Kotak Mahindra Bank SDE1 - Backend [Rejected]

Status: Passout Batch of 2022 {Tier 3 College}
Position: Project Engineer at Wipro
Location: Hyderabad
Date: Septemebr 15, 2023

I saw a post on Twitter stating that Kotak Mahindra Bank has Internal Openings accross all Software Development Roles, So Considering My Years of Experince I decided to Give it a Try For SDE1 - Backend (JAVA), So I reached out to Multiple People on Linkedin to refer me for this role, along with their HR Team.
On 11th September 2023, I received a call from HR stating that My resume has Been Shortlisted, and he asked about the current Tech Stack, Current CTC and a few Questions Relatied to Time and Space Complexities and mutually we agreed for the Interview to be scheduled on the following Friday.
I doubted the call, so I decided to Start Preparing only after the Interview was Scheduled (As I was Almost ready with Data Structures and Algorithms), Being not received a mail on any Update, I called the HR on Wednesday Asking for the Interview to be Scheduled.
Sometime later he called me and asked about the Availability and Scheduled the same on Friday. Now this was scheduled by BarRaiser Team, Apparently they Outsource their Interview Process.
So, First thing I did was to find the previous Interview Experinces, But I was Unable to find a single Interview Experince for this Company.
So, I started Preparing Covered up all the concept of SDE Sheet of Striver, The Last Moment Interview Sheet by Striver, and Core Subjects.
The day had Come, the Interview day, I joined the meeting, The Interviewer told that he was a 6 year Experienced Professional, and Asked me about me.
Post That he gave me a Question:
Given Two Strings, say s1 = "ab" and s2 = "hdeuudba", Now you need to tell me if any Permutation of string s1 occurs in String s2 or not.
I told him that In the Brute force way I will generate all the permutations of string s1 and then check manually for each, post that Time and Space Complexity of approach was Discussed, Time Complexity being exponential, He was Unhappy with the approach and asked me to Optimize, It took a few hints from him to figure out that it was Based on Sliding Window, I was Oply Surprised that how could I not figure this out.
Post this he asked me to code it up, I coded it up, First a n^2 Solution then, Optimized to O(N), He was Happy with this.
Now comes the question which I didn't expect, He asked me about the Internal Working of Hashmap, as I used Hashmap in the previous question, I tried a lot, But I was unable to figure it out, all I remembered was about the Collision Detection Stuff in Hashmap. So I was blank for a minute or so, he asked me about the internal working of a Hashset, which I explained that it Bsed on a Red- Black Tree, Post this He asked me about Lambda Functions in JAVA, Which I answered, then he Jumped to another Section Which I didn't knew was being asked at SDE - 1 LEvel that is System Design. He asked me 3 questions based on it, and they were the terms that I heard for the first time, so I politely Told that I dont know about it.
Then we discussed about feedback and all, then the Interview ended, On Monday Morning (17th Sept 2023), I received a Rejection Mail from the HR. Still it had been a great Experience Interviewing with them.