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DISHOWN - Editorial

7
1

PROBLEM LINK:

Practice
Contest

Author: Vivek Hamirwasia
Tester: Shang Jingbo and Gerald Agapov
Editorialist: Devendra Agarwal

DIFFICULTY:

Easy

PREREQUISITES:

Set Union Data Structure

PROBLEM:

N chef's are there. Each chef has a dish which is given a value (denoted by S[i]). You need to answer 2 queries :
1. 0 x y : Chef containing dish x competes with chef containing dish y. If both dishes belong to same chef print "Invalid Query!" , otherwise the winner of the round will obtain all the dishes of the loser which will then be eliminated.
2. 1 x : Output the chef which contains the dish x.

Explanation

This problem can be solved using Disjoint set data structures.

Union Find or Disjoint Set Data structures allow you to merge two sets of items together dynamically and maintain for each item - to which set set does it belong.

This is exactly what we need in the problem , i.e. dynamically merging two sets and querying which set does an element belong.

We will be using Disjoint Set Data Structure that supports the following :

  1. find(A) : Get the id of the disjoint set to which A belongs(i.e. the chef id which contains that dish).
  2. union(A,B) : Merge the two disjoint sets in which A and B belong respectively.

Initially, we will create N disjoint set , each set contains 1 dish and it's owner.
Let dish x be in setA and dish y be in setB.
If the dish contained by the owner of setA has score higher than the dish contained by the owner of setB , then we will merge setA and setB and set the owner of setA as the overall owner of the merged set.
If the dish contained by the owner of setB has score higher than the dish contained by the owner of setA , then we will merge setA and setB and set the owner of setB as the overall owner of the merged set.

Note : You can easily prove from this that the owner of the set has dish whose score is higher than all other dish of the set.
We will be using only Path Compression heuristics to solve the problem.

Pseudo Code

Initialize parent[i] = i  
Let S[i] denote the initial array.

int find(int i)
    int j
    if(parent[i]==i)
                return i
    else
        j=find(parent[i])
        //Path Compression Heuristics
        parent[i]=j
        return j

set_union(int i,int j)
    int x1,y1
    x1=find(i)
    y1=find(j)
    //parent of both of them will be the one with the highest score
    if(S[x1]>S[y1])
        parent[y1]=x1
    else if ( S[x1] < S[y1])
        parent[x1]=y1

solve()
    if(query == 0)
        Input x and y
        px = find(x)
        py = find(y)
        if(px == py)
            print "Invalid query!"
        else
            set_union(px,py)
    else
        Input x.
        print find(x)

AUTHOR'S and TESTER'S Solution:

Author's solution
Tester's solution

This question is marked "community wiki".

asked 14 Jul '14, 15:01

devuy11's gravatar image

4★devuy11 ♦♦
46273842
accept rate: 0%

edited 29 Jan, 16:41

admin's gravatar image

0★admin ♦♦
19.0k348495533

2

I have solved with the same approach but was getting TLE as i was using Console.WriteLine to print all the output in C# . When i used the StringBuilder to store the messages and output in the last it passed the test cases. I am not sure if we have to consider this small optimization also

(14 Jul '14, 15:22) dev85462★

I have used same approach but I am getting TLE http://www.codechef.com/viewsolution/4294855 please help me!

(14 Jul '14, 15:31) mtbar1312★

@mtbar131 : You didnt use path compression. That's what I could notice.

(14 Jul '14, 15:33) himanshujaju6★

@dev8546: Thanks for your comment. Now I understand the reason that I never got it AC. I couldn't think that Console.WriteLine() might be causing the problem. I kept on hitting my head thinking there might be a better optimized way to solve the problem and finally gave up on it after trying some micro optimizations :(

(14 Jul '14, 15:35) ig_quasar1★

Did the same in Java as dev8546 described. By printing System.out.println() it gets TLE. If i use a stringbuilder and output it at the end it passes. (I used both path compression and union by rank).

(14 Jul '14, 15:43) szilard4★

Just a quick check - Is this true with all languages or just the managed ones like C#/JAVA?

(14 Jul '14, 16:13) ig_quasar1★

author's solution is giving TLE. i think path compression technique is not working. i also used that method and got tle. pls check it.

(14 Jul '14, 23:56) imrajeshpal3★

this code is giving TLE i cant understand why. http://www.codechef.com/viewsolution/4333486

(15 Jul '14, 19:48) abhishekjhaji2★
2

Authors' solution for this problem is wrong. It gives TLE because it doesn't use path compression.

(23 Jul '14, 02:12) nitinj5★

What is the need of using find() function again in set_union() when we are already passing set_union() the parents of the 2 dishes?

(03 Sep '14, 18:42) samdcoder2★
showing 5 of 10 show all

123next »

@kuruma thanks for this question i learnt something new.

first tiny mistake is the output of second query type you should print find(x)+1 as your numbers are 0-based .

secondly you have used "ios_base::sync_with_stdio(false)" that seems to cause your output to be buffered and is printed together only when the program terminates.

this would have been correct had you used puts/printf in output of 2nd query as well. since you used cout that output is not buffered and printed instantly while the output of all first queries is printed in the end.

So what you can do:

1.use fflush(stdout) after puts statement

2.use cout

3.NOT use ios_base::sync_with_stdio(false);

4.use puts/printf in the output of 2nd query as well

link

answered 20 Jul '14, 01:36

shubham12's gravatar image

3★shubham12
4421610
accept rate: 13%

edited 20 Jul '14, 01:49

Thanks a lot @shubham12 , I switched puts to cout and used the +1 trick and got AC :D

Really, really thanks a lot :) And the seudo-code from editorial was also really well written :D Hope I can start doing this more often

(20 Jul '14, 01:48) kuruma2★

glad i could help :). though i had some incorrect understanding of the effect of "ios_base::sync_with_stdio(false)". i have to read it up.

(20 Jul '14, 01:51) shubham123★

Hello,

can anyone tell why TLE: http://www.codechef.com/JULY14/status/DISHOWN,squal

Thanks.

link

answered 14 Jul '14, 15:29

squal's gravatar image

4★squal
1.2k41533
accept rate: 14%

2

don't use System.out.println(), instead use StringBuilder to store answers and then in the end use System.out.println() to print StringBuilder object. I have modified your code it has got accepted. Link to ur code: http://www.codechef.com/status/DISHOWN,sousnake (see the accepted solution).

(14 Jul '14, 16:03) sousnake2★

@sousnake: thanks :) bad that i missed this because of System.out.println() :(

(14 Jul '14, 16:49) squal4★

admins...plz see my code ..http://www.codechef.com/viewsolution/4328720 ..used same approach ..stll TLE.!!..its really annoying ..

link

answered 14 Jul '14, 15:37

rachilies's gravatar image

4★rachilies
114147
accept rate: 11%

1

yeah ..the path compression heuristics...really had an opportunity to learn.. btw real thanx. :)

(14 Jul '14, 16:32) rachilies4★

Hai, I don't know whether I can post this or not ,Here are some unofficial test cases categorised into small,medium,large.Who are getting mainly WA but also RTE,TLE,... can also check.You may need not worry about the constraints.the values are guarenteed to be in the given constraints.

link

answered 14 Jul '14, 15:38

achaitanyasai's gravatar image

5★achaitanyasai
2.2k61747
accept rate: 10%

i have tried all your test files for DISHOWN and got all correct output.. but NZEC in codechef ..

see the below comment /!\

(14 Jul '14, 22:04) aq_faridi5★

Can anybody tell me why i was getting Runtime Error(NZEC) in java (i have used the same union-find logic)??
here is the code => http://www.codechef.com/viewsolution/4303110

When i transformed the same logic in c++, it got accepted... here is ACed c++ code => http://www.codechef.com/viewsolution/4303214

Thanx in Advance :)

link

answered 14 Jul '14, 15:39

aq_faridi's gravatar image

5★aq_faridi
1
accept rate: 0%

I think the problem was in your function owner. I have modified that function just a bit and it got accepted. Here's the link: http://www.codechef.com/viewsolution/4329762

(14 Jul '14, 17:39) sousnake2★

what is the difference between these two functions?? :

function 1 : gave AC

static int owner(int x)
{
    if(p[x] == x)
        return x;
    else
    {
        int var = owner(p[x]);
        p[x] = var;
        return p[x];
    }
}

function 2: gives NZEC

static int owner(int x)
{
    if(p[x] == x)
        return x;
    else
    {
        p[x] = owner(p[x]);
        return p[x];
    }
}
(14 Jul '14, 19:19) aq_faridi5★

no I think the problem was that: (this was your original code) static int owner(int x) { if(p[x] == x) return x; else return p[x]=owner(p[x]); }

here may be the problem was with assignment operator (i am not sure how it works but it works differently in java and C++).

(15 Jul '14, 18:58) sousnake2★

I used UF data structure with path compression but still got TLE. Why would the program run into TLE when using System.out.println()? Can anybody explain that?

link

answered 14 Jul '14, 16:54

nastra's gravatar image

2★nastra
15335
accept rate: 0%

System.out.println() is slow .. use BufferedWriter or BufferedOutputStream for fast IO see my code for reference : http://www.codechef.com/viewsolution/4330077

(15 Jul '14, 21:37) aq_faridi5★

Can anyone tell what's wrong with my code?Getting WA. Code

link

answered 14 Jul '14, 17:14

itsmepsk's gravatar image

3★itsmepsk
162126
accept rate: 0%

anyone would tell wats wrng in my code .. its same as that of the editorial !!!!! the link of the solution is http://www.codechef.com/viewsolution/4326802 .. any help is seriously welcomed

link

answered 14 Jul '14, 19:18

meintoo's gravatar image

5★meintoo
33136
accept rate: 16%

http://ideone.com/jN6HUM You code gives WA for the test case Mentioned on this link. Also i have commented in one line where i have done the edit. After this, use fast I/O and it should give AC.

(14 Jul '14, 23:53) damn_me3★

i don't know about how the test cases were designed but still if this algorithm is implemented with union by rank it will give better performance.I am a bit disappointed that my code that should have been accepted with better performance gave TLE if anybody can explain the reason for this do reply....

link

answered 14 Jul '14, 19:28

vaibhav9518s's gravatar image

2★vaibhav9518s
462
accept rate: 0%

In Java BufferedReader is giving TLE.
Even taking only input without processing anything it gives TLE.Why this happened for java Language. It run smoothly for scanf in c if it fastio problem then it should give TLE for scanf also.
This is my code in which only input is getting take by program and for this it is giving TLE.

link

answered 14 Jul '14, 20:46

shivamsharma's gravatar image

3★shivamsharma
12
accept rate: 0%

edited 14 Jul '14, 20:48

1

i think you should use BufferdOutputStream instead of System.out.println see my code for reference : http://www.codechef.com/viewsolution/4330077

(15 Jul '14, 21:41) aq_faridi5★

@aq_faridi I took only input then also I got TLE.

(15 Jul '14, 22:06) shivamsharma3★
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question asked: 14 Jul '14, 15:01

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last updated: 25 Mar, 19:55