Recently, many students preparing for Fintech positions (like Stripe, PayPal, Affirm, Bloomberg) and Google/Amazon have encountered this type of "Command Processor" problem.
The characteristics of this type of problem: extremely simple to start, but follow-ups layer on complexity, ultimately testing your micro System Design and OOD (Object-Oriented Design).
Today we'll deconstruct this Merchant and Payment Processor problem, a textbook-level "perfect score roadblock".
🔍 Problem Analysis: From CRUD to Temporal Logic
This problem simulates a real payment settlement system, requiring you to process a series of command streams.
✅ Part 1 & 2: Basic Warm-up (The Trap)
The first two parts are very straightforward:
- INIT: Initialize merchant balance
- CREATE: Create payment, add money
- REFUND: Refund, deduct money
Trap: Many students here directly use a simple Map<String, Integer> to store merchant balances and think they're done.
Wrong! If you don't encapsulate Payment as a separate object for storage, later requirement changes will force you to rewrite the entire code architecture.
✅ Part 3: Hidden State Management
Although the problem only says "handle more commands", this hints at introducing a State Machine.
- Does the payment exist?
- Has the payment already been refunded? (Double Refund problem)
- Does the payment belong to this merchant?
At this stage, comprehensive Error Handling mechanisms must be introduced.
🤯 Part 4: Timestamps and Business Rules (The Killer)
This is where most people fail.
Command format changed: timestamp INIT merchant_id balance refund_limit
Refund logic changed: Current Timestamp - Payment Timestamp <= Refund Limit
Why is it difficult?
- Data Structure Refactoring: You need to store not only payment amounts but also payment timestamps
- Temporal Validation: Every command carries time, you must strictly validate time windows
- Parsing Complexity: String parsing logic needs to be compatible with both timestamped and non-timestamped cases (if the problem requires backward compatibility), or completely rewrite the parser
🏆 oavoservice Exclusive Approach
The optimal solution for this problem is to use Command Pattern combined with Ledger Design.
We need two core Maps:
MerchantMap: Store MerchantID -> {Balance, RefundLimit}
PaymentMap: Store PaymentID -> {MerchantID, Amount, CreatedTime, Status}
Only by designing data structures with sufficient "decoupling" can you easily handle Part 4's time constraints by simply adding one line: if (now - createdTime > limit) return.
💡 Why Do These Problems Often Not Get Strong Hire?
Interviewers/OA systems test not just outputting correct numbers, but also:
Code Extensibility: When I add a Part 5 requiring "partial refunds", does your code need rewriting?
Edge Case Handling: What if refund amount exceeds balance? What about refunding a non-existent ID?
Code Style: Are you writing scripts or engineering code?
🚀 oavoservice: Your Perfect Score Guarantee
Facing OAs with progressively complex logic and expanding code volume, you need not just answers, but professional technical team support.
oavoservice specializes in providing top-tier written test/interview assistance services for North American students:
✅ Full-Stack Coverage: We're familiar with various Command Processing, Log Parsing, Graph and other high-frequency problem types
✅ Clean Code: Not just AC, we write Production-Level code that meets OOD standards and impresses interviewers
✅ Direct to Perfect Score: Whether Stripe's FinTech logic or Google's algorithm challenges, we ensure optimal solutions
Don't let a system design problem block your path to a High Package Offer.
📩 Contact us for any needs.
We consistently provide professional online assessment services for major tech companies like TikTok, Google, and Amazon, guaranteeing perfect scores.
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