

is there a hack phase ?

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EN_SA

11 месяцев назад, # ^ |

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Ladamalina

11 месяцев назад, # |

Solved D1+D2 just 5 seconds after final :'(((

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arash.

11 месяцев назад, # |

E is a really standard problem

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ShivanshJ

11 месяцев назад, # ^ |

Can you give a slight hint?

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arash.

11 месяцев назад, # ^ |

+23

Solve it offline and do a dfs.

When you are in vertex v try to have a segment tree maintaining the distances of other vertices from v.

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BrAin

11 месяцев назад, # |

Anyone plz give the idea of A

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anil9717

11 месяцев назад, # ^ |

Let Sq be set of positions from where queen(xq,yq) can be attacked. (xq+a,xq-b) is one such postion. Let Sk be set of positions from where king(xk,yk) can be attacked.

Find the size of intersection set of Sq and Sk.

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ma_da_fa_ka

11 месяцев назад, # ^ |

True, couldn't find better way to implement this

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vstiff

11 месяцев назад, # ^ |

https://codeforces.me/contest/1904/submission/236576012 :)

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cf_my_gf

11 месяцев назад, # |

why my solution for B gets wrong

https://codeforces.me/contest/1904/submission/236570576

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Hasib.Nabid

11 месяцев назад, # |

Couldn’t solve A pajn. Nice problemset tho

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kyuukyuusha

11 месяцев назад, # |

+11

Can someone hack this $$$O(n^2)$$$ solution for D2? https://codeforces.me/contest/1904/submission/236575342

I feel like it shouldn't work.

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kriepiekrollie

11 месяцев назад, # ^ |

I hacked it

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LaughingBuddha

11 месяцев назад, # ^ |

How can we hack any solution now?

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aureliony

11 месяцев назад, # ^ |

← Rev. 2 →

Unfortunately they made uphacking available only for div1 participants (1900+)

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mnasser02

11 месяцев назад, # |

Spoiler

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ALnQ417

11 месяцев назад, # ^ |

+27

Skill issue tbh

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Charararar

11 месяцев назад, # ^ |

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avighnakc

11 месяцев назад, # ^ |

Np, happens

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Prajwalrayal

11 месяцев назад, # |

This failed on system testing Submission can anyone tell me what is the mistake? n^2logn solutions were not meant to pass for this problem or something else ?

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https://codeforces.me/contest/1904/submission/236581716

11 месяцев назад, # ^ |

just changing $$$j$$$ from $$$i+1$$$ to $$$n$$$ instead of $$$0$$$ to $$$n$$$ in case $$$k=2$$$. your code runs in 1400 ms. 236577461

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Prajwalrayal

11 месяцев назад, # ^ |

Bruh :) this small mistake took me from Rank 1700 to 2800, Thanks!!

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LaughingBuddha

11 месяцев назад, # |

System testing completed for this contest? Any idea anyone?

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great_fortune

11 месяцев назад, # |

236573136 I believe that my D1 solution shouldn't work in D2 (tl) on max test like this:

10 9 8 7 6 5 4 3 2 1

10 10 10 10 10 10 10 10 10 10

But it gets accepted with 187/4000ms

can someone try to hack it with

200000

200000 199999...

200000 200000... pls? I have some kind of problem with hacking.

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TryingToBeEnough

11 месяцев назад, # ^ |

+10

Hacked. Really weak test data in D2.

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great_fortune

11 месяцев назад, # ^ |

Thanks TryingToBeEnough!

so....

REALLY weak D2 tests lol

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great_fortune

11 месяцев назад, # ^ |

i wonder how many n^2 "solutions" got accepted

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oursaco

11 месяцев назад, # ^ |

Sorry for weak tests. We managed to kill some optimized n^2 solutions during testing, but seems it was not enough :(

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aureliony

11 месяцев назад, # ^ |

← Rev. 2 →

My O(n^2) solution works as well, should have submitted during contest :(

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11 месяцев назад, # ^ |

Hacked. sorry

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Deep1809

11 месяцев назад, # ^ |

I had same doubt, many O(n^2) solution worked in D2, example is 236567513

Can someone please verify if it can be hacked too with these given testcases

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11 месяцев назад, # ^ |

Hacked.

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Kawaii

11 месяцев назад, # |

-6

why problem A is so hard

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Rakesh_898

11 месяцев назад, # ^ |

I think the given picture made it so hard

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biomat

11 месяцев назад, # ^ |

← Rev. 2 →

I don't think it was that hard but I really started lauging when I understood I have fallen into the well set airhead trap...

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defnotmee

11 месяцев назад, # |

Didnt particularly like the contest, too structure heavy for me. A-D were easy and E was annoying. C also had a stupid corner that wasnt in the samples and A had the wrong picture. F also looks annoying but ill check edi to see if its cool.

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Igor_Parfenov

11 месяцев назад, # ^ |

Why do you look at pictures?)

I usually don't look at samples.

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defnotmee

11 месяцев назад, # ^ |

+23

My sol was bugged so i thought i misunderstood the problem, it ended up making me understand the problem less.

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cosenza

11 месяцев назад, # ^ |

what was the corner case on C?

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Igor_Parfenov

11 месяцев назад, # ^ |

For me it was "no operations, the answer is in initial array".

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defnotmee

11 месяцев назад, # ^ |

yeah, this

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LaughingBuddha

11 месяцев назад, # |

← Rev. 2 →

Can someone hack my D1? 236559856

Pretests were damn weak it seems.

Testcase —

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eklavya_k

11 месяцев назад, # |

5 solves for the first time in a div 2

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elseecay

11 месяцев назад, # |

Why doesnt work? C

    if (k == 2)
    {
        sort(fullrng(arr));
        i64 m = numeric_limits<i64>::max();
        
        update_min(m, *std::min_element(fullrng(arr)));

        for (int i = 1; i < arr.size(); ++i)
            update_min(m, arr[i] - arr[i - 1]);

        for (int i = 0; i < arr.size(); ++i)
        {
            for (int j = i + 1; j < arr.size(); ++j)
            {
                auto d = arr[j] - arr[i];
                auto lbnd = lower_bound(fullrng(arr), d);
                update_min(m, abs(d - *lbnd));
                if (++lbnd < arr.end())
                    update_min(m, abs(d - *lbnd));
            }
        }

        solved(m);
    }

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elseecay

11 месяцев назад, # ^ |

got it, if element not present lower bound returns element that greater

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Blagoj

11 месяцев назад, # |

Surprisingly good contest! Good job!

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34z12000

11 месяцев назад, # ^ |

It was kind of boring

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Blagoj

11 месяцев назад, # ^ |

+20

Hey, anyone can have different opinions

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34z12000

11 месяцев назад, # ^ |

-10

Yes, why not

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Jhin

11 месяцев назад, # |

Not sure why everyone thought B was that easy, I solved it but it took longer than A and C.

if they didn't mess up the photos in A I would have solved it faster, I stared at the wrong picture for 10 minutes wondering if I became so stupid to not understand an A problem after not solving problems for more than 2 months

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Blagoj

11 месяцев назад, # ^ |

Maybe B wasn't that easy but you just needed one observation to solve it

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11 месяцев назад, # ^ |

Can you give me an edge case for this solution why it is not working B

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EvenToWorldFinal

11 месяцев назад, # |

Great contest! Lost some ratings tho...

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dolphin2025

11 месяцев назад, # |

Why does my submission https://codeforces.me/contest/1904/submission/236601220 pass tests? It fails when the difference between two elements is exactly an element in the array. Example:

Input:

1
2 2
1 2

Output:

troll

Expected Output:

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11 месяцев назад, # ^ |

Seems like missing test cases.

If k is 2 and there exist three elements a, b, c in the list such that a + b = c, the answer should be 0.

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dolphin2025

11 месяцев назад, # ^ |

Exactly ...

BTW, how did hack it? I tried to but couldn't find an option anywhere.

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11 месяцев назад, # ^ |

I heard that it is only open to people with rating 1900+

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11 месяцев назад, # ^ |

Can you give me an edge case for this solution why it is not working B

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11 месяцев назад, # ^ |

← Rev. 2 →

You can try this case

1
6
1 1 1 1 3 6

The correct answer will be

5 5 5 5 5 5

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