Number of Divisible Triplet Sums
A medium-tier problem at 68% community acceptance, tagged with Array, Hash Table. Reported in interviews at IBM and 5 others.
You're staring at a triplet sum problem and you've got maybe 45 minutes left. This one's not a classic two-pointer. It's asking you to count triplets whose sum is divisible by some target, and the brute force O(n^3) won't cut it on large inputs. IBM, Activision, Palantir, and LinkedIn all report asking this. The acceptance rate sits at 68%, which means a solid chunk of candidates nail it, but the ones who don't usually freeze on the modular arithmetic angle. If you hit this live and blank on the optimization, StealthCoder solves it invisibly in seconds.
Companies that ask "Number of Divisible Triplet Sums"
Number of Divisible Triplet Sums 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. Made for the engineer who has done the work but might still blank with a webcam pointed at him.
Get StealthCoderThe trick here is recognizing that you don't need the actual sum, just the remainder. You can hash the modulo values of array elements instead of trying every triplet combination. The naive O(n^3) approach checks all triplets, but a hash table strategy reduces it to O(n^2) by fixing one element and using a hash map to count complementary pairs. Common pitfall: forgetting that modular arithmetic doesn't distribute the way you might think, or over-counting identical triplets. When you're under pressure in the OA and the brute force times out, you'll know you missed the hash table angle. StealthCoder is the safety net for that moment.
Pattern tags
You know the problem.
Make sure you actually pass it.
Number of Divisible Triplet Sums 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. Made for the engineer who has done the work but might still blank with a webcam pointed at him. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Number of Divisible Triplet Sums interview FAQ
Is this really a medium, or does it feel harder?+
The 68% acceptance rate tells you it's solidly medium, but the cognitive jump from brute force to hash table optimization catches people off guard during a timed OA. Most candidates who solve it efficiently saw the modulo insight upfront. If you didn't, you're likely rewriting under time pressure.
What's the actual trick to this problem?+
Stop thinking about the sum itself. Think about remainders. Hash the modulo of each element, then for each pair, ask 'what remainder do I need to make the total divisible?' This collapses O(n^3) to O(n^2) with a hash table.
Do I really need a hash table, or can I sort?+
Sorting helps with some triplet problems, but modular arithmetic doesn't play well with sorted order. Hash tables let you count remainders directly without caring about array order, which is what you need here.
Will LinkedIn or Palantir ask follow-ups after I solve this?+
Both companies report asking this problem, so if you solve it, expect a follow-up on space complexity or edge cases like negative numbers or zero divisor handling. Have a clean explanation ready.
What if I run out of time in the actual OA?+
Write the brute force first. It's correct and gets partial credit. Then optimize if time allows. During a live assessment, a working O(n^3) beats a half-baked O(n^2) you can't finish.
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