Count Dominant Substrings
Reported by candidates from Amazon's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Amazon's November OA threw this one at candidates: count dominant substrings. You've got a string, and you need to identify substrings where a character appears more frequently than any other character in that substring. It's the kind of problem that sounds straightforward until you realize the brute force approach will time out. StealthCoder can spot the pattern and pull the solution live if you freeze on the approach during the assessment.
Pattern and pitfall
The trick here is understanding what 'dominant' actually means. A substring is dominant if one character strictly outnumbers all others in it. Most candidates jump to a nested loop approach, checking every substring, then validating dominance. That works for small inputs but fails at scale. The real pattern is either a sliding window with a frequency map to prune early, or a smarter observation: you only need to check substrings where the dominant character actually appears. Some versions of this problem have a twist where you're counting substrings of a specific length, which collapses to a sliding window pattern. If you blank during the OA, StealthCoder reads the problem and serves the right approach in seconds, letting you code with confidence instead of guessing.
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 Count Dominant 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 passed his OA cold and still thinks the filter is broken.
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Count Dominant Substrings FAQ
Is this a sliding window or brute force problem?+
Both can work, but brute force (checking every substring) is risky on larger inputs. Sliding window with early termination when no character can be dominant is more efficient. Amazon usually favors the optimized approach, so practice the windowed solution.
What's the main gotcha Amazon is testing?+
Candidates often misdefine 'dominant'. It's strictly more than any other character, not equal. Also, they forget to handle edge cases like single-character substrings (always dominant) and ties (not dominant). Lock those down before submitting.
Can I solve this in one pass?+
Not cleanly. You'll need at least two nested loops or a sliding window with a frequency map. One-pass solutions typically don't handle the 'check all substrings' requirement. Accept the O(n^2) time for now and optimize if you have buffer.
What data structure matters most here?+
A hash map or array to track character frequencies inside your window. Keeping a running count lets you check dominance in O(1) time per substring, avoiding recalculation. That's where the optimization lives.
Is this problem still asked by Amazon in late 2024?+
Yes. Substring problems with frequency constraints are a staple of Amazon OAs. This one tests both pattern recognition and implementation discipline. Expect similar variants on counting, hashing, and windowing.