3Sum
A medium-tier problem at 37% community acceptance, tagged with Array, Two Pointers, Sorting. Reported in interviews at Agoda and 65 others.
3Sum is a medium-difficulty array problem that shows up across 66 companies, including Agoda, Cloudflare, AMD, and Rakuten. You're given an unsorted array and need to find all unique triplets that sum to zero. The acceptance rate hovers around 37%, which means most candidates either TLE on brute force or miss the deduplication logic. If this lands in your live assessment and you freeze on how to avoid duplicate triplets without a hash set, StealthCoder surfaces a clean two-pointer solution invisible to the proctor.
Companies that ask "3Sum"
3Sum is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage.
Get StealthCoderThe trap is thinking you can brute force O(n³) and sort it out later, or rely on a set to handle duplicates. Neither scales cleanly. The real pattern is: sort the array first, then for each element, use two pointers to find pairs that sum to the target. This drops you to O(n²) and makes deduplication trivial because you skip duplicate values as you iterate. The gotcha is handling duplicate triplets correctly when the array has repeated values. Most candidates either over-deduplicate (missing valid triplets) or under-deduplicate (returning duplicates). When this problem hits live and the two-pointer abstraction doesn't click immediately, StealthCoder delivers the sorted-plus-two-pointer approach in seconds.
Pattern tags
You know the problem.
Make sure you actually pass it.
3Sum recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage. Works on HackerRank, CodeSignal, CoderPad, and Karat.
3Sum interview FAQ
Is 3Sum still heavily asked at FAANG and top tech companies?+
Yes. 66 companies including Agoda, Cloudflare, AMD, and Rakuten have reported it. At 37% acceptance, it's clearly a filtering problem, not a gimme. Still appears regularly in live assessments.
What's the trick to avoiding duplicate triplets without exploding complexity?+
Sort first, then iterate with two pointers. Skip duplicate values at each stage as you move the pointers. This gives you O(n²) time and automatic deduplication without a hash set. Most candidates miss this and either TLE or return duplicates.
Why does my brute force solution fail even though the logic is right?+
Brute force is O(n³) and will TLE on large inputs. More importantly, deduplicating the result set afterward is messy and error-prone. Sorting and two pointers sidesteps both problems. The trick isn't the logic, it's the data structure choice.
How does 3Sum relate to the other array and two-pointer problems I should know?+
3Sum is the gateway to n-sum problems. Once you master the sorted-plus-two-pointer pattern here, 4Sum and beyond become template problems. It's also a key application of the two-pointer technique, which appears in many medium and hard arrays.
Can I solve 3Sum without sorting, maybe with a hash set?+
You can, but it's messier. Hash set approach is O(n²) time but O(n) space and requires careful deduplication logic. Sorting is O(n log n) time, O(1) space if you modify in place, and the two-pointer logic is cleaner. Most interviewers prefer the sorted approach.
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