Find First Unique
Reported by candidates from Amazon's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Amazon's Find First Unique hit the OA circuit in March 2024, and it's a straightforward pattern that trips up candidates who overthink it. You're looking at a problem that demands you identify the first character or element that appears exactly once in a sequence. The catch isn't algorithmic complexity. It's tracking frequency efficiently and preserving insertion order. This is the kind of problem where a hash table and a single pass solve it cleanly. StealthCoder keeps you moving if you freeze on the implementation details during the live assessment.
Pattern and pitfall
The core pattern here is hash-table frequency counting combined with order preservation. Build a map of each element to its count in one pass, then iterate through the original sequence again to find the first element with a count of one. The trick candidates miss is thinking they need two separate data structures when the order is already baked into the sequence itself. Some overthink it as a string problem when the logic scales to any sequence. Common pitfall: using a set to track 'seen' elements instead of actual frequency counts. That fails immediately on repeated elements. StealthCoder flags this kind of logic error in real time, catching it before you submit a wrong solution.
If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.
You can drill Find First Unique 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 would have shipped this the night before his JPMorgan OA if he'd had it.
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This OA pattern shows up on LeetCode as first unique character in a string. If you have time before the OA, drill that.
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Amazon reuses patterns across OAs. Built by an Amazon engineer who would have shipped this the night before his JPMorgan OA if he'd had it. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Find First Unique FAQ
What's the actual trick in Find First Unique?+
There isn't a trick. It's hash-table frequency counting plus a second pass. Count all occurrences, then iterate the original sequence and return the first element with count equal to one. O(n) time, O(n) space. The 'trick' is not overthinking it.
Do I need a special data structure to preserve order?+
No. The original sequence already preserves order. You iterate through it a second time after building your frequency map. Python dicts maintain insertion order in Python 3.7+. You don't need extra complexity.
What if there's no unique element?+
Return a default value like -1, null, or an empty string depending on the problem spec. Amazon usually makes this explicit in the problem statement. Check the exact wording on your OA.
Is this still a common Amazon OA question?+
Yes. It's a breadboard problem that appears frequently because it tests basic hash-table fluency without being a trap. It's safe to assume you'll see frequency-counting problems on Amazon assessments.
How do I prepare for this in 48 hours?+
Know your hash table operations by heart. Write the two-pass solution three times in your target language. Test on strings and arrays both. You don't need a special algorithm. You need clean code and zero typos.