## Converting Java BufferedImage to OpenCV Mat and vice versa

The code below allows us to convert BufferedImage to Mat (in OpenCV) and vice versa.

This becomes handy when getting images from network or certain other sources:

## HackerRank: Lisa’s Workbook

### Problem

Lisa just got a new math workbook. A workbook contains exercise problems, grouped into chapters.

• There are n chapters in Lisa’s workbook, numbered from 1 to n.
• The i-th chapter has ti problems, numbered from 1 to ti.
• Each page can hold up to k problems. There are no empty pages or unnecessary spaces, so only the last page of a chapter may contain fewer than k problems.
• Each new chapter starts on a new page, so a page will never contain problems from more than one chapter.
• The page number indexing starts at 1.

Lisa believes a problem to be special if its index (within a chapter) is the same as the page number where it’s located. Given the details for Lisa’s workbook, can you count its number of special problems?

Note: See the diagram in the Explanation section for more details.

Input Format

The first line contains two integers n and k — the number of chapters and the maximum number of problems per page respectively.
The second line contains n integers t1,t2,,tn where ti denotes the number of problems in the ii-th chapter.

Constraints

• 1n,k,ti100

Output Format

Print the number of special problems in Lisa’s workbook.

Sample Input

5 3
4 2 6 1 10


Sample Output

4


Explanation

The diagram below depicts Lisa’s workbook with n=chapters and a maximum of k=3 problems per page. Special problems are outlined in red, and page numbers are in yellow squares.

There are 4 special problems and thus we print the number 4 on a new line.

## HackerRank: Manasa and Stones

### Problem

Manasa is out on a hike with friends. She finds a trail of stones with numbers on them. She starts following the trail and notices that two consecutive stones have a difference of either a or b. Legend has it that there is a treasure trove at the end of the trail and if Manasa can guess the value of the last stone, the treasure would be hers. Given that the number on the first stone was 0, find all the possible values for the number on the last stone.

Note: The numbers on the stones are in increasing order.

Input Format
The first line contains an integer T, i.e. the number of test cases. T test cases follow; each has 3 lines. The first line contains nn (the number of stones). The second line contains a, and the third line contains b.

Output Format
Space-separated list of numbers which are the possible values of the last stone in increasing order.

Constraints
1T10
1n,a,b10^3

Sample Input

2
3
1
2
4
10
100


Sample Output

2 3 4
30 120 210 300


Explanation

All possible series for the first test case are given below:

1. 0,1,2
2. 0,1,3
3. 0,2,3
4. 0,2,4

Hence the answer 2 3 4.

Series with different number of final steps for second test case are the following:

1. 0, 10, 20, 30
2. 0, 10, 20, 120
3. 0, 10, 110, 120
4. 0, 10, 110, 210
5. 0, 100, 110, 120
6. 0, 100, 110, 210
7. 0, 100, 200, 210
8. 0, 100, 200, 300

Hence the answer 30 120 210 300.

### Solution

Original solution source

System design is a very broad topic. Even a software engineer with many years of working experience at top IT company may not be an expert on system design. If you want to become an expert, you need to read many books, articles, and solve real large scale system design problems. This repository only teaches you to handle the system design interview with a systematic approach in a short time. You can dive into each topic if you have time. Of course, welcome to add your thoughts!

• Clarify the constraints and identify the user cases Spend a few minutes questioning the interviewer and agreeing on the scope of the system. Remember to make sure you know all the requirements the interviewer didn’t tell your about in the beginning. User cases indicate the main functions of the system, and constraints list the scale of the system such as requests per second, requests types, data written per second, data read per second.
• High-level architecture design Sketch the important components and the connections between them, but don’t go into some details. Usually, a scalable system includes web server (load balancer), service (service partition), database (master/slave database cluster plug cache).
• Component design For each component, you need to write the specific APIs for each component. You may need to finish the detailed OOD design for a particular function. You may also need to design the database schema for the database.

### Basic Knowledge about System Design:

Here are some articles about system design related topics.

Of course, if you want to dive into system related topics, here is a good collection of reading list about services-engineering, and a good collection of material about distributed systems.

Company Engineering Blogs:

If you are going to have an onsite with a company, you should read their engineering blog.

### Products and Systems:

The following papers/articles/slides can help you to understand the general design idea of different real products and systems.

### Hot Questions and Reference:

There are some good references for each question. The references here are slides and articles.
Design a CDN network Reference:

Design a Google document system Reference:

Design a random ID generation system Reference:

Design a key-value database Reference:

Design the Facebook news feed function Reference:

Design the Facebook timeline function Reference:

Design a function to return the top k requests during past time interval Reference:

Design an online multiplayer card game Reference:

Design a graph search function Reference:

Design a picture sharing system Reference:

Design a search engine Reference:

Design a recommendition system Reference:

Design a tinyurl system Reference:

Design a garbage collection system Reference:

Design a scalable web crawling system Reference:

Design the Facebook chat function Reference:

Design a trending topic system Reference:

Design a cache system Reference:

### Object Oriented Design:

#### Tips for OOD Interview

Clarify the scenario, write out user cases Use case is a description of sequences of events that, taken together, lead to a system doing something useful. Who is going to use it and how they are going to use it. The system may be very simple or very complicated. Special system requirements such as multi-threading, read or write oriented.
Define objects Map identity to class: one scenario for one class, each core object in this scenario for one class. Consider the relationships among classes: certain class must have unique instance, one object has many other objects (composition), one object is another object (inheritance). Identify attributes for each class: change noun to variable and action to methods. Use design patterns such that it can be reused in multiple applications.

Useful Websites

Original Source

## UniRest

Unirest is a set of lightweight HTTP libraries available in multiple languages, built and maintained by the Mashape team.

Unirest.post("http://httpbin.org/post")
.queryString("name", "Mark")
.field("last", "Polo")
.asJson()

## Features

• Make GET, POST, PUT, PATCH, DELETE, HEAD, OPTIONS requests
• Both syncronous and asynchronous (non-blocking) requests
• It supports form parameters, file uploads and custom body entities
• Easily add route parameters without ugly string concatenations
• Supports gzip
• Supports Basic Authentication natively
• Customizable timeout, concurrency levels and proxy settings
• Customizable default headers for every request (DRY)
• Customizable HttpClient and HttpAsyncClient implementation
• Automatic JSON parsing into a native object for JSON responses
• Customizable binding, with mapping from response body to java Object

## Installing

Is easy as pie. Kidding. It’s about as easy as doing these little steps:

### With Maven

You can use Maven by including the library:

<dependency>
<groupId>com.mashape.unirest</groupId>
<artifactId>unirest-java</artifactId>
<version>1.4.7</version>
</dependency>

There are dependencies for Unirest-Java, these should be already installed, and they are as follows:

<dependency>
<groupId>org.apache.httpcomponents</groupId>
<artifactId>httpclient</artifactId>
<version>4.3.6</version>
</dependency>
<dependency>
<groupId>org.apache.httpcomponents</groupId>
<artifactId>httpasyncclient</artifactId>
<version>4.0.2</version>
</dependency>
<dependency>
<groupId>org.apache.httpcomponents</groupId>
<artifactId>httpmime</artifactId>
<version>4.3.6</version>
</dependency>
<dependency>
<groupId>org.json</groupId>
<artifactId>json</artifactId>
<version>20140107</version>
</dependency>

If you would like to run tests, also add the following dependency along with the others:

<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>2.4</version>
<scope>test</scope>
</dependency>

### Without Maven

Alternatively if you don’t use Maven, you can directly include the JAR file in the classpath:http://oss.sonatype.org/content/repositories/releases/com/mashape/unirest/unirest-java/1.4.7/unirest-java-1.4.7.jar

Don’t forget to also install the dependencies (org.json, httpclient 4.3.6, httpmime 4.3.6,httpasyncclient 4.0.2) in the classpath too.

There is also a way to generate a Unirest-Java JAR file that already includes the required dependencies, but you will need Maven to generate it. Follow the instructions at http://blog.mashape.com/post/69117323931/installing-unirest-java-with-the-maven-assembly-plugin

## Creating Request

So you’re probably wondering how using Unirest makes creating requests in Java easier, here is a basic POST request that will explain everything:

HttpResponse<JsonNode> jsonResponse = Unirest.post("http://httpbin.org/post")
.queryString("apiKey", "123")
.field("parameter", "value")
.field("foo", "bar")
.asJson();

Requests are made when as[Type]() is invoked, possible types include Json, Binary, String, Object.

If the request supports and it is of type HttpRequestWithBody, a body it can be passed along with.body(String|JsonNode|Object). For using .body(Object) some pre-configuration is needed (see below).

If you already have a map of parameters or do not wish to use seperate field methods for each one there is a.fields(Map<String, Object> fields) method that will serialize each key – value to form parameters on your request.

.headers(Map<String, String> headers) is also supported in replacement of multiple header methods.

Full Documentation @ unirest.io

## jSoup: Java HTML Parser

jsoup is a Java library for working with real-world HTML. It provides a very convenient API for extracting and manipulating data, using the best of DOM, CSS, and jquery-like methods.

jsoup implements the WHATWG HTML5 specification, and parses HTML to the same DOM as modern browsers do.

• scrape and parse HTML from a URL, file, or string
• find and extract data, using DOM traversal or CSS selectors
• manipulate the HTML elements, attributes, and text
• clean user-submitted content against a safe white-list, to prevent XSS attacks
• output tidy HTML

jsoup is designed to deal with all varieties of HTML found in the wild; from pristine and validating, to invalid tag-soup; jsoup will create a sensible parse tree.

## Example

Fetch the Wikipedia homepage, parse it to a DOM, and select the headlines from theIn the news section into a list of Elements (online sample):

Document doc = Jsoup.connect("http://en.wikipedia.org/").get();
Elements newsHeadlines = doc.select("#mp-itn b a");

## Open source

jsoup is an open source project distributed under the liberal MIT license. The source code is available at GitHub.

## Getting started

1. Download the jsoup jar (version 1.8.3)
2. Read the cookbook introduction
3. Enjoy!

## Problem Statement

Given an array of integers, every element appears twice except for one. Find that single one.

Note: Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

Example :

Input : [1 2 2 3 1]
Output : 3

## Solution

Logic:

The basic logic that A XOR A = 0 means that means all the doubles will be XOR’ed out to 0 and the remaining number will be the result of the XOR.

## Problem Statement

Sherlock Holmes suspects his archenemy, Professor Moriarty, is once again plotting something diabolical. Sherlock’s companion, Dr. Watson, suggests Moriarty may be responsible for MI6’s recent issues with their supercomputer, The Beast.

Shortly after resolving to investigate, Sherlock receives a note from Moriarty boasting about infecting The Beastwith a virus; however, he also gives him a clue—a number, NN. Sherlock determines the key to removing the virus is to find the largest Decent Number having NN digits.

A Decent Number has the following properties:

1. Its digits can only be 3‘s and/or 5‘s.
2. The number of 3‘s it contains is divisible by 5.
3. The number of 5‘s it contains is divisible by 3.
4. If there are more than one such number, we pick the largest one.

Moriarty’s virus shows a clock counting down to The Beast‘s destruction, and time is running out fast. Your task is to help Sherlock find the key before The Beast is destroyed!

Constraints
1T201≤T≤20
1N1000001≤N≤100000

Input Format

The first line is an integer, TT, denoting the number of test cases.

The TT subsequent lines each contain an integer, NN, detailing the number of digits in the number.

Output Format

Print the largest Decent Number having NN digits; if no such number exists, tell Sherlock by printing -1.

Sample Input

4
1
3
5
11


Sample Output

-1
555
33333
55555533333


Explanation

For N=1, there is no decent number having 1 digit (so we print 1−1).
For N=3, 555 is the only possible number. The number 5 appears three times in this number, so our count of 5‘s is evenly divisible by 3 (Decent Number Property 3).
For N=5, 33333 is the only possible number. The number 3 appears five times in this number, so our count of 3‘s is evenly divisible by 5 (Decent Number Property 2).
For N=11, 55555533333 and all permutations of these digits are valid numbers; among them, the given number is the largest one.

## Easy Rules: Java™ rules engine

Easy Rules is a Java rules engine inspired by an article called Should I use a Rules Engine? by Martin Fowler in which he says:

You can build a simple rules engine yourself. All you need is to create a bunch of objects with conditions and actions, store them in a collection, and run through them to evaluate the conditions and execute the actions.

## Core features

• Lightweight library and easy to learn API
• POJO based development with annotation programming model
• Useful abstractions to define business rules and apply them easily with Java
• The ability to create composite rules from primitive ones
• Dynamic rule configuration at runtime using JMX

# Hello World tutorial

This tutorial shows how to use Easy Rules in a very simple application. The goal is to ask the user if he is a friend of duke and says ‘Hello duke’s friend!’ only if he replies ‘yes’.

Based on this requirement, the rule is pretty straightforward :

• The condition is that the user input must be equal to ‘yes’
• The action is to say ‘Hello duke’s friend!’ to the user

First, let’s create a rule class:

@Rule(name = "Hello World rule",
description = "Say Hello to duke's friends only")
public class HelloWorldRule {

/**
* The user input which represents the data
* that the rule will operate on.
*/
private String input;

@Condition
public boolean checkInput() {
//The rule should be applied only if
//the user's response is yes (duke friend)
return input.equalsIgnoreCase("yes");
}

@Action
public void sayHelloToDukeFriend() throws Exception {
//When rule conditions are satisfied,
//prints 'Hello duke's friend!' to the console
System.out.println("Hello duke's friend!");
}

public void setInput(String input) {
this.input = input;
}

}


Then, we have to register an instance of this rule in a Easy Rules engine and launch the tutorial with the following class:

public class Launcher {

public static void main(String[] args) {

Scanner scanner = new Scanner(System.in);
System.out.println("Are you a friend of duke?[yes/no]:");
String input = scanner.nextLine();

/**
* Declare the rule
*/
HelloWorldRule helloWorldRule = new HelloWorldRule();

/**
* Set business data to operate on
*/
helloWorldRule.setInput(input.trim());

/**
* Create a rules engine and register the business rule
*/
RulesEngine rulesEngine = aNewRulesEngine().build();

rulesEngine.registerRule(helloWorldRule);

/**
* Fire rules
*/
rulesEngine.fireRules();

}
}

You would get the following output:

Are you a friend of duke? [yes/no]:
yes
INFO: Rule 'Hello World rule' triggered.
Hello duke's friend!
INFO: Rule 'Hello World rule' performed successfully.


Source