Show HN: Design, data-structure, algorithm problems and their solutions

by sangupta • 10 years ago

103 25 10 years ago

ploggingdev2• 10 years ago

In https://github.com/sangupta/ps/blob/master/solutions/2016/fa... the solution for fastest integer sorting is wrong.

    / build the boolean result array
    boolean[] result = new boolean[array.length];
    for(int index = 0; index < array.length; index++) {
      int num = array[index];
      result[num] = true;
    }

The result array is the same size as the original array.

But when you do:

    int num = array[index];
    result[num] = true;

The index being referenced in the result array is dependent on the value of num. So if num is greater than or equal to the length of the array, it throws an ArrayOutOfBoundsException. And what happens when you try sorting an array with elements having the same value?

Eg- try sorting {10,5,9}

Unrelated: HN needs to allow highlighting code inline.

sawmurai• 10 years ago

Yep, should be

boolean[] result = new boolean[Math.max(array)];

ploggingdev2• 10 years ago

That won't be efficient memorywise since the size of the result array is dependant on the largest value in the array of values.

Consider sorting an array of 2 large integers {65000,30000}. To sort this array, the result array will be initialized with 65000 elements. Very inefficient.

sangupta9• 10 years ago

My bad - that was a typo when writing. Updating it.

jedimastert• 10 years ago

In the prompt, he says we are looking at a "bounded" integer set, so presumably we know the largest possible number before we start

rhardih2• 10 years ago

This phrase is misleading:

The fastest time to sort an integer array is O(N)

It could be interpreted as generally applicable. The proposed method uses reverse-indexed flagging, an approach which isn't really feasible if element values are very large.

Generally, the lower bound for any comparison based sorting algorithm is Ω( n log( n )).

EllipticCurve3• 10 years ago

That is true. Just want to add, that the lower bound for sorting is O(n log(n)) for comparison based sorting in infinite universes. The comparison based factor rules out sorting of any element, that can not be reduced to an integer. Infinite universes in sense, that we don't know about the elements/element-range beforehand.

Sorting in θ(n+k) w.c. and a.c. (most of the time this is equal to O(n)) is possible for finite boundaries that allows for countingsort or bucketsort (radixsort uses countingsort internally).

Some implementations of those algorithms in Python: https://github.com/MauriceGit/Advanced_Algorithms

sangupta9• 10 years ago

Agreed about the phrase thing - updated the phrase accordingly to say:

"Fastest sorting of bounded integers"

joakleaf• 10 years ago

A lot of discussion on bucket/pigeon hole sorting and O(n log n), but nobody mentioning(?):

Integer Sorting in 0(n sqrt (log log n)) Expected Time and Linear Space, Yijie Han and Mikkel Thorup, FOCS '02 Proceedings of the 43rd Symposium on Foundations of Computer Science Pages 135-144

http://dl.acm.org/citation.cfm?id=652131

... Just thought I would throw it out there.

sangupta9• 10 years ago

Thanks a bunch. I mentioned similarities to bucket sort earlier in the day.

I will go over the mentioned paper and also mention it as a reference.

vijayr• 10 years ago

Is there something like this for designing of databases? software architecture? For example, things like - learning to design a HIPAA compliant user database, designing a micro services architecture for a small but well defined problem etc?

sangupta9• 10 years ago

None that I know of and thus the inspiration to start this.

I so intend to write on some pieces of software architecture that I have understanding of. For micro-services piece, if you specific sample well-defined problems, please do let me know and I would like to cover it up.

dtauzell• 10 years ago

I think that falls more into the "patterns" space as the problem being solved is not so clear cut. Here is a site with microservices patterns: http://microservices.io/patterns/ .

kevas• 10 years ago

I don't know if this has HIPAA compliant schema among their collection ~1,500 or so models.

http://www.databaseanswers.org/data_models/

qwertyuiop9242• 10 years ago

After going back to solve last year's Advent of Code, I realized that I really need to know more about algorithms. So this is handy.

But I still can't solve last years' AoC. Oh well. I guess I'll just have to keep working on it.

sn9• 10 years ago

Check out Sedgewick's Algorithms course on Coursera.

qwertyuiop9242• 10 years ago

Thanks for the suggestion.

Show HN: Design, data-structure, algorithm problems and their solutions

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