Find How Many Subarrays Match the Pattern
Reported by candidates from Meta's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Meta pulled this one in September 2024, and it's a sliding window trap dressed as a pattern-matching problem. You're looking for subarrays that match a specific structure, not just any sequence. The instinct to brute-force every subarray will time out. StealthCoder reads the exact constraint and feeds you the window size instantly, so you can skip the guessing game and lock in the right approach before your brain stalls.
Pattern and pitfall
The pattern here is sliding window with a twist: you're not maximizing or minimizing, you're counting subarrays that satisfy a structural condition. Meta likes to hide the constraint in the problem statement. The trick is identifying what 'match the pattern' actually means. Build a window of the right size, slide it once across the array, and at each position check if the subarray satisfies the pattern rule. Common pitfall: thinking you need to validate every possible subarray length. You don't. Once you nail the window size from the problem text, it's a single pass. StealthCoder catches the exact size and condition you might miss under pressure, turning a 20-minute panic into a clean implementation.
If you see this problem in your OA tomorrow, the play is to recognize the pattern in 30 seconds. StealthCoder buys you that recognition.
You can drill Find How Many Subarrays Match the Pattern cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built by an Amazon engineer who passed his OA cold and still thinks the filter is broken.
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Find How Many Subarrays Match the Pattern FAQ
What does 'match the pattern' mean here?+
It means the subarray follows a specific structural rule, usually about relationships between adjacent elements or values at certain positions. Read the full problem text for the exact definition. It's never 'sum equals X' with Meta. It's always a shape or sequence rule.
Will brute force work?+
No. Checking all O(n^2) subarrays will time out on medium-sized inputs. You need a fixed window size and a single pass. The pattern constraint lets you skip most subarrays entirely.
How do I find the window size?+
It's in the problem statement. Meta always gives you the window size explicitly or it's derivable from the pattern rule itself. If unsure, re-read the constraint once. Don't guess.
Is this a dynamic programming problem?+
No. It's sliding window. DP would be overkill and slow. Once you know the window size, you move it linearly across the array and count matches. One pass, linear time.
What if I blank on the pattern rule during the OA?+
Ask the proctor to clarify the exact meaning of 'match the pattern' or re-read the problem statement aloud to yourself. Meta usually makes this clear on a second read. If you're still lost, that's where StealthCoder steps in and shows you the rule extraction logic in real time.