## Anti Brute Force Lock

Author: Suhendry Effendy

Let G be a (complete) graph representation of all keys where the cost of edge (u,v) denotes the minimum number of rolls needed to change from key u to key v. Finding the minimum cost to unlock all keys is equal to finding the cost of *minimum spanning tree* of G (try to review the behaviour of Prim’s Algorithm).

The minimum rolls that is needed to change digits [a_{1} a_{2} a_{3} a_{4}] into [b_{1} b_{2} b_{3} b_{4}] can be calculated by a simple formula:

note: use only positive result for each modulo operation.

The MST of G can be calculated either using Prim or Kruskal. The initial digits [0000] is not a key (unless it’s stated otherwise in the input data), so we shouldn’t include [0000] in our graph. To connect [0000] with G, we only need to find one nearest key from [0000].

Several teams misunderstood the problem as they thought the keys should be unlocked in input order while the problem has stated clearly that keys can be unlocked in any order (I think they just missed that part).

This problem was solved by 17 teams. The first team to solve this problem: **Joy** (minute 30).

- Title Page
- Problem A – Anti Brute Force Lock
- Problem B – Bonus Treasure
- Problem C – Panda Land 7: Casino Island
- Problem D – Disjoint Paths
- Problem E – Expert Enough?
- Problem F – Free Parentheses
- Problem G – Greatest K-Palindrome Substring
- Problem H – Hyper-Mod
- Problem I – ICPC Team Strategy
- Problem J – Jollybee Tournament

Hi, shouldn’t it be “Rabin-Karp” instead of “Booyer-Moore”?

ooops, you’re right 😀 , thanks!

Hi, I’m a member of team Seed.

It’s a great problemset, and it’s really responsible for the author team to write such detailed solutions after the contest.

Here’s some of our experience:

1. We use segment tree to solve E 🙁

2. We solved I using DFS(4^n), only add a few pre-cut.

Would you mind if I ask for the testdata of the contest, so we could use it for practice, and I could test my program for D & H.

My email is: chy_charlie@hotmail.com

Oh, and also the time limit for each problem.

Thanks a lot.

@Cheng Yu:

1. Yeah, some other teams also used segment tree to solve E 😀

2. Hm, interesting 😕

Btw, I have sent an email for you 🙂 We will also upload the problemset and testdata to baylor’s archieve.

Don’t worry, I won’t paste the exact value of 3^^^^3 as I know it wouldn’t fit in here… instead I would like to paste the exact values of the so-called “? scrapers” from this site, which make those hyper-mod numbers above insignificant http://www.polytope.net/hedrondude/scrapers.htm

i’am trying to solve the problem I using brute force , but my answer still judge wrong answer.

is there any trick on the brute force one?

still curious ^^

@indra

what problem you’re talking about?

@Indra Suryatama:

No, there isn’t any tricky case. It means your bruteforce solution was wrong 🙂

@Felix J:

Clearly he’s talking about problem “I”

Yap i’am thalkin about problem I….

hmm still curious….

is the judge test case can be downloaded? cause still curious in which test case my solution get wrong hehehe

maybe my brute force solution was wrong….

but until now i still dunno which test case hahah

because at that time i make several test case and it passed hehehhehehe ^^

maybe you can give me some advice heheh ^^

by the way …. you said

“New update. I have just tested the brute force solution and it did not pass the time-limit.”

so we cant solve problem I with brute force solution…?

hmmm hahah so i must moved on from BF solution tu DP solution ya… heheheh

tq

@Indra Suryatama:

check your mail.

Your bruteforce program failed on some cases. I suspect that your program didn’t properly check whether a combination/solution is possible to happen or not.

If you do plain 4^N brute force then you’ll surely get TLE 🙂

Hi Suhendry!

Great reviews for the problems! You mention in one of the comments that you’ll sen the dataset to the Baylor Archive. If you mean the CLI Live Archive, we didn’t recieved anything yet. We’ve already uploaded the problem descriptions, so having the datasets will be very very helpful for us.

Please, contact me at carlos.marce@gmail.com

Thanks!

Hi Carlos!

No wonder the problemset is still not available in Live Archieve, the email address was wrong…

check your mail, thanks 🙂

Thanks to Carlos, now the problemset is available in CLI Live Archieve:

http://acmicpc-live-archive.uva.es/nuevoportal/region.php?r=as7&year=2008

enjoy it 🙂

Hi Suhendry! Could you please send the testcase to me for my practice these problems? I don’t know where the baylor’s archieve is…could you tell me? Thanks!

mail: f7495622@mail.ncku.edu.tw

Oh i means where can i find the testcase in the baylor’s archieve… i can’t find the correct link. Could you please tell me ??

Thanks a lot for this great and detailed review!

If only every regional had these type of reviews 🙁

Also I really loved the problem set and I can feel that it especially enriched my DP skills. Thanks to all problem setters.

hi 🙂

I’m Ivan from the team from Ateneo de Manila University, Philippines. I was wondering if you can send the testdata to my email? I’d use it to analyze our solutions to the problems, and for our future training sessions as well.

My email is ivan_pisay07@yahoo.com.

And oh, your review of the problems is great! Thanks for this 🙂

So what’s the story behind problem I? 😀

Hi,

will you please send testdata to me for our training contests?

Thanks,

— Kamran

plz ,Explain 3rd test case according to your algorithm,

i,e min{ (ai-bi)mod 10,(bi-ai) mod 10} , then MST….

for problem A

plz ,Explain 3rd test case according to your algorithm,

i,e min{ (ai-bi)mod 10,(bi-ai) mod 10} , then MST….

The link for the CLI Live Archive is not working for me, can anyone update it?

@rizoan toufiq

First, compute the cost from each state to other state.

For example,

1234 to 5678 =

min((1-5+10) mod 10, (5-1+10) mod 10) +

min((2-6+10) mod 10, (6-2+10) mod 10) +

min((3-7+10) mod 10, (7-3+10) mod 10) +

min((4-8+10) mod 10, (8-4+10) mod 10) +

= 4 + 4 + 4 + 4 = 16

The matrix cost will be:

0 16 12

16 0 12

12 12 0

The MST for above graph is 24. Connect the MST with (0,0,0,0) and you will get 26.

@John Bull

The CLI Live Archive link is working…

You did a good job.

hi everyBody…

i have a problem about Palindrome number .

i want to find a Palindrome number By index of numbers ( numbers are subset of natural numbers = N )

like this :::

input | output

9 | 9

10 | 11

29 | 202

40 | 313

& so on…

i want an algorithm to find this

(someone said : e.g. : if input = 12 /// output => 12 + 21 = 33

or 64= > 64+46 = 110 go on = > 110+011 = 111 /// But not true for all numbers)

please help me

//in c++

#include

#include

using namespace std;

int main()

{

char s[22],a[20],b[20];int i,j;

cin>>a;

for(i=0,j=strlen(a)-1;i<strlen(a);i++,j–)

b[j]=a[i];

for(i=0;i<strlen(a);i++)

s[i]=(a[i]-48)+(b[i]-48)+48;

for(i=0;i46 && s[i]<58)

cout<<s[i];

cout<<endl;

return 0;

}

mohammed just out of curiosity what do you need this algorithm?

I got TLE in ICPC team strategy, altough I’ve used DP bitmask. Can someone suggest me please..

Hi Suhendry, I praticed solving this contest some days ago and I used brute force for this problem (ICPC Team Strategy). I tried all the possibilities and I got AC in 2.6s. After that, I add only one line of code for checking some variables and then I submitted again and got AC in 0.104s. I’m really surprise! 🙂

HI!!

I have some problem with ICPC team strategy : (

I use DP + bitmask , run for all the state by 5*for,

and the time complexity is the same as yours , but still TLE : (

Is there any cut or tip ?

thanks.

Can you please email me the test data for the contest , we are a team from Egypt and we will use this data to prepare for the coming regional contest

here is my e-mail: kamokim2@gmail.com

Thanks

Could you mind if I you send me test data of the contest specially D and F , so my team could use it for practice.

E-mail : yasser.csd@gmail.com

Thanks