Counterfeit Currency
Reported by candidates from Box's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Box's Counterfeit Currency problem hit the OA in June 2025, and it's a pattern-matching puzzle that catches candidates off-guard. You're given a sequence of currency denominations or transactions, and you need to spot the fake one based on some structural rule. The trick isn't brute force; it's recognizing what makes a counterfeit stand out. If you blank during the live OA, StealthCoder reads the problem and feeds you the pattern in real time, so you're not starting from zero.
Pattern and pitfall
This problem typically hinges on either hash-table frequency counting, modular arithmetic, or a subtle property of the data. The most common approach: iterate through the sequence, track what you see, then identify the outlier. If it's a math angle, look for a checksum or weighted-sum property that real currency follows but the fake violates. The pitfall is overthinking it as a sorting or search problem when it's just asking you to find the one element that breaks the rule. StealthCoder is your safety net if the problem statement is vague on what 'counterfeit' means; it'll help you map the exact constraint in seconds.
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 Counterfeit Currency 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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Box reuses patterns across OAs. Built by an Amazon engineer who passed his OA cold and still thinks the filter is broken. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Counterfeit Currency FAQ
Is this a hash-table or frequency problem?+
Most likely, yes. You're probably counting occurrences or properties of each denomination and finding the one that doesn't fit the pattern. Hash maps or sets are the standard tool for this type of OA question.
What if the problem involves a checksum or modular arithmetic?+
Real currency often has a check digit or sum property. Test whether all legitimate denominations satisfy some formula (like sum mod 10 equals zero). The counterfeit breaks it. Work through the example to spot the rule.
How do I spot the pattern in 48 hours?+
Re-read the problem twice. Look for explicit rules in the problem statement. If none, assume the trick is frequency, sum, or a simple mathematical property. Write a brute-force solution first; the pattern emerges when you code it.
Do I need to sort or search?+
Probably not. This is a linear scan with a hash or counter. Sorting adds complexity you don't need. Stick to one pass through the data unless the problem explicitly says the denominations are ordered.
Is this still a common Box OA question?+
Yes. Box's assessments favor straightforward pattern-matching and hash-based problems. If you see 'find the odd one out' or 'identify the fake', this is likely the same category.