How to pass Amazon SDE interview

Amazon is considered to be one for the most wanted employers among software engineers who don’t work for any of the tech giants. Standing in one line with Google, Microsoft, Facebook and maybe some smaller like Twitter, Uber, Dropbox etc., it has unstoppable flow of CV’s from people passionate of working on big scale.

But is it really that cool, demanding and, in the end, rewarding? A lot of people would disagree with that, others will be neutral and there will be only few of those who will agree. For example, typical everyday job of server-side SDE II responsible for customer experience with purchasing goods can only consist of sending/receiving requests to/from internal web-services, validating input data, fixing small bugs and that’s all. Oh no, there’s one more thing – on-call rotations. So one week every few months (that depends on a team, but just to give you an idea) that employee despite of his “interesting and challenging” duties will be responsible for fixing bugs on production asap which literally means ASAP – during the weekend, in the evening, in the night – doesn’t matter.

That is why Amazon looks for people who won’t whine about such lifestyle. Amazon has a dozen of so-called “principles” (read “search criteria for new employees”) where some are contradictory to the others. Like they need employees who have a “bias for action” but are “insisting on highest standards” or who are “frugal” but “think big” and stuff like that. Interviewers will ask you about how do you match with these principles and what they’re really interested in is if you had experience working overtime, on the weekends, under pressure, overnight – in order to deliver results in short terms and fix bugs. They clearly tell you about it – if you’re weak in programming or algorithms – it does not matter if on the other hand you’re used to working overtime just to deliver results.

So how to pass Amazon interview? They will ask you about your experience and definitely will ask you to give them example where you had tight deadlines and half-finished task. They want to hear how did you work overnights and did not complain for that. If they will – you’ve passed even if your solution for their O(N^2) dynamic programming puzzle is NP-complete.

Replacing QNetworkAccessManager for the great good

Everybody using Qt for networking for small tasks will sometimes face oddities of QNetworkAccessManager. This class aims to be useful and convenient while having few quite sensible drawbacks. First one of couse is inability to use it in blocking way. What you should do instead is to create instance of QEventLoop and connect it’s quit() signal with network manager.

QNetworkAccessManager networkManager;
QEventLoop loop;
QNetworkReply *netReply = networkManager.get(resource);
connect(netReply, SIGNAL(finished()), &loop, SLOT(quit()));

This is overkill and overengineering of course. This inconveniency strikes also when you try to use it from background thread for downloading something – QNetworkAccessManager needs an event loop and it will launch one more thread – it’s own to do all the operations required.

Also it has a lot of data, methods and abilities not needed for “everyday simple network operations” like querying some API or downloading files. I don’t know anybody who wasn’t looking for a substitude for it at least once. But fortunately the solution exists.

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Resources to learn and understand parallel programming. The hard way

There’s no way other than the hard way. (c)

Parallel programming is considered as not easy or even advanced topic by many programmers. It’s the starting point for even more advanced stuff like distributed computations, reliability, CAP theorem, consensus problems and much more. Besides, deep understanding of how CPU and operating system works can help you to write less buggy software and parallel programming can help you with that too.

In this post I will focus on books describing parallel programming using 1 computer and 1 CPU using classical approaches. Neither they contain SSE instructions guides nor you will find matterials on CUDA or OpenCL. Similary you will find no resourced about Hadoop and/or MapReduce technologies and nothing about technologies supporting parallel programming out of the box like Go or Erlang.

So I will go now through all the resources which I find more or less useful. I’m not going to stick to any technology in general – the point is to understand the topic from different perspectives. The materials I’m refering to in general should not be considered as entry-level –  they require fair amount of knowledge, but nevertheless, list goes sorted starting from “easier” things.

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Implementing spellchecking in desktop application in C++

When user is supposed to enter significant amount of text in your application, it’s better to help him/her to control it with checking spelling. Basically, to check spelling you need a dictionary with words and algorithm to order these words. Also it might be useful to provide user with possible corrections for any spelling error. Here where Hunspell comes handy. It’s an open source library built on top of MySpell library and used in a significant number of projects varying from open source projects like Firefox to proprietary like OS X. It contains bindings to a number of platforms (.NET, Ruby etc.) and should be fairly easy to integrate to your project. In this post I’ll discuss how to integrate it to C++/Qt project.

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Classic Producer-Consumer in Qt/C++

Producer-Consumer is a classic pattern of interaction between two or more threads which share common tasks queue and workers who process that queue. When I came to similar task first I googled for standard approaches in Qt to solve this problem, but they were based on signals/slots plus synchronization primitives while I wanted simple and clear solution. Of course, in the end I’ve invented my own wheel and I invite you to take a look at it.

For the synchronization in Producer-Consumer it’s useful to use Mutex and some kind of WaitingEvent for synchronous waiting until mutex is acquired. In Qt you have QMutex and QWaitCondition which are all that we need.

Let’s suppose we have following data structures:

        QWaitCondition m_WaitAnyItem;
        QMutex m_QueueMutex;
        QVector<T*> m_Queue;

where T is type of messages we’re producing/consuming. So we have queue of elements being processed, mutex to secure access to the queue and wait condition to wait if the queue is empty.

For Producer-Consumer usually we need methods produce() and consume(). Let’s see how we can implement them.

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Implementing autocomplete for English in C++

When it comes to implementing autocompletion in C++ in some type of input field, the question is which algorithm to choose and where to get the source for completion. In this post I’ll try to answer both questions.

As for the algorithm, SO gives us hints about tries, segment trees and others. You can find good article about them. Author has implemented some of them in a repository called FACE (fastest auto-complete in the east). You can easily find it on GitHub. This solution is used for the autocompletion in search engine Duck-Duck-Go which should tell you how good it is. Unfortunately their solution requires dependencies on libuv and joyent http-parser, which is not good in case you need just to integrate autocompletion functionality into your C++ application, but not build auto-complete server and send queries to it. Another drawback – libuv and cpp-libface itself fails to compile in Windows which is bad in case you’re building cross-platform solution.

You can find out how to built FACE into your cross-platform C++ application below.

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Handling drag’n’drop of files in Qt under OS X

If you ever tried to handle drag’n’drop files in your Qt application, you would usually come up with the code like the following.
First of all you will need a Drop Area somewhere in your application, which will handle drops

DropArea {
  anchors.fill: parent
  onDropped: {
    if (drop.hasUrls) {
      var filesCount = yourCppModel.dropFiles(drop.urls)
      console.log(filesCount + ' files added via drag&drop')

Where yourCppModel is a model exposed to Qml in main.cpp or wherever like this:

QQmlContext *rootContext = engine.rootContext();
rootContext->setContextProperty("yourCppModel", &myCppModel);

and int dropFiles(const QList<QUrl> &urls) is just an ordinary method exposed to QML via Q_INVOKABLE attribute.

You will sure notice everything works fine unless you’re working under OS X. In OS X instead of QUrls to local files you will get something like this: file:///.file/id=6571367.2773272/. There’s a bug in Qt for that and it even looks closed, but it still doesn’t work for me that’s why I’ve implemented my own helper using mixing of Objective-C and Qt-C++ code.

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How to download huge folder from Dropbox

If you face a problem to download folder from Dropbox which contains tons of files, no known browser extension can help you. Dropbox moves each file download to it’s separate page and you can’t do it directly.

When I faced this problem I knew I would need to create my own solution and quick googling just confirmed that.

I opened javascript console and extracted all links from the folder. Then I replaced “dl=0” to “dl=1” to get actual download link.

var links = document.querySelectorAll("div.filename a")
var processed =, 
  function(link) { 
    return link["href"].replace("dl=0", "dl=1"); 

After I copied those to file links_to_download. If your processed array is too big, you can print it to console by chunks, using slice(start, end) method from Javascript. Now the problem is to download them.

I came up with wget for such problem. Linux and OS X users should have wget available (OS X users can install it via e.g. homebrew). Windows users have to download it separately, install and add to the PATH environmental variable. Additionaly I used –trust-server-names and –content-disposition parameters to save real filenames instead of dropbox hashed url. Then I faced a problem that it fails to download a file on first request and request timeout is quite big so I’ve set it to 5 seconds. Now it makes several timed-out requests, but they quickly resolve to the successful one.

wget --content-disposition --trust-server-names 
  --timeout=5 -i links_to_download

Also in order to download via https in Windows you probably will need to use “–no-check-certificate“.

Tips and tricks to improve performance of your ACM solution

Here I gathered system-programming tricks that can improve performance of your solution in C++ dramatically!

  • Use scanf/printf functions for standard IO instead of cin/cout
  • Memory-align buffers and structures to WORD size of your architecture (4 bytes for 32-bit and 8 bytes for 64-bit)
  • Use arrays instead of linked lists (to use memory block caching)
  • Avoid “if” stamements in loops
  • If-close should contain code, which is more likely to execute (if-condition == true)
  • Use inlining for short functions
  • Use objects allocated on stack but not on heap (local objects for functions instead of allocated with malloc/new)
  • Use pre-calculated hardcoded data (e.g. you can store first N prime numbers or first N Fibonacci numbers in order not to calculate them every time you need one)

Interesting issues and features of Qt programming

In this post I enlist all interesting facts and issues I’ve experienced while developing my first project in Qt (from 5.3 to 5.4 5.6)

18.02 – QSettings interface in Qml transformed bool to string and was always true on deserialization

18.02 – QByteArray returned from local scope crashed with heap corruption on return of function (destructor of QByteArray)

Qt can delete your object in it’s gui loop. QObject should have CppManaged attribute and it should be set before returning object to UI code.

26.02 QTimer can be only started when EventLoop is already running (e.g. with app.exec()) otherwise you get an error “QTimer can be started in QThread”

Drag and Drop files in OS X inserts NSUrl instead of QUrl and you have to convert it using Objective C to real filepath. You can add .m file to Qt project and write C++/Objective-C code.

23.03 QtConcurrent::mapped can accept a struct with operator() but only with inner typedef T return_type; where T is mapped type for correct QFuture<T> conversion. Also, QtConcurrent::mapped cannot be cancelled.

30.05 Qt Column allows to do animations, but has issues with stretching and ColumnLayout has no issues with stretching, but doesn’t allow you to do animations. I had to use simple anchors layout and States with Transitions to animate properties I wanted

1.06 Qt ListView has Transitions add/remove/removeDisplaced/displaced/etc, which allow to create nice animations for adding/deleting items from listview and to make UI really nice

6.06 Qt lacks standard Zip/Unzip functionality. QuaZip makes life easier, but has minor issues with compilation (linking etc).

22.07 TextInput’s EditingFinished signal fires twice in OS X

6.09 Windeployqt does not pack qml dlls into the bundle, you have to do it manually from your <Qt-bin-dir>/qml. You need to copy QtQuick and QtQuick2 directories and others which you use

19.09 Tab control is a loader and you can’t access it’s child object by Id. You have to create a property of Tab and do double binding inside and outside Tab to that property.

25.10 Repeater in GridLayout does not respect QAbstractItemModel changes, but does respect once put into Flow

23.11 Always initialize boolean fields, volatile or not. Always do that because in other case you will get tons of unpredictable behavior.

28.11 Use QVector instead of QList for most types. If you have released public version with wrong type and users depend on it, you’re screwed

12.12 QML ListView fails to update itself after sophisticated filter/remove operations. You have to manually positionViewAtBeginning() stuff

20.03 QFile in Windows does not respect flag QIODevice::Unbuffered because of “lack of native support in Windows” (bullshit) as of Qt 5.6

21.03 QProcess has super strange problems. Exiftool does not work with unicode from QProcess but works when launched via cmd.exe

2.05 QWaitCondition destructor produced a warning that it’s destroyed while inner Mutex is still locked. Was able to debug that using export QT_FATAL_WARNINGS=true in the running environment

20.07 QString::fromLatin1() truncated buffer of QByteArray parameter if the latter contained some eol/nl characters

16.12 virtual inheritance from the QObject is not supported.. Need to have stubs for calling signals

17.03 Timer in QML can have very high CPU utilization. Need to use with care