Software Engineer · Onsite · Coding, System Design

Palo Alto Networks — Software Engineer ✅ Passed

Level: Senior-Level

Round: Onsite · Type: Coding, System Design · Difficulty: 6/10 · Duration: 60 min · Interviewer: Unfriendly

Topics: Set Operations, Data Streams, Bloom Filter, Caching, Database

Location: San Francisco Bay Area

Interview date: 2025-08-15

Summary

I had an onsite interview that started with a coding question and evolved into a system design discussion through follow-up questions.

Details

The initial coding question involved finding the difference between two lists: identifying elements present in list1 but not in list2.

My approach was to convert both lists into sets, standardizing the elements (e.g., converting to lowercase) during the set creation. Then, I used the set difference operation to find the elements unique to list1.

Follow-up 1: If list1 becomes a stream, how would you handle it?

I suggested converting list2 into a set and, for each element received from the list1 stream, checking if it exists in the set. If not, I would add it to the result set.

Follow-up 2: If the lists are too large to be fully stored in memory, what would you do?

I proposed a few ideas:

1. Store list2 in a cache like Redis.
2. Store list2 in any type of database and query which elements from list1 are not present in the database.
3. Use a Bloom filter, acknowledging the possibility of false positives.