4. MongoDB

19. What is a compound index in MongoDB?

20. Why does field ordering matter in a compound index?

Key concept discussed:

Compound Index:
{ name: 1, age: 1 }

The interviewer was probing the leftmost-prefix rule.

5. DSA — BFS / DFS

21. Difference between BFS and DFS

22. What are their use cases?

23. What data structures do BFS and DFS use?

Answer:

BFS → Queue
DFS → Stack / Recursion

24. Why does BFS use a Queue and DFS use a Stack?

6. DSA — Quick Sort

25. Does Quick Sort work / how does Quick Sort work?

26. Why does Quick Sort degrade to O(n²)?

27. What happens if the pivot is the last element of an already sorted array?

28. Why exactly does that case become O(n²)?

Example discussed:

[1, 2, 3, 4, 5]

Pivot = 5
        ↓
[1, 2, 3, 4] | 5

Pivot = 4
        ↓
[1, 2, 3] | 4

Pivot = 3
        ↓
[1, 2] | 3

So partitions become:

n - 1, 0
n - 2, 0
n - 3, 0
...

7. DSA — Anagrams

29. What are anagrams?

30. Given two strings, check whether they are anagrams.

31. Give brute-force, better and optimal approaches.

We discussed:

Brute     → O(n²)
Better    → Sorting → O(n log n)
Optimal   → Frequency Map/Array → O(n)

32. What if the input contains Unicode characters?

33. What is Unicode?

34. Why can't we simply use freq[26] for Unicode?

Overall Interview Flow

                 INTERVIEW
                     |
        ┌────────────┼────────────┐
        ↓            ↓            ↓
      DSA           CS          Development
        |            |            |
   BFS / DFS       OS          Node.js
   Quick Sort      Deadlock    JavaScript
   Anagrams        Networking  TypeScript
                   HTTPS       Payments
                   DNS         Security
                   MongoDB
                   Indexes