3Sum With Multiplicity
A medium-tier problem at 46% community acceptance, tagged with Array, Hash Table, Two Pointers. Reported in interviews at Quora and 0 others.
3Sum With Multiplicity is a medium-difficulty array problem that trips up candidates who memorize the classic 3Sum but miss the counting twist. You're looking for triplets that sum to a target value, but the catch is that you need to count how many ways you can pick them from an array where elements can repeat. Quora has asked this. The 40% acceptance rate reflects that most people either over-count (treating [1,1,1] as six ways when it's one) or under-count by ignoring order. If this problem hits your live OA and you blank on the counting logic, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "3Sum With Multiplicity"
3Sum With Multiplicity 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 Amazon engineer who used it to pass JPMorgan's OA and system design loop.
Get StealthCoderThe trap is thinking this is just 3Sum with a modulo operation tacked on. It's not. You're counting distinct triplets, and when the array has duplicates, you have to manage that carefully. The two-pointer approach works, but the counting breaks people. After sorting, iterate through potential first elements, then use two pointers on the remainder. When you find a sum that matches, count how many times the left pointer value appears and how many times the right pointer value appears, then multiply. Edge case: when all three elements are the same (left equals target minus left equals right), count combinations, not permutations. Hash table and counting approaches exist too, but two pointers with careful multiplication of counts scales better. When you hit this live and your first attempt over-counts or misses the combination math, StealthCoder runs silently and shows you the exact counting formula.
Pattern tags
You know the problem.
Make sure you actually pass it.
3Sum With Multiplicity 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 Amazon engineer who used it to pass JPMorgan's OA and system design loop. Works on HackerRank, CodeSignal, CoderPad, and Karat.
3Sum With Multiplicity interview FAQ
Is this really just 3Sum with counting?+
No. Standard 3Sum finds triplets; this counts distinct triplets from a multiset. Duplicates in the array matter for the count, not the uniqueness. You're multiplying frequencies, not just flagging 'found a triplet.' That's the core difference most people miss on first attempt.
How do two pointers handle duplicates here?+
After sorting, fix one element, then expand/contract left and right pointers. When the sum matches, count occurrences of the left value and right value, multiply them together, then move both pointers past duplicates. This avoids double-counting and scales linearly through the remainder.
What's the trick when all three elements are identical?+
If target is 9 and you're picking three 3s from [3,3,3,3], you're choosing 3 items from 4, which is C(4,3)=4, not 4*4=16. Use the combination formula n*(n-1)*(n-2)/6 when left equals right. Miss this and your count balloons.
Why is 40% acceptance so low for a medium?+
Candidates confuse counting distinct triplets with finding them. They either permute when they should combine, mishandle the modulo arithmetic, or iterate inefficiently over duplicates. The math is simple once you see it, but the pattern isn't obvious from the problem statement.
Should I use a hash table or sorting plus two pointers?+
Two pointers scales better and keeps the counting logic simpler. Hash table works but requires more bookkeeping to avoid double-counts and handle the combination math. Given time pressure on an OA, two pointers with sorted input and smart duplicate skipping is the safer path.
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