Money, Precision, and Ledgers

A crypto exchange moves other people's money every second, and the code that touches a balance is graded harder than almost anything else in the loop. The failure is rarely an algorithm. It is a float that should have been an integer, a fee that rounded the wrong way, a transfer that ran twice on a retry. Each one is invisible in a demo and expensive in production, which is exactly why Coinbase probes for them. A solution that finds the optimal answer but stores a dollar as 19.99 can still fail the round.

The bar here is correctness under money rules, with clever big-O second. Hold value in the smallest whole unit so arithmetic stays exact. Make every transaction balance so the books cannot drift. Tag every operation with an idempotency key so a network retry does not pay twice. Decide where the rounding remainder goes before a fee is ever computed. The three problems below are crypto-shaped versions of these habits: assembling a block to maximize fees, validating a double-entry ledger, and splitting an amount so no unit goes missing.

CoinbasePremium lesson · Coding round

The 3 Coinbase problems are below.

1. 1A Premium questionCoding · Medium
2. 2A Premium questionCoding · Medium
3. 3A Premium questionCoding · Easy

Each with hints, the follow-up and a tested solution.

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