Count the Hidden Sequences
A medium-tier problem at 57% community acceptance, tagged with Array, Prefix Sum. Reported in interviews at Zomato and 0 others.
Count the Hidden Sequences is a medium-difficulty problem that Zomato has reportedly asked. With an acceptance rate just above 56 percent, most candidates who attempt it don't land a clean solution on the first pass. The trick isn't complex, but it's easy to miss if you're stressed or haven't drilled prefix sums recently. You'll be working with an array, and the problem forces you to think about cumulative state rather than local values. If this one hits your live OA and you blank on the pattern, StealthCoder solves it in seconds, invisible to the proctor.
Companies that ask "Count the Hidden Sequences"
Count the Hidden Sequences 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 problem uses Array and Prefix Sum as its core mechanics. The 'hidden sequences' part means you're not just scanning the array once; you're counting subsequences or subarrays that meet some condition. Prefix sums let you answer range-query questions in O(1) time after O(n) preprocessing, and that's almost always the unlock here. Most candidates try to brute-force nested loops and hit time limits. The real move is precomputing cumulative sums so you can check conditions on any range instantly. Once you set up the prefix array correctly, the counting logic becomes straightforward. If you walk into the assessment without that mental model locked in, you'll waste 15 minutes on a TLE solution. StealthCoder is the hedge for that exact moment when the brute-force approach isn't passing the test cases.
Pattern tags
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Make sure you actually pass it.
Count the Hidden Sequences 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.
Count the Hidden Sequences interview FAQ
Is Count the Hidden Sequences really medium, or does it feel harder?+
The acceptance rate of 56.7 percent suggests it's genuinely medium but with a sharp drop-off for candidates who don't spot the prefix sum angle fast enough. The algorithm itself isn't complex once you see it, but the setup and implementation details trip people up under time pressure.
Do I need to precompute the entire prefix sum array, or can I do it on the fly?+
Depends on the exact problem statement, but typically precomputing saves you from bugs and makes the counting loop cleaner. With an array-based problem at this difficulty, building the full prefix array first is the safest approach and gives you O(n) time overall.
How often does Zomato actually ask array and prefix sum problems?+
Zomato has this problem on record. Range-query and cumulative-state problems are standard in backend and systems-heavy companies, so expect prefix sum techniques to come up in their assessments.
What's the most common mistake candidates make here?+
Trying to solve it with nested loops and not realizing prefix sums cut the complexity. Another trap is off-by-one errors in the prefix array indexing. Get that indexing wrong and all your counts are garbage.
Should I code this in Python or C++ for speed?+
Language doesn't matter much for this problem. The bottleneck is the algorithm, not the runtime. Pick whichever language you're fastest and most confident in during a live assessment.
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