Product of the Last K Numbers
A medium-tier problem at 63% community acceptance, tagged with Array, Math, Design. Reported in interviews at Target and 1 others.
Product of the Last K Numbers is a stream-design problem that hits different in a live assessment because it requires you to think about prefix sums in reverse. You're building a data structure that accepts numbers one at a time, then answers queries about the product of the most recent K elements. Target and Tekion have asked this. The trap is trying to recalculate products on every query instead of precomputing them. With a 62.7% acceptance rate, it's not a gimme, but it's entirely learnable if you spot the pattern. StealthCoder surfaces the prefix-sum trick in seconds if you blank on it during screen share.
Companies that ask "Product of the Last K Numbers"
Product of the Last K Numbers 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 at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoderThe naive approach multiplies K elements for each query, which tanks on large K or frequent queries. The real solution uses prefix products: store the product of all numbers seen so far. Then the product of the last K numbers is current_product divided by the product from K steps ago. The wrinkle is handling zeros: they reset the prefix product to zero, so you need to track the last zero's position and only divide if all K elements fall after it. Most candidates see 'product query' and jump to maintaining a window. That works but scales poorly. Once you realize you're really doing division on a prefix product, the problem flattens. If this shows up live and the zero-handling logic breaks your flow, StealthCoder gives you the exact structure in real time.
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Product of the Last K Numbers 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 at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Product of the Last K Numbers interview FAQ
Is this really asked at FAANG or just Target and Tekion?+
Input data shows Target and Tekion have asked it. Stream-design problems with prefix patterns are staples across big tech, so the algorithmic concept is broader than just those two. The specific problem is less universal than, say, Two Sum, but the pattern is solid interview material.
What's the actual trick that makes it fast?+
Prefix products with division. Store cumulative product at each step. Query answer is current_product divided by the prefix product from K positions ago. Zeros break division, so track zero position and skip division if a zero is in your K-window. That's it.
How does this differ from other prefix-sum problems?+
Most prefix-sum problems sum past values. This one multiplies and must handle zeros, which zero out everything before them. Division adds complexity most Array or Prefix Sum basics don't have. Design element forces you to think about state and edge cases, not just the algorithm.
What happens if I just use a deque and recalculate on each query?+
Deque works but is O(K) per query. Prefix products drop it to O(1) per query. In an OA with tight time limits or large K and many queries, the slow solution may time out. The problem is testing if you think in terms of precomputation, not just data structures.
How hard is zero-handling versus the core algorithm?+
The prefix product idea is the hard part. Zeros are a straightforward edge case once you see it: reset prefix to 1, record the index, and check if your K-window overlaps that index before you divide. Most rejections come from missing the prefix strategy, not botching zeros.
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