Solution to Coding Challenges in Hacker Rank

This repository contains the solutions to some of the coding challenges published in Hacker Rank. I do coding challenges in my spare time and I keep my solutions all in one place.

In order to speed up the writing and testing of the solution during development I have added a couple of wrapper classes simplify the execution of test cases via JUnit. The main method of the solution is a function that has the same signature (except for public static which became protected) of the method stub provided in the Hacker Rank's code editor to solve the challenge. Hence, it can be easily copied in.

Running Tests

You need to have Java and Maven installed (JDK 11 and Maven 3.0.2 will do).

    mvn test

Object Model

Challenge Solution

This interface abstracts away the contract of a solution and it is specialised by two template types: ChallengeInput and ChallengeResult.

public interface ChallengeSolution<I extends ChallengeInput, R extends ChallengeResult> {

    public boolean accept(String testCaseId);
    public R execute(I input);
}

There are two main methods in this interface:

• accept(String) which can be used to skip some test-cases by a given implementation (for instance in order to avoid long-runs or marginal cases not covered by a solution);
• execute(I) which unpacks the input executes the method that implements the solution and packs the output into a ChallengeResult implementation.

The ChallengeInput and ChallengeResult interfaces act only as type constraints and are used to provide a generic approach to manage the variety of parameters required by the specific challenges from within the testing environment.

This interface is implementedd by DefaultChallengeSolution which provides a default implementation for accept(String) always returning true and DefaultChallengeResult<V> for the result class. This class implements a simple container for the one single parameter and overrides the equals(Object) method to perform equality check by checking that the wrapped value is equal or both of the two values are null.

This leaves the implementation of a specific problem solution limited to define the execute(Input input) and the solution method itself. An example of the pattern is the following:

public class MySolution extends DefaultChallengeSolution<Input, Integer> {


    public DefaultChallengeResult<Integer> execute(Input input) {

        int result = this.mySolutionMethod(input.getN(), input.getS());
        return new DefaultChallengeResult<>(result);
    }

    protected int mySolutionMethod(int n, String s) {... }
}

Here Input is defined as a simple pojo with two attribute (n and s) and the associated getters:

public class Input implements ChallengeInput {

    protected int n;
    protected String s;
    
    public Input(int n, String s) {
       this.n = n;
       this.s = s;
    }
    
    public int getN() { return this.n; }
    public String getS() { return this.s; }
}

Unit Test Automation

The unit test automation is primarily implemented via the class SolutionTestBase. The class is defined below:

public abstract class SolutionTestBase<I extends ChallengeInput, R extends ChallengeResult> {


    protected InputStream getTestCase(String testCaseId) { ... }

    protected void runTestCase(String testCaseId) throws IOException { .... }

    protected abstract TestCase<I,R> readTestCase(InputStream testInput) throws IOException;
    
    protected abstract ChallengeSolution<I,R> getSolution();
}

The first two methods provide capabilities for retrieving the input stream associated to a test case file and execute the test with the given test case. The method runTestCase(String) does the heavy lifting of:

• accessing the input stream associated to the test case identifier
• retrieving an instance of ChallengeSolution<I,R> and verifying that the current instance can execute the specified test case
• invoking the method readTestCase(InputStrem) which produces an instance of TestCase<I,R> which contains both the input and the expected output
• invoking ChallengeSolution.execute(I) with the given input and verifying that the result matches the expected result.

The methods readTestCase(InputStream) and getSolution() are extension points for concrete test classes to provide the logic to read the content of the file in an instance of TestCase<I,R> and the instance of the solution under test. A TestCase instance is container for both the test input data and the expected result as defined below:

public class TestCase<I extends ChallengeInput, R extends ChallengeResult> {
    
    protected I input;
    protected R result;
    
    public TestCase(I input, R result) {
        this.input = input;
        this.result = result;
    }
    
    public I getInput() { return this.input; }
    public R getResult() { return this.result; }
}

To implement a specific test class for a given solution it is sufficient to implement the two extension points and define a @Test method for each of the test cases to be run, which simply invoke runTestCase(String). An example follows below:

public class MySolutionText extends SolutionTestBase<Input, DefaultChallengeResult<Integer>> {

    @Test
    public void testCase0() throws IOException {
        this.runTestCase("my-solution/testCase0.test");
    }

    @Override
    protected TestCase<I, DefaultChallengeResult<Integer>> readTestCase(InputStream testInput) throws IOException {


            // logic to create a TestCase instance from the input stream
            // the example is based on the previous definition of input
            // which takes one integer and one string, and the result 
            // wrapping an integer (i.e. expected)

            return new TestCase<>(new Input(n,s), new DefaultChallengeResult<>(expected));
    }

    @Override 
    protected ChallengeSolution<Input, DefaultChallengeResponse<Integer>> getSolution() {
        return new MySolution();
    }
}

The string passed to the runTestCase(String) method is interpreted as a path relative to the root of the src/test/resources folder.