Interview Pro Tip
Red Flag: Using a nested loop (O(n²)) or not handling edge cases (empty, all repeating). Pro-Move: 'I'd use Counter for readability; in production we'd add a docstring and handle the case where the input might be bytes vs. str for encoding safety.'
**Approach**: Two-pass—first pass counts characters; second pass finds first with count 1. Preserves order of first occurrence. **Code**: def first_non_repeating(s): counts = {}; for c in s: counts[c] = counts.get(c, 0) + 1; return next((c for c in s if counts[c] == 1), None). Alternative with Counter: def first_non_repeating(s): from collections import Counter; cnt = Counter(s); return next((c for c in s if cnt[c] == 1), None). **Complexity**: O(n) time, O(k) space (k = unique chars)....
The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations. It covers performance optimization strategies, common pitfalls to avoid, and real-world examples from companies like Delivery Hero, Dunnhumby. The answer also includes follow-up discussion points that interviewers commonly explore.
Continue Reading the Full Answer
Unlock the complete expert answer with code examples, trade-offs, and pro tips - plus 1,863+ more.
Or upgrade to Platform Pro - $39
Engineers who used these answers got offers at
AmazonDatabricksSnowflakeGoogleMeta
According to DataEngPrep.tech, this is one of the most frequently asked Python/Coding interview questions, reported at 2 companies. DataEngPrep.tech maintains a curated database of 1,863+ real data engineering interview questions across 7 categories, verified by industry professionals.