Experience - 3 YOE(2.5 relevant), SDE at Product Based SaSS Unicorn

The recruiter reached out to me and asked to apply for the Associate role if interested and I did

First Round(Hackerrank)-
2 Easy Array based questions. 2 Hours time, doable in an hour itself.
Was informed the next working day that I had cleared it and I will be informed for the next round.

The Recruiter called me after 10 days and asked to schedule the CoderPad round in the next couple of days and I agreed for the next day itself.

CoderPad Round-
2 questions - 1 Hour

1. Given a String as "aaabbcbbb" output as "a3b2c1b3".
2. Trapping Rainwater - the problem was reworded but the same solution.
   Was able to wrap it up in 30 mins

Super day - 3 tech rounds in 1 day.. all elimination and lastly HM round

Round 1 (>1 hour)
Only pseudo code -

1. Longest common substring
2. given an array of elements with each element occurring twice and one element occurring once.
   Special condition- The repeating once will occur in pair
   Example - 3 3 7 7 4 4 5 6 6
   Discussed diff O(N) solution and ultimately with a hint got to the O(logN) solution. The trick was- if mid is not the answer then apart from the mid and its repeating guy, count no of elements on the right and left of the mid element. move towards the side having odd numbers.

Then Internal implementation of HashMap - hashcode, equals, collision, and all. How after getting the hash we map to the correct bucket. All these stuff.

Then questions on OOP and figuring out how the code would behave and what type of polymorphism is in place for the given snippet.
Garbage collection in java, why it is not done in C++ automatically. And some snipped in C++ and what's wrong with it? I last used C++ in college so he gave some hints.

Questions were asked on how I do monitoring of our applications (resume based it was coz the same tech is used by the interviewer so he was keen)

Round 2 (1 hr)
Tree related questions - easy questions around searching in BST and BT and its complexity and kind of traversals and stuff
Different kind of Version control systems(not to get confused with Gitlab or Bitbucket ) and which one do I use.
Questions around git and various situation for which how would I maintain the codebase.
Questions around Hashmap and Arraylist vs LinkedList
Questions around projects and how we do SDLC and also how we do testing and gave scenario-based questions and how should we go about the testing
Questions around Enum and what purpose does it serve apart from being as a substitute for Const .
And some more work and Java-related questions here and there.

Round 3(1 hr)

First 30-35 mins
Asked about my projects and stuff,
Then asked to implement serialisation for a sample class - I surrendered here
Then was asked LLD for a chatbot - where we can ask for financial information from and it replies with predefined answers
Then started adding condions like will be have one chatbot instance or not for each user/session. Then brought in singleton concepts and stuff
Then was asked 1 HLD kind of question to it - what sort of queuing service shall we use for the chatbot

last 25-30 mins
2nd interviewer asked me another LLD question
Question - Do the LLD for - on browser we have move forward and move backward keys , on top left side , which is used to navigate to and from your search history in that particular tab.

Rejected after this round.

The last round was the Hiring manager round of 30 mins to which I couldn't make it.