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STICKS - Editorial

PROBLEM LINK:

Practice
Contest

Author: Hasan Jaddouh
Tester: Misha Chorniy
Editorialist: Pushkar Mishra

DIFFICULTY:

Simple

PREREQUISITES:

Greedy

PROBLEM:

Given an array of $A$ of $N$ integers which represent stick lengths, we have to report the maximal area over all the rectangles that we can form with the given sticks without breaking any.

EXPLANATION:

The problem is a simple one based on a very basic property of all rectangles: the opposite sides of a rectangle are parallel, and hence, equal in length. The area of a rectangle is given by base $B$ multiplied by height $H$. Now, to maximise the area, we simply need to maximise the length of sticks we choose as our $B$ and $H$. This is pretty intuitive but we can also invoke the exchange argument method (page 3 of link) to give a more formal argument as proof of why this works (left to reader as a simple learning exercise).

Now we come to the implementation detail of the above mentioned algorithm. Note that $N$ and $A_{max}$ both range between 1 and $10^3$. So, one way is that we simply keep an array $Count$ of length $10^3$ wherein $Count[i]$ is the number of sticks of length $i$ that we have. Once we have populated this structure, we can simply scan from $10^3$ down to 1. If while scanning, we can find one field which has more than 3 sticks, then that $i$ can be our height and base both; at this point we terminate our scan. If we find a field that has more than 1 stick, we can make it either our height or base and continue our scan to find a value for the other one. If at the end of scan we don't find at least two $i$ for which the count is more than 1, then we output -1 since no rectangle can be formed.

The other implementation can be by using a map instead of an array. In that case, if the $N$ is much smaller than $A_{max}$, we can get a better performance. Both of the implementations easily pass for the given constraints.

Please see editorialist's/setter's program for implementation details.

COMPLEXITY:

$\mathcal{O}(N\log N)$ or $\mathcal{O}(A_{max})$ per test case.

SAMPLE SOLUTIONS:

Author
Tester
Editorialist

This question is marked "community wiki".

asked 25 Jun '16, 12:31

pushkarmishra's gravatar image

4★pushkarmishra
1.3k156381
accept rate: 4%

edited 27 Jun '16, 00:14

admin's gravatar image

0★admin ♦♦
19.0k348495533


1234next »

link

answered 27 Jun '16, 10:08

easy_'s gravatar image

2★easy_
1286
accept rate: 0%

whats wrong with my implementation ? link- https://www.codechef.com/viewsolution/10624567

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answered 27 Jun '16, 00:15

mkfeuhrer's gravatar image

2★mkfeuhrer
213
accept rate: 0%

Try this test: 1 4 1 2 3 3 Your solution output 0, but must be -1

BTW, change for(i=0;i<10002;i++) to for(i=0;i<10001;i++), your arrays size only 10001

(27 Jun '16, 01:33) mgch6★

I implemented in c++ and java. In c++ I am getting run-time error when I am running the code in the code-compile-run here. And in Java it's wrong answer.

Java Implementation:

http://pastebin.com/gAFVG2Fs

C++ Implementation:

http://pastebin.com/nf4zuNSM

If possible please tell me what is wrong with these two codes.

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answered 27 Jun '16, 00:28

priyabrata1992's gravatar image

2★priyabrata1992
1
accept rate: 0%

You forget about square: 1

4

1 1 1 1

Your solution output -1, but must be 1.

(27 Jun '16, 01:35) mgch6★

A different implementation of the above logic.

https://www.codechef.com/viewsolution/10616757

First enter all the length of the sticks in an array, sort it in O(nlogn) and then traverse the array from the right end, checking for multiple instances of any stick. This also covers the special case of a square.

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answered 27 Jun '16, 00:32

utkarsh1997's gravatar image

4★utkarsh1997
7749
accept rate: 11%

edited 27 Jun '16, 11:28

hey,can anyone just help me to find error in my code.. https://www.codechef.com/viewsolution/10623972

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answered 27 Jun '16, 00:58

nanhe's gravatar image

3★nanhe
413
accept rate: 0%

You forget about square case too

(27 Jun '16, 01:37) mgch6★

can u elaborate more ,which case i have omitted

(27 Jun '16, 01:53) nanhe3★

1

4

1 1 1 1

(27 Jun '16, 01:57) mgch6★

ok,I got it ..thank U

(27 Jun '16, 02:00) nanhe3★

A different approach,which has O(N*logN) time complexity but uses Only One Array is as follows:

After storing the array containing the lengths of sticks, sort it. We must note that the same lengths will automatically come together after sorting. For example,

the array 5 2 8 3 1 2 4 3 5

becomes 1 2 2 3 3 4 5 5 8

Then if we traverse the array from backwards picking up the first two occurrences of a pair of same numbers(for e.g (5,5) would be the first occurrence and (3,3) would be the second in this case.Don't forget to break the loop here. :P ) the answer would be 5*3=15.If we are unable to find two occurrences we print -1.

The JAVA solution of the problem using this method is:

https://www.codechef.com/viewsolution/10620032.

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answered 27 Jun '16, 02:50

code_blooded_'s gravatar image

4★code_blooded_
211
accept rate: 0%

https://www.codechef.com/viewsolution/10618548 Please let me know what is wrong with this solution.

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answered 27 Jun '16, 10:46

sreedishps's gravatar image

4★sreedishps
1
accept rate: 0%

@sreedishps if N<3 , then you are not reading sticks length

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answered 27 Jun '16, 11:41

arunmittal53's gravatar image

4★arunmittal53
92
accept rate: 0%

https://www.codechef.com/viewsolution/10627623 Hey, Can anybody tell why I am getting WA ? I used the algorithm mentioned in editorial.

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answered 27 Jun '16, 12:13

chandan5's gravatar image

2★chandan5
11
accept rate: 0%

edited 27 Jun '16, 12:25

1

8

1 1 2 2 2 2 3 3

Answer is 6 = 2 * 3, but your output will be 2 * 2 = 4

(27 Jun '16, 12:26) mgch6★

@mgch Thanks :)

(27 Jun '16, 14:50) chandan52★

https://www.codechef.com/viewsolution/10621785 Can somebody please look into it,I am getting a WA.

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answered 27 Jun '16, 15:59

pada1211's gravatar image

1★pada1211
1
accept rate: 0%

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question asked: 25 Jun '16, 12:31

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last updated: 06 Jul, 15:24