I appeared in 5 rounds of interview including one round of HackerRank test. The questions are as follows:

HackerRank Test: 4 questions. Can't remember the question exactly.

• 2 LC Easy - One question from array, other question Valid Email counts given in an array with the given pattern
• 1 LC Medium - Greedy algorigm array
• 1 LC Hard - DP

Out of 4 I was able to solve first 3. Few test cases I missed for the 4th question.

Interview 1: System Design
Design a instant messaging system (desktop/mobile application). Design sender and receiver. Scale up the system to handle a milion of message per second. Keeping a history of the message and search functionality. Secure the messages with a encryption. After the encryption how search functionality is affected and how it can be resolved.

Interview 2: DS & Algo
Given a char array with some words separated by a space. Reverse the positions of the words without using any inbuilt function and space complexity should be O(1).
Input: "Hello! Have a good day."
output: "day. good a Have Hello!"

Interview 3: DS & ALgo
Design a growing list (like arraylist in Java). Implement CRUD operation on the list and their complexities. Performance improvent areas. Any other design if you can think of..

Interview 4: Project specific discussion
Architecture of my current project. Design pattern of Microservices. Few question on Asynchronous messaging system (kafka).

After completing 4 rounds of interview, I followed up multiple times but did not get any response from the recruitment HR. Any suggestions?