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This is a Medium difficulty Coding & Algorithms question, commonly asked during Onsite rounds at Coinbase.

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This question is commonly asked for Software Engineer candidates at Coinbase during technical interviews.

Implement a Simple Banking Ledger | Coinbase Coding Question

Quick Overview

This question evaluates competence in designing an efficient, stateful ledger API and associated data structures, emphasizing correctness under invalid inputs, performance at scale, integer overflow handling, unit testing for edge cases, and transactional (atomic batch) behavior.

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Quick Overview

Medium
Coinbase
Coding & Algorithms
Software Engineer

Implement a Simple Banking Ledger

Company: Coinbase

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: Medium

Interview Round: Onsite

Design and implement a banking ledger for n accounts labeled 1..n with initial balances. Provide an API: deposit(id, amount) -> bool, withdraw(id, amount) -> bool, transfer(fromId, toId, amount) -> bool. Rules: return false for invalid account IDs or insufficient funds; otherwise apply updates and return true. Constraints: up to 200,000 operations, amounts up to 10^12, target O( 1) per operation and O(n) space; assume single-threaded execution. Sub-questions: write unit tests for edge cases (invalid IDs, zero/negative amounts, overflow), extend to support atomic batches of operations with rollback on partial failure, and analyze time/space complexity and common pitfalls (e.g., integer overflow).

Quick Answer: This question evaluates competence in designing an efficient, stateful ledger API and associated data structures, emphasizing correctness under invalid inputs, performance at scale, integer overflow handling, unit testing for edge cases, and transactional (atomic batch) behavior.

Process deposit, withdraw, and transfer operations on 1-indexed accounts with validation.

Constraints

Accounts are labeled 1..n
Amounts must be positive

Examples

Input: ([100, 50], [('deposit', 1, 25), ('withdraw', 2, 70), ('transfer', 1, 2, 50)])

Expected Output: {'results': [True, False, True], 'balances': [75, 100]}

Explanation: Valid and invalid operations.

Input: ([0], [('withdraw', 1, 1), ('deposit', 2, 5), ('deposit', 1, 0)])

Expected Output: {'results': [False, False, False], 'balances': [0]}

Explanation: Invalid ids and nonpositive amounts.

Input: ([10, 0], [('transfer', 1, 1, 5)])

Expected Output: {'results': [True], 'balances': [10, 0]}

Explanation: Self-transfer is valid and leaves balance unchanged.

Hints

Validate before mutating so failed operations leave balances unchanged.

Last updated: Jun 27, 2026