Count Special Substrings
Reported by candidates from Amazon's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Amazon's May 2025 OA threw a substring counting problem at candidates, and it's the kind of trap that catches you if you brute-force it. You need to recognize that "special" usually means either palindromic, character-frequency rules, or some modular arithmetic constraint. The real play is spotting whether you can solve this in linear or near-linear time with a hash table or prefix sum, not by testing every substring. StealthCoder will catch the pattern if you blank on the constraint definition during your live session.
Pattern and pitfall
Substring counting problems at Amazon almost always hinge on a hidden constraint that makes a naive O(n^3) or even O(n^2) solution fail under time limits. The "special" label means you're tracking either character counts, palindrome properties, or numeric divisibility. Hash tables and prefix sums are your primary tools here. You'll likely need to iterate once, maintain a rolling count or map of states, and recognize when you've hit your target condition. The common pitfall is solving the right problem too slowly. If you're iterating through every substring, you're already wrong. StealthCoder sitting on your second monitor as a safety net means you won't panic if the exact wording escapes you mid-assessment.
If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.
You can drill Count Special Substrings 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 would have shipped this the night before his JPMorgan OA if he'd had it.
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Count Special Substrings FAQ
What makes a substring 'special' in this problem?+
The problem statement defines it, but typical patterns are: all characters appear the same number of times, the substring is a palindrome, characters follow a specific frequency rule, or the substring length or content satisfies a modular constraint. Look for that definition in the problem text immediately.
Is brute force ever fast enough?+
Not typically. O(n^2) substring enumeration plus O(n) validation per substring will time out. You need O(n) or O(n log n). Hash tables tracking character states or prefix sums are the lever.
How do I avoid off-by-one errors on substrings?+
Use inclusive start and end indices, test with small examples (length 1, 2, 3), and verify your loop bounds before submission. Amazon OAs often have weak test cases, so manual validation on paper is worth the 90 seconds.
What if I can't figure out the pattern in 10 minutes?+
Code a brute-force O(n^2) or O(n^3) solution that's correct but slow. Pass the small examples. Then optimize if time allows. Partial credit beats zeros. StealthCoder can guide the optimization path if you're stuck.
Do I need to handle empty strings or single characters?+
Always check edge cases in the problem constraints. Single-character substrings often trivially satisfy "special" conditions. Empty strings usually don't count. Confirm before you code.