Number of Valid Subarrays
A hard-tier problem at 79% community acceptance, tagged with Array, Stack, Monotonic Stack. Reported in interviews at Hulu and 0 others.
You're staring at a hard problem with a 79% acceptance rate, and it's backed by Hulu. That's a good sign it's learnable, not some obscure corner case. Number of Valid Subarrays is a monotonic stack problem that looks deceptively simple on the surface. Most candidates see the constraint (absolute difference between any two elements is at most 2) and think brute force iteration. That's where you get stuck. The trap is that once you understand the stack pattern, the problem collapses into a few lines. If you hit this on your live OA and the brute force feels slow, StealthCoder surfaces the stack-based approach in seconds while the proctor stays blind.
Companies that ask "Number of Valid Subarrays"
Number of Valid Subarrays 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 insight is that you're not just counting subarrays, you're counting *maximal valid ranges*. A monotonic stack tracks elements in order, and when you encounter an element that violates the constraint, you know exactly how many subarrays ending at that position are valid. The trick most candidates miss is that the answer accumulates incrementally as you process each element. Common pitfall: trying to validate every subarray independently, which is O(n^2) or worse. The stack approach is O(n) because each element enters and exits the stack once. You'll need to track indices and understand how the stack height translates to valid subarray count. This is pure pattern recognition. If you've drilled monotonic stack problems before, this clicks fast. If not, the live assessment is where you either know it or you're guessing. StealthCoder is your hedge for that moment.
Pattern tags
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Make sure you actually pass it.
Number of Valid Subarrays recycles across companies for a reason. It's hard-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 Valid Subarrays interview FAQ
Is this really a hard problem, or is the rating inflated?+
The 79% acceptance rate suggests it's hard in theory but fair in practice. Most solvers have seen monotonic stack problems before. If you haven't, it feels impossible. If you have, it's a pattern match. There's no hidden trick after the stack logic clicks, so the difficulty is front-loaded on recognizing the approach.
Why does monotonic stack work here?+
A monotonic stack maintains elements in a specific order and lets you instantly know when an element breaks the constraint. As you push elements, the stack height tells you how many valid subarrays end at the current position. You avoid rechecking constraints by storing the relative order already.
What's the pitfall with brute force?+
Checking every subarray is O(n^2) and won't pass large inputs. You think you're being thorough, but the stack approach counts valid subarrays in O(n) by leveraging the constraint structure. One validates everything, the other is surgical.
How does this relate to other monotonic stack problems?+
It follows the same template as largest rectangle in histogram or daily temperatures. You're using the stack to find ranges where a property holds, not just finding next greater element. The key difference is how you map the stack state to the answer you're counting.
Will Hulu ask this again, or should I prep something else?+
Hulu has asked it at least once, which means it's in their rotation. Don't assume they won't ask it again. Monotonic stack is a core data structure for interview rounds, so drilling this is time well spent regardless of which company asks.
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