Get Number of Redundancy Free
Reported by candidates from Amazon's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Amazon's December 2024 OA threw this one at candidates: Get Number of Redundancy Free. You don't have the full problem text yet, but the title alone tells you something's being counted or deduplicated. It's a filtering problem. The pattern is almost certainly about identifying non-redundant elements, which means hash tables, sets, or deduplication logic. If you blank on the exact algorithm, StealthCoder reads the problem statement and feeds you the approach in real time. You'll want to know the trick before you sit down.
Pattern and pitfall
The title 'Redundancy Free' points toward a counting or filtering task where you need to isolate elements that don't repeat or don't satisfy some redundancy condition. The algorithm typically hinges on set operations, frequency maps, or a two-pass scan to mark what's redundant and count what remains. Amazon loves these because they test whether you can think clearly about state and deduplication under time pressure. Common pitfall: overthinking which definition of 'redundant' applies, or missing that you need to count output, not just filter. If the problem text is ambiguous in the OA itself, StealthCoder's real-time read of the prompt becomes your safety net to nail down the exact requirement before you code.
If you see this problem in your OA tomorrow, the play is to recognize the pattern in 30 seconds. StealthCoder buys you that recognition.
You can drill Get Number of Redundancy Free cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built by an Amazon engineer who passed his OA cold and still thinks the filter is broken.
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Amazon reuses patterns across OAs. Built by an Amazon engineer who passed his OA cold and still thinks the filter is broken. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Get Number of Redundancy Free FAQ
What does 'redundancy free' actually mean in an Amazon OA?+
Without the full problem text, it likely means elements that appear exactly once, or elements that don't violate some uniqueness or relationship constraint. Use a hash map to track frequency or a set to track seen values. Test both interpretations against the examples once the OA loads.
Is this a hard problem or a warm-up?+
Title-only, it reads like a medium-difficulty filtering or counting problem. Not a graph or DP problem. Hash tables and sets are your toolkit. Execution matters more than algorithmic novelty. Amazon's December OAs tend to punish off-by-one errors and edge cases.
How do I prep for this in 24 hours if I don't know the full problem?+
Brush up on hash map / set patterns: frequency counting, deduplication, finding unique elements. Know how to iterate twice (once to gather state, once to filter or count). Practice returning the count vs. the elements themselves. Both are common variants.
What's the most common pitfall candidates hit on Amazon deduplication problems?+
Forgetting to account for the order of operations or misreading whether you return count, indices, or elements. Read the return type carefully. Also, not handling empty input or single-element input. Test those edge cases immediately.
Can I solve this without knowing the exact problem text before the OA?+
Mostly no. But if you know set and hash map patterns cold, you can pivot fast once you see the examples. That's what StealthCoder covers: it reads the full problem and examples, so you don't waste the first 90 seconds guessing intent.