Interview Questions

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  what happens in the background from the time you press the Power button until the Linux login prompt appears?
  operating system linux kernel bios bash •

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  It's very useful.. Thanks!

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  Design Uber Backend
  object-oriented design system design •

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  I found a video about how to design a uber https://www.youtube.com/watch?v=oLmAEsMZZP0, but it's in Chinese.

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  Design a Logistics System
  object-oriented design system design •

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  @shashibk11 Very nice. Patiently waiting

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  Parking Lot Design Using OO Design
  object-oriented design amazon google •

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  I didn't follow the "Current Scenario" because I had this done from another forum.

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  Google onsite interview questions
  google •

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  @wtb_sw

  If it's a number instead of an array, we can do this:

  def findMissingNum(num): while num: if ((num%10)*111) != (num%1000): return num%10 num //= 1000 return -1
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  Difference between forking and multithreading
  operating system •

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  @WenhuiZhang *whether

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  5 flavors of singleton!
  object-oriented design design pattern singleton enum thread-safety •

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  When I first learnt all five flavors 10 months ago, I was struggling to memorize all of them. 10 months later, I use only enum, simple and short.

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  Difference between BIOS and Kernel
  operating system •

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  BIOS is literally the first piece of (programmable) software that run on the CPU when the power button is pushed (i.e. reset vector, chipset initialization, board/platform specific initialization, etc.). It can be described as a bootloader for other bootloaders since there are various hardware/chipset components (e.g. USB controller, AHCI/SATA controller) that have to be initialized before we can even search for the bootloader (e.g. grub) on removable media (e.g. USB sticks, SATA drives).

  After the bootloader has been found and loaded into memory by the BIOS, it (the bootloader) finds and load the OS kernel into memory and executes that.

  So in short, the BIOS functions similarly to a bootloader, and the kernel is a (relatively) high-level system resource and process management software.

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  This topic is deleted!
  

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  Design an online hotel booking system
  object-oriented design •

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  It is my first time practicing the ood question. I don't quite understand, why under hotel class, we use userid and hotelid of type int instead of user of type User and hotel of type Hotel, i thought thats the reason why we have those classes.

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  Social Network Most active users
  object-oriented design design pattern •

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  How to/best practices to write a thread safe code?
  operating system •

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  @andro14
  First objective to thread safe code is to not write concurrent code in the first place. If you can avoid it, you can eliminate risk.

  So if you do have to write threaded code, then apply functional programming where ever possible. Functional programming is trying to solve the issue of state changes. If your program can only create and delete objects and can never modify objects. You have functional programming. Look up event sourcing, banks use this.

  Next step, say you did have two threads, you have to identify what data is being transferred between the two threads. Hence, shared memory. That is the critical section or possible race conditions that could occur. In general you protect them with locks and mutexes, semaphore.

  To test conccurency, the best method to date is to use a shuffle technique. You want to mimic how the operating system gives time to your thread to execute. So you can create two threads, and tell them to stop, start, sleep randomly shuffled between each other. Then in between each shuffle, a random number of milliseconds to wait. For more, read clean code, there is a chapter just for concurrency.

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  Design an online book reader system
  object-oriented design •

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  @vishal.gupta4081 I also would like to know how to handle multiple active users on the same book. Thank you.

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  Design Pastebin
  system design pastebin •

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  I don't understand why there need to be two tables in the database. It seems that a key-value storage is sufficient -- the key is the hash of the text and the value contains the text, the upload time, and the expiration period.

  What is in my mind is that we need a Web server, which accepts the uploaded text and saves the text into the above key-value storage. We also need a daemon, which scans over the storage pass and pass. For each item, if it is expired (current_time >= upload_time + expiration_period), the daemon removes the item; otherwise, the daemon creates a timer to wake it up at the expiration time to remove the item.

  package main import ( "timer" ) type TimeoutHandler struct { actionList map[string]int nextPass chan int } func NewTimeoutHandler() *TimeoutHandler { return &TimeoutHandler{ actionList: make(map[string]int), nextPass: make(chan int, 100), } } func (th *TimeoutHandler) RemoveItemWhenExpired(string key, Value value) { th.actionList[key] = 1 t := timer.NewTimer(value.ExpireTime) go func(key string) { <-t.C storage.Delete(key) delete(th.actionList, key) if len(th.actionList) <= 0 { th.nextPass <- 1 // trigger the scan of the next pass. } t.Close() }(key) } func (th *TimeoutHandler) Run() { for { for s := storage.NewScanner(); !s.Done(); s.Next() { th.RemoveItemWhenExpired(s.Key(), s.Value()) } <-th.nextPass } } func main() { th := NewTimeoutHandler() th.Run() }
• A

  Design the build tool like gradle
  object-oriented design amazon interview •

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  Design Reddit
  system design object-oriented design •

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  If I decide to store all the data in some NoSQL database like Firebase, then I might want to have list<comments_id> under Post, and list<comments_id> under Comment if it has children comments.

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  Design BookMyShow
  system design bookmyshow •

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  e-Commerce Design and Implementation
  object-oriented design python interview interview-question •

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  Designing a Ecommerce Website in 3 months will have its own challenges. First of we need to decide on the stack to be used. For current question, i am going with NoSQL/Java and NodeJS Stack.

  I am assuming the following system will already be bought from off market.

  Merchandising System (Pricing/Promotion) Auditing System Inventory Management System. Messaging System (Integration between various system) Content Delivery Network Content Merchandising System

  We will have a NoSQL DB at the back-end which will have the following structure:

  Key/Value pair for the following:
  a. Product Id to Product Info
  b. Category Id to Category Info
  c. Product to Category Mapping
  d. UPC Number to Product Mapping
  e. UPC Number to Availability Mapping
  f. Promotion Id to PromotionInfo
  g. Promotion Id to UPC Number Mapping
  h. Category Id to Assets Mapping
  i. Product Id to Assets Mapping
  j. Bag Id to Customer Id Mapping
  h. Customer Id to Customer Profile Mapping
  k. Bag Id to Bag detail Mapping
  l. Order ID to Order Detail Creating a back end Application layer using Java API to create the front end micro services (REST base Microservices). The components will be broken down based on the functionality Product/Category/Assets/Promotion/Bag/Checkout/Profile (this will help in long run to make changes faster and have separate team for each component which will help in CI/CD process and testing/debugging issue in production. Create a common Core Component which will have the following things:
  a. Logging Framework
  b. Authentication
  c. REST Core
  d. Contextual Validation
  e. API Validation
  f. Filters for REST
  g. Pagination for REST.
  Making sure that this common component is injected in each module component as mentioned in step 2. Creating a Nodejs layer which will do Isomorphic UI, more like single page app(To be SEO friendly) and to improve the page performance.

  My website wont be having real time availability but that can be implemented using the messaging framework while CDN can help in improving the performance of the site.
  One thing that will be different than others will be the Search functionality which will be based on Googles Tensor Flow Machine Learning where based on the segment of a customer i.e. age/location/etc. We would add a feature that the search result have a Like button below each Product Thumbnail and as customer likes a product the search result can provide more products in the same attributes along with the existing customer profile.

  Again there can be more improvement but this is what i was in the first 3 months.
  Search a product -> Like --> Improve the Search result --> Add to Bag --> Profile Login --> Checkout --> Email or Text Confirmation.

  Hopefully i can levrage Amazon fulfillment service to deliver the product to customer door.

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  New question - Google interview
  google •

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  https://www.geeksforgeeks.org/minimum-number-of-swaps-required-for-arranging-pairs-adjacent-to-each-other/

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  Design an elevator system
  object-oriented design •

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  In my approach in Java I calculated the waiting time of the elevator user by letting the elevator wait at the floor from which it has been called the most.
  Further ideas and improvements are very welcome :)

  /** * Creates an {@link Elevator} object and calls the {@link Elevator#callElevator} method */ class LiftController { public static void main(String... aArgs) { Elevator elevator = new Elevator(); elevator.callElevator(Floor.FIRST, Floor.SIXT); elevator.callElevator(Floor.THIRD, Floor.SEVENTH); elevator.callElevator(Floor.FIRST, Floor.SECOND); elevator.callElevator(Floor.FIRST, Floor.SEVENTH); elevator.callElevator(Floor.FIRST, Floor.FIFTH); elevator.callElevator(Floor.SECOND, Floor.SEVENTH); elevator.callElevator(Floor.SEVENTH, Floor.GROUND); elevator.callElevator(Floor.THIRD, Floor.FOURTH); elevator.callElevator(Floor.SECOND, Floor.GROUND); elevator.callElevator(Floor.SEVENTH, Floor.GROUND); elevator.callElevator(Floor.FIRST, Floor.FIFTH); } } import java.util.*; public class Elevator { private static Floor waitingFloorPosition; protected static boolean firstUsage = true; private static Map<Floor,Integer> usageMap; /** * If the elevator is called for the first time, the {@link #initializeElevator()} is called * Writes the console output with the waiting time computed by {@link #computeWaitingTime(Floor calledFrom)} * Sets the {@link #waitingFloorPosition} based on the most called calledFrom floor, computed by computeWaitingTime(Floor calledFrom) * * @param calledFrom * @param calledTo */ public void callElevator(Floor calledFrom, Floor calledTo){ if(firstUsage) { initializeElevator(); } System.out.println("Drove from "+ calledFrom.toString() + " floor to "+ calledTo.toString() + " floor. The waiting time was " + computeWaitingTime(calledFrom)); computeWaitingFloorPosition(calledFrom); } /** * Puts the floors and the number this floors have been called from into a Map<Floor,Integer> * Calls {@link MapUtils#sortByValue} to sort the map by its values * Sets the last entry in the sorted {@link #usageMap} as {@link #waitingFloorPosition} * * @param calledFrom */ private void computeWaitingFloorPosition(Floor calledFrom) { if(usageMap.containsKey(calledFrom)){ usageMap.put(calledFrom,usageMap.get(calledFrom) + 1); } else if (!usageMap.containsKey(calledFrom)){ usageMap.put(calledFrom,1); } usageMap = MapUtils.sortByValue(usageMap); waitingFloorPosition = MapUtils.getLast(usageMap).getKey(); } /** * Calculates the waiting time of the elevator user by substracting the {@link #waitingFloorPosition} from the calledFrom floor position * @param calledFrom * @return Long the waiting time */ private Long computeWaitingTime(Floor calledFrom){ return Math.abs(getNumericValueOfFloor(calledFrom) - getNumericValueOfFloor(waitingFloorPosition)); } /** * Assigns a long value to every floor so its possible to compute a waiting time * * @param floor * @return */ private Long getNumericValueOfFloor(Floor floor){ Long numericValue = 0l; switch (floor) { case GROUND: numericValue = 0l; break; case FIRST: numericValue = 1l; break; case SECOND: numericValue = 2l; break; case THIRD: numericValue = 3l; break; case FOURTH: numericValue = 4l; break; case FIFTH: numericValue = 5l; break; case SIXT: numericValue = 6l; break; case SEVENTH: numericValue = 7l; break; default: numericValue = 0l; break; } return numericValue; } /** * If the elevator is called for the first time: * The {@link #waitingFloorPosition} is set to {@link Floor#GROUND} * A new instance of {@link #usageMap} Map<Floor,Integer> is created * The static boolean {@link #firstUsage} is set to false so that the elevator can't be initialized a second time */ private void initializeElevator() { waitingFloorPosition = Floor.GROUND; usageMap = new HashMap<>(); firstUsage = false; } } import java.util.Collections; import java.util.Comparator; import java.util.Iterator; import java.util.LinkedHashMap; import java.util.LinkedList; import java.util.List; import java.util.Map; /** * Contains static methods for: * * - Sorting a map by its values * - Returning the last element of a map */ public class MapUtils { /** * Returns the last entry in a Map<K, V> * @param map * @param <K> * @param <V> * @return Map.Entry<K, V> */ public static <K,V> Map.Entry<K,V> getLast(Map<K,V> map) { Iterator<Map.Entry<K,V>> iterator = map.entrySet().iterator(); Map.Entry<K, V> result = null; while (iterator.hasNext()) { result = iterator.next(); } return result; } /** * Sorts a map by its values * @param map * @param <K> * @param <V> * @return Map<K, V> */ public static <K, V extends Comparable<? super V>> Map<K, V> sortByValue(Map<K, V> map) { List<Map.Entry<K, V>> list = new LinkedList<Map.Entry<K, V>>(map.entrySet()); Collections.sort( list, new Comparator<Map.Entry<K, V>>() { public int compare(Map.Entry<K, V> o1, Map.Entry<K, V> o2) { return (o1.getValue()).compareTo( o2.getValue() ); } }); Map<K, V> result = new LinkedHashMap<K, V>(); for (Map.Entry<K, V> entry : list) { result.put(entry.getKey(), entry.getValue()); } return result; } } /** Enum class with all possible floors the elevator can go to or be called from*/ public enum Floor { GROUND, FIRST, SECOND, THIRD, FOURTH, FIFTH, SIXT, SEVENTH }