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  • Great Debugging skills weak problem solving

    - by Mahmoud
    For the 5 years I worked for various companies, I worked in large software like computer vision kits, embedded, games. I found myself very good at debuggins skills, I've even found and fixed bugs in frameworks and I solved them. The problem is that I'm very weak at problem solving. I got interview with Qualcomm, and they said you're fine at software, but you have a limited problem solving, I also had the same results with Google. I'm very bad at solving puzzles and brain teasers. During the interviews I solve all of the software related problems on the blackboard, but when I went to the GM and face math problems and probabilities, I struggle. How can I improve my problem solving skills? Edit Some of the problems: A cake that is cut from anywhere and needs just one cut to halved in equal. I told him cut it horizontally, he said No, consider it as a 2D Problem!. Consider a concenteric 3 circles, each one can get a color, but not matched with the other circle, how many blobs you can make out of those circles ? this was with the GM ( Augmented Reality SDK) Consider a train, an infinite one, and you looked at the window, and there are two cars, one big, and one small, what is the probability of having only a big car, I said 50%, he said, what if that two cars you dont know their length, and you want to get the probability of getting the biggest one, I struggled, didn't solve it... I was really exahusted after long day of interviews prob of having a number divisible by 5 in numbers from 1 to 100.. struggled!! All coding questions I solved them like reverse a string, detect a cycle in a linked list,..etc.

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  • Does it matter the direction of a Huffman's tree child node?

    - by Omega
    So, I'm on my quest about creating a Java implementation of Huffman's algorithm for compressing/decompressing files (as you might know, ever since Why create a Huffman tree per character instead of a Node?) for a school assignment. I now have a better understanding of how is this thing supposed to work. Wikipedia has a great-looking algorithm here that seemed to make my life way easier. Taken from http://en.wikipedia.org/wiki/Huffman_coding: Create a leaf node for each symbol and add it to the priority queue. While there is more than one node in the queue: Remove the two nodes of highest priority (lowest probability) from the queue Create a new internal node with these two nodes as children and with probability equal to the sum of the two nodes' probabilities. Add the new node to the queue. The remaining node is the root node and the tree is complete. It looks simple and great. However, it left me wondering: when I "merge" two nodes (make them children of a new internal node), does it even matter what direction (left or right) will each node be afterwards? I still don't fully understand Huffman coding, and I'm not very sure if there is a criteria used to tell whether a node should go to the right or to the left. I assumed that, perhaps the highest-frequency node would go to the right, but I've seen some Huffman trees in the web that don't seem to follow such criteria. For instance, Wikipedia's example image http://upload.wikimedia.org/wikipedia/commons/thumb/8/82/Huffman_tree_2.svg/625px-Huffman_tree_2.svg.png seems to put the highest ones to the right. But other images like this one http://thalia.spec.gmu.edu/~pparis/classes/notes_101/img25.gif has them all to the left. However, they're never mixed up in the same image (some to the right and others to the left). So, does it matter? Why?

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  • How do I produce "enjoyably" random, as opposed to pseudo-random?

    - by Hilton Campbell
    I'm making a game which presents a number of different kinds of puzzles in sequence. I choose each puzzle with a pseudorandom number. For each puzzle, there are a number of variations. I choose the variation with another pseudorandom number. And so on. The thing is, while this produces near-true randomness, this isn't what the player really wants. The player typically wants what they perceive to be and identify as random, but only if it doesn't tend to repeat puzzles. So, not really random. Just unpredictable. Giving it some thought, I can imagine hacky ways of doing it. For example, temporarily eliminating the most recent N choices from the set of possibilities when selecting a new choice. Or assigning every choice an equal probability, reducing a choice's probability to zero on selection, and then increasing all probabilities slowly with each selection. I assume there's an established way of doing this, but I just don't know the terminology so I can't find it. Anyone know? Or has anyone solved this in a pleasing way?

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  • Do Not Optimize Without Measuring

    - by Alois Kraus
    Recently I had to do some performance work which included reading a lot of code. It is fascinating with what ideas people come up to solve a problem. Especially when there is no problem. When you look at other peoples code you will not be able to tell if it is well performing or not by reading it. You need to execute it with some sort of tracing or even better under a profiler. The first rule of the performance club is not to think and then to optimize but to measure, think and then optimize. The second rule is to do this do this in a loop to prevent slipping in bad things for too long into your code base. If you skip for some reason the measure step and optimize directly it is like changing the wave function in quantum mechanics. This has no observable effect in our world since it does represent only a probability distribution of all possible values. In quantum mechanics you need to let the wave function collapse to a single value. A collapsed wave function has therefore not many but one distinct value. This is what we physicists call a measurement. If you optimize your application without measuring it you are just changing the probability distribution of your potential performance values. Which performance your application actually has is still unknown. You only know that it will be within a specific range with a certain probability. As usual there are unlikely values within your distribution like a startup time of 20 minutes which should only happen once in 100 000 years. 100 000 years are a very short time when the first customer tries your heavily distributed networking application to run over a slow WIFI network… What is the point of this? Every programmer/architect has a mental performance model in his head. A model has always a set of explicit preconditions and a lot more implicit assumptions baked into it. When the model is good it will help you to think of good designs but it can also be the source of problems. In real world systems not all assumptions of your performance model (implicit or explicit) hold true any longer. The only way to connect your performance model and the real world is to measure it. In the WIFI example the model did assume a low latency high bandwidth LAN connection. If this assumption becomes wrong the system did have a drastic change in startup time. Lets look at a example. Lets assume we want to cache some expensive UI resource like fonts objects. For this undertaking we do create a Cache class with the UI themes we want to support. Since Fonts are expensive objects we do create it on demand the first time the theme is requested. A simple example of a Theme cache might look like this: using System; using System.Collections.Generic; using System.Drawing; struct Theme { public Color Color; public Font Font; } static class ThemeCache { static Dictionary<string, Theme> _Cache = new Dictionary<string, Theme> { {"Default", new Theme { Color = Color.AliceBlue }}, {"Theme12", new Theme { Color = Color.Aqua }}, }; public static Theme Get(string theme) { Theme cached = _Cache[theme]; if (cached.Font == null) { Console.WriteLine("Creating new font"); cached.Font = new Font("Arial", 8); } return cached; } } class Program { static void Main(string[] args) { Theme item = ThemeCache.Get("Theme12"); item = ThemeCache.Get("Theme12"); } } This cache does create font objects only once since on first retrieve of the Theme object the font is added to the Theme object. When we let the application run it should print “Creating new font” only once. Right? Wrong! The vigilant readers have spotted the issue already. The creator of this cache class wanted to get maximum performance. So he decided that the Theme object should be a value type (struct) to not put too much pressure on the garbage collector. The code Theme cached = _Cache[theme]; if (cached.Font == null) { Console.WriteLine("Creating new font"); cached.Font = new Font("Arial", 8); } does work with a copy of the value stored in the dictionary. This means we do mutate a copy of the Theme object and return it to our caller. But the original Theme object in the dictionary will have always null for the Font field! The solution is to change the declaration of struct Theme to class Theme or to update the theme object in the dictionary. Our cache as it is currently is actually a non caching cache. The funny thing was that I found out with a profiler by looking at which objects where finalized. I found way too many font objects to be finalized. After a bit debugging I found the allocation source for Font objects was this cache. Since this cache was there for years it means that the cache was never needed since I found no perf issue due to the creation of font objects. the cache was never profiled if it did bring any performance gain. to make the cache beneficial it needs to be accessed much more often. That was the story of the non caching cache. Next time I will write something something about measuring.

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  • Binomial test in Python

    - by Morlock
    I need to do a binomial test in Python that allows calculation for 'n' numbers of the order of 10000. I have implemented a quick binomial_test function using scipy.misc.comb, however, it is pretty much limited around n = 1000, I guess because it reaches the biggest representable number while computing factorials or the combinatorial itself. Here is my function: from scipy.misc import comb def binomial_test(n, k): """Calculate binomial probability """ p = comb(n, k) * 0.5**k * 0.5**(n-k) return p How could I use a native python (or numpy, scipy...) function in order to calculate that binomial probability? If possible, I need scipy 0.7.2 compatible code. Many thanks!

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  • When are you truly forced to use UUID as part of the design?

    - by Pyrolistical
    I don't really see the point of UUID. I know the probability of a collision is effectively nil, but effectively nil is not even close to impossible. Can somebody give an example where you have no choice but to use UUID? From all the uses I've seen, I can see an alternative design without UUID. Sure the design might be slightly more complicated, but at least it doesn't have a non-zero probability of failure. UUID smells like global variables to me. There are many ways global variables make for simpler design, but its just lazy design.

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  • Binomial test in Python for very large numbers

    - by Morlock
    I need to do a binomial test in Python that allows calculation for 'n' numbers of the order of 10000. I have implemented a quick binomial_test function using scipy.misc.comb, however, it is pretty much limited around n = 1000, I guess because it reaches the biggest representable number while computing factorials or the combinatorial itself. Here is my function: from scipy.misc import comb def binomial_test(n, k): """Calculate binomial probability """ p = comb(n, k) * 0.5**k * 0.5**(n-k) return p How could I use a native python (or numpy, scipy...) function in order to calculate that binomial probability? If possible, I need scipy 0.7.2 compatible code. Many thanks!

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  • Mathematics for Computer Science

    - by jiewmeng
    I am going into university next year. I think maths would be one of the more important aspects of computer science? I recently saw the MIT Intro to Algorithms video on YouTube and the maths required is quite hardcore. I wonder what parts of maths do i need, probability, calculus, trigo etc. Will the book Concrete Mathematics - it claims to be foundation for computer science - on Amazon cover most of whats required?

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  • Pirate Problem In Interview Question

    - by Hafiz
    Some one asked me this question in an interview, so I want to know that what can be technical or algorithmic or strategical solution can we provide? If I am a leader of Pirates who looted 100kg gold, now every pirate has 1 bullet in gun and every pirate wants to get each other's share. They are 5 in number including me. So what strategy I will use to get to kill others while being safe or is there way to decrease probability?

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  • Keyword Research - Four Key Criteria For Selecting "Money" Keywords

    Keyword research is absolutely essential to the success of any website because keywords are what people use to find websites and are what search engines use as the basis for their rankings. Good keyword research increases the probability that search engines will rank the pages on your website high for your target keyword; and bad keyword research (which often means no keyword research) will doom your website to the search engine cellars.

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  • Using only password to authenticate user (no "username" field)

    - by Guy
    I am creating a client access system, to allow manage invoices, make payments, access information about their products and information/functionality alike. Supposedly there are less than 1000 clients. Would there be any security threat to use only password (UUID v4 strings) to authenticate user? My thoughts: There is virtually no probability of collision or success with brute-force attack. http://en.wikipedia.org/wiki/UUID#Random%5FUUID%5Fprobability%5Fof%5Fduplicates User friendly (one click go) It is not intended to be remembered

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  • Quicker Loading of Your Website Ensures Improved SE Rankings

    Now that Google has released news regarding the inclusion of website's speed as one of the factors responsible for website search engine ranking, it is obvious that most website owners would get want delve into matter, a bit more deep. According to the statement, it is clear that the websites having lower loading speed would have less probability of making their way to the search engine result pages than those that load up faster.

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  • Possible applications of algorithm devised for differentiating between structured vs random text

    - by rooznom
    I have written a program that can rapidly (within 5 sec on a 2GB RAM desktop, 2.33 Ghz CPU) differentiate between structured text (e.g english text) and random alphanumeric strings. It can also provide a probability score for the prediction. Are there any practical applications/uses of such a program. Note that the program is based on entropy models and does not have any dictionary comparisons in its workflow. Thanks in advance for your responses

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  • Multiple database accesses or one massive access?

    - by DudeOnRock
    What is a better approach when it comes to performance and optimal resource utilization: accessing a database multiple times through AJAX to only get the exact information needed when it is needed, or performing one access to retrieve an object that holds all information that might be needed, with a high probability that not all is actually needed? I know how to benchmark the actual queries, but I don't know how to test what is best when it comes to database performance when thousands of users are accessing the database simultaneously and how connection pooling comes into play.

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  • Should the test and the fix be written by different people?

    - by Nutel
    There is a common practice in TDD to write a test before fix to avoid regression and simplify fixing. I just wonder what if the test and fix will be written by different people, total spent time will be almost the same but as now three people will think about possible failures (+tester) we increase probability that fix will cover all possible failure scenarios. Does this practice make sense or it will just waste additional time needed for one more person to familiarize with bug?

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  • Which of these courses are the hardest and why?

    - by DSL Client
    Which of these courses are the hardest and why? What should I watch out for? Probability and Statistics for Computer Science Introduction to Software Engineering Data Structures and Algorithms Operating Systems Introduction to Theoretical Computer Science System Hardware Advanced Program Design with C++ Information Systems Security Computer Architecture Databases Web Programming Computer Graphics Digital System Design

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  • Importance of Inbound Links For a Website

    In Off-Page optimization, inbound links are the most important factor for your website to get displayed in search engine results page. If you are getting links from relevant sites then you have probability to get more relevant traffic to your websites.

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  • How Does SEO Marketing Work?

    SEO is a process to increase traffic towards a specific website or web page. The basic concept behind this is that number of clicks is directly proportional to your search result list. It means that the more clicks you get, the probability of your website coming at the earliest increases.

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  • Generating strongly biased radom numbers for tests

    - by nobody
    I want to run tests with randomized inputs and need to generate 'sensible' random numbers, that is, numbers that match good enough to pass the tested function's preconditions, but hopefully wreak havoc deeper inside its code. math.random() (I'm using Lua) produces uniformly distributed random numbers. Scaling these up will give far more big numbers than small numbers, and there will be very few integers. I would like to skew the random numbers (or generate new ones using the old function as a randomness source) in a way that strongly favors 'simple' numbers, but will still cover the whole range, I.e. extending up to positive/negative infinity (or ±1e309 for double). This means: numbers up to, say, ten should be most common, integers should be more common than fractions, numbers ending in 0.5 should be the most common fractions, followed by 0.25 and 0.75; then 0.125, and so on. A different description: Fix a base probability x such that probabilities will sum to one and define the probability of a number n as xk where k is the generation in which n is constructed as a surreal number1. That assigns x to 0, x2 to -1 and +1, x3 to -2, -1/2, +1/2 and +2, and so on. This gives a nice description of something close to what I want (it skews a bit too much), but is near-unusable for computing random numbers. The resulting distribution is nowhere continuous (it's fractal!), I'm not sure how to determine the base probability x (I think for infinite precision it would be zero), and computing numbers based on this by iteration is awfully slow (spending near-infinite time to construct large numbers). Does anyone know of a simple approximation that, given a uniformly distributed randomness source, produces random numbers very roughly distributed as described above? I would like to run thousands of randomized tests, quantity/speed is more important than quality. Still, better numbers mean less inputs get rejected. Lua has a JIT, so performance can't be reasonably predicted. Jumps based on randomness will break every prediction, and many calls to math.random() will be slow, too. This means a closed formula will be better than an iterative or recursive one. 1 Wikipedia has an article on surreal numbers, with a nice picture. A surreal number is a pair of two surreal numbers, i.e. x := {n|m}, and its value is the number in the middle of the pair, i.e. (for finite numbers) {n|m} = (n+m)/2 (as rational). If one side of the pair is empty, that's interpreted as increment (or decrement, if right is empty) by one. If both sides are empty, that's zero. Initially, there are no numbers, so the only number one can build is 0 := { | }. In generation two one can build numbers {0| } =: 1 and { |0} =: -1, in three we get {1| } =: 2, {|1} =: -2, {0|1} =: 1/2 and {-1|0} =: -1/2 (plus some more complex representations of known numbers, e.g. {-1|1} ? 0). Note that e.g. 1/3 is never generated by finite numbers because it is an infinite fraction – the same goes for floats, 1/3 is never represented exactly.

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  • Procedual level generation for a platformer game (tilebased) using player physics

    - by Notbad
    I have been searching for information about how to build a 2d world generator (tilebased) for a platformer game I am developing. The levels should look like dungeons with a ceiling and a floor and they will have a high probability of being just made of horizontal rooms but sometimes they can have exits to a top/down room. Here is an example of what I would like to achieve. I'm refering only to the caves part. I know level design won't be that great when generated but I think it is possible to have something good enough for people to enjoy the procedural maps (Note: Supermetrod Spoiler!): http://www.snesmaps.com/maps/SuperMetroid/SuperMetroidMapNorfair.html Well, after spending some time thinking about this I have some ideas to create the maps that I would like to share with you: 1) I have read about celular automatas and I would like to use them to carve the rooms but instead of carving just a tile at once I would like to carve full columns of tiles. Of course this carving system will have some restrictions like how many tiles must be left for the roof and the ceiling, etc... This way I could get much cleaner rooms than using the ussual automata. 2) I want some branching into the rooms. It will have little probability to happen but I definitely want it. Thinking about carving I came to the conclusion that I could be using some sort of path creation algorithm that the carving system would follow to create a path in the rooms. This could be more noticiable if we make the carving system to carve columns with the height of a corridor or with the height of a wide room (this will be added to the system as a param). This way at some point I could spawn a new automa beside the main one to create braches. This new automata should play side by side with the first one to create dead ends, islands (both paths created by the automatas meet at some point or lead to the same room. It would be too long to explain here all the tests I have done, etc... just will try to summarize the problems to see if anyone could bring some light to solve them (I don't mind sharing my successes but I think they aren't too relevant): 1) Zone reachability: How can I make sure that the player will be able to reach all zones I created (mainly when branches happen or vertical rooms are created). When branches are created I have to make sure that there will be a way to get onto the new created branch. I mean a bifurcation that the player could follow. Player will follow the main path or jump to a platform to get onto the other way). On the other hand if an island is created by the meeting of both branches I need to make sure the player will be able to get onto the island too. 2) When a branch is created and corridors are generated for each branch how can I make then both merge or repel to create an island or just make them separated corridors. 3) When I create a branch and an island is created becasue both corridors merge at somepoint or they lead to the same room, is there any way to detect this and randomize where to create the needed platforms to get onto the created isle? This platforms could be created at the start of the island or at the end. I guess part of the problem could be solved using some sort of graph following the created paths but I'm a bit lost in this sea of precedural content creation :). On the other hand I don't expect a solution to the problem but some information to get me moving forward again. Thanks in advance.

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  • Map large integer to a phrase

    - by Alexander Gladysh
    I have a large and "unique" integer (actually a SHA1 hash). I want (for no other reason than to have fun) to find an algorithm to convert that SHA1 hash to a (pseudo-)English phrase. The conversion should be reversible (i.e., knowing the algorithm, one must be able to convert the phrase back to SHA1 hash.) The possible usage of the generated phrase: the human readable version of Git commit ID, like a motto for a given program version (which is built from that commit). (As I said, this is "for fun". I don't claim that this is very practical — or be much more readable than the SHA1 itself.) A better algorithm would produce shorter, more natural-looking, more unique phrases. The phrase need not make sense. I would even settle for a whole paragraph of nonsense. (Though quality — englishness — of a paragraph should probably be better than for a mere phrase.) A variation: it is OK if I will be able to work only with a part of hash. Say, first six digits is OK. Possible approach: In the past I've attempted to build a probability table (of words), and generate phrases as Markov chains, seeding the generator (picking branches from probability tree), according to the bits I read from the SHA. This was not very successful, the resulting phrases were too long and ugly. I'm not sure if this was a bug, or the general flaw in the algorithm, since I had to abandon it early enough. Now I'm thinking about attempting to solve the problem once again. Any advice on how to approach this? Do you think Markov chain approach can work here? Something else?

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  • Linear regression confidence intervals in SQL

    - by Matt Howells
    I'm using some fairly straight-forward SQL code to calculate the coefficients of regression (intercept and slope) of some (x,y) data points, using least-squares. This gives me a nice best-fit line through the data. However we would like to be able to see the 95% and 5% confidence intervals for the line of best-fit (the curves below). What these mean is that the true line has 95% probability of being below the upper curve and 95% probability of being above the lower curve. How can I calculate these curves? I have already read wikipedia etc. and done some googling but I haven't found understandable mathematical equations to be able to calculate this. Edit: here is the essence of what I have right now. --sample data create table #lr (x real not null, y real not null) insert into #lr values (0,1) insert into #lr values (4,9) insert into #lr values (2,5) insert into #lr values (3,7) declare @slope real declare @intercept real --calculate slope and intercept select @slope = ((count(*) * sum(x*y)) - (sum(x)*sum(y)))/ ((count(*) * sum(Power(x,2)))-Power(Sum(x),2)), @intercept = avg(y) - ((count(*) * sum(x*y)) - (sum(x)*sum(y)))/ ((count(*) * sum(Power(x,2)))-Power(Sum(x),2)) * avg(x) from #lr Thank you in advance.

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  • mysql/algorithm: Weighting an average to accentuate differences from the mean

    - by Sai Emrys
    This is for a new feature on http://cssfingerprint.com (see /about for general info). The feature looks up the sites you've visited in a database of site demographics, and tries to guess what your demographic stats are based on that. All my demgraphics are in 0..1 probability format, not ratios or absolute numbers or the like. Essentially, you have a large number of data points that each tend you towards their own demographics. However, just taking the average is poor, because it means that by adding in a lot of generic data, the number goes down. For example, suppose you've visited sites S0..S50. All except S0 are 48% female; S0 is 100% male. If I'm guessing your gender, I want to have a value close to 100%, not just the 49% that a straight average would give. Also, consider that most demographics (i.e. everything other than gender) does not have the average at 50%. For example, the average probability of having kids 0-17 is ~37%. The more a given site's demographics are different from this average (e.g. maybe it's a site for parents, or for child-free people), the more it should count in my guess of your status. What's the best way to calculate this? For extra credit: what's the best way to calculate this, that is also cheap & easy to do in mysql?

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  • regressions with many nested categorical covariates

    - by eric
    I have a few hundred thousand measurements where the dependent variable is a probability, and would like to use logistic regression. However, the covariates I have are all categorical, and worse, are all nested. By this I mean that if a certain measurement has "city - Phoenix" then obviously it is certain to have "state - Arizona" and "country - U.S." I have four such factors - the most granular has some 20k levels, but if need be I could do without that one, I think. I also have a few non-nested categorical covariates (only four or so, with maybe three different levels each). What I am most interested in is prediction - given a new observation in some city, I would like to know the relevant probability/dependent variable. I am not interested as much in the related inferential machinery - standard deviations, etc - at least as of now. I am hoping I can afford to be sloppy. However, I would love to have that information unless it requires methods that are more computationally expensive. Does anyone have any advice on how to attack this? I have looked into mixed effects, but am not sure it is what I am looking for.

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  • algorithm - How to sort a 0/1 array with 2n/3 comparisons?

    - by Jackson Tale
    In Algorithm Design Manual, there is such an excise 4-26 Consider the problem of sorting a sequence of n 0’s and 1’s using comparisons. For each comparison of two values x and y, the algorithm learns which of x < y, x = y, or x y holds. (a) Give an algorithm to sort in n - 1 comparisons in the worst case. Show that your algorithm is optimal. (b) Give an algorithm to sort in 2n/3 comparisons in the average case (assuming each of the n inputs is 0 or 1 with equal probability). Show that your algorithm is optimal. For (a), I think it is fairly easy. I can choose a[n-1] as pivot, then do something like in quicksort partition, scan 0 to n - 2, find the middle point where left side is all 0 and right side is all 1, this take n - 1 comparisons. But for (b), I can't get a clue. It says "each of the n inputs is 0 or 1 with equal probability", so I guess I can assume the numbers of 0 and 1 equal? But how can I get a result which is related to 1/3? divide the whole array into 3 groups? Thanks

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