Leetcode Coding Challenge Using Lambda

• Ability to register and sign in to the application
• Present the problem (questions)
• Provide an interface for the end-users to write, compile and execute code.

Guidelines

• The whole system should be implemented in 3 to 4 days
• Reuse as much as you can when it comes to technology or service. DON'T build anything new unless until it doesn't exist
• ZERO standing cost for the infrastructure. Pay ONLY for what you use and pay as MINIMAL you can

Technologies & Runtime Services

• Bootstrap + JQuery (UI)
• AWS Lambda
• AWS API Gateway
• AWS Dynamodb
• AWS Dynamodb streams
• AWS Simple email service (SES)
• AWS S3 (For static site hosting)

High-level Architecture

• Registration & sign-in
• Compile & Execute
• Score Calculator
• Winner Predictor
  

Registration & Sign-in Flow

We have a single lambda function that takes care of registering a new user
and authenticating an existing user in the application.

• Part of registration flow user enters “FirstName”, “LastName” & “Email
  address” which is passed in as “QueryParam” to API gateway. Gateway
  invokes a Lambda function which takes care of saving the user details in
  DynamoDb, generating a random registration code (UUID based) and
  sending it in an email to the registered email address
• The sign-in flow will authenticate the user with the passed in email address and UUID using the Lambda function
  

Compile & Execute

• QuestionsId
• User types code
• Registration code

API gateway will invoke the lambda function which will get information
about the question/problem statement from the “Questions” table in
Dynamodb (data looks like below).

From the table data, we take the “GeneratedCode” attribute value and
replace various tokens like — name — , — codeworking — & — testcases
— with the class name (generated), the user typed code, and test cases
(from columns “Test_1”… “Test_N”). Here is how the replaced code will
look:

// --name--
public class f_sdsds_a11223_dd {
    
    // --codeworking--
    static class Solution{
      public static int calculate(String s){
       int count=0;
       int index=0;
          while(true){
              try{
                  s.charAt(index++);
                  count++;
              }catch(Exception e){
                  break;
              }
          }         
          return count;
      }
   }
public static void main(String args[]) {        
        int actual=0;
        int expected = 3;
        String input = null;
        // --testcases--       
        input = "Hi1";
        expected = 3;
        actual = Solution.calculate(input);        
        assert actual == expected :  String.format("--helper-- Input %s -> Expected = %s but got = %s", input, expected, actual);
    }
}

Since Lambda containers don't have JDK installed we have to package
“tools.jar” inside the deployment package so we can run the compilation
inside the containers, here is the code snippet on how we run compile
inside lambda:

try (StringInputStream inputStream = new StringInputStream(data);
     ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
     ByteArrayOutputStream errorOutputStream = new ByteArrayOutputStream()) {
    Class<?> javacTool = Class.forName("com.sun.tools.javac.api.JavacTool");
    Method create = javacTool.getMethod("create");
    JavaCompiler compiler = (JavaCompiler) create.invoke(null);
    compiler.run(inputStream, outputStream, errorOutputStream, path.toFile().getAbsolutePath());
    return new ProcessResponse(replaceNewLine(outputStream), replaceNewLine(errorOutputStream));
} catch (Exception e) {
    return new ProcessResponse(null, "Compilation error --> " + e.getMessage());
}

java -ea -cp /tmp/f_sdsds_a11223_dd 

Score Calculator

We utilize DynamoDB streams to update the total points and total execution time for any given user. A lambda function listens to all updates done on “UserSubmission” table and performs these calculations once the
submission is successful.

Winner Predictor

To find the winners, we run a simple query to get the top 3 rows in
“UserStastics” table generated by sorting “TotalPoints” column in
descending order and “TotalExecution” column in ascending

Demo

I know this doesn't solve all the use cases that a popular code challenge websites like “leetcode” or “topcoder” solves. But this is a good start & foundation which we can keep adding on.

Previously published at