Stream Processing with Stop Words Filtering

Last updated: September 4, 2025

Quick Overview

Process a high-throughput token stream, filter stop words, maintain frequency counts, and support sliding window operations.

Perplexity
Coding & Algorithms
Software Engineer
Perplexity
September 4, 2025
Software Engineer
Coding Round
Coding & Algorithms
Medium

7

11

2,425 solved


Process a high-throughput token stream, filter stop words, maintain frequency counts, and support sliding window operations.

Perplexity processes millions of tokens in search and LLM pipelines. This tests efficient data structure design for streaming data.

What the Interviewer Expects
  • Process tokens in a single pass
  • Maintain frequency counts with O(1) updates
  • Implement sliding window for recent token analysis
  • Filter stop words efficiently using a hash set
  • Handle high throughput without memory bloat
Key Topics to Cover
Stream processing
Sliding window
Frequency counting
Stop word filtering
Deque data structure
How to Approach This
  1. Clarify input constraints and edge cases before writing code.
  2. Walk through your approach verbally and confirm with the interviewer before coding.
  3. Start with a brute force solution, then optimize. Mention time and space complexity.
  4. Test your solution with examples, including edge cases like empty input or duplicates.
  5. Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
  • How would you extend this to detect trending topics?
  • What if the stream is distributed across multiple workers?
  • How would you add approximate counting for memory savings?
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Sample Answer
Problem Analysis

The problem involves processing a high-throughput stream of tokens while filtering out stop words and maintaining frequency counts over a sliding window. This suggests the use of a sliding window ...

Approach
  1. Initialize Data Structures: We will use a deque to maintain the sliding window of tokens, a defaultdict to count the frequencies of valid tokens, and a set to hold the stop words for O(1)...

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