Implement deposit, withdraw, and transfer in a class

Q: Implement deposit, withdraw, and transfer in a class

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Question

Problem

Implement a class AccountService that supports basic money operations on accounts.

Operations

deposit(accountId, amount)
withdraw(accountId, amount)
transfer(fromAccountId, toAccountId, amount)
getBalance(accountId)

Rules

amount is a positive integer.
withdraw must fail (return false or throw) if the account has insufficient funds.
transfer must be atomic : either both balances change, or neither changes.
If an account does not exist, treat its initial balance as 0 (or explicitly create on first use).

Concurrency (if discussed)

Assume these methods may be called concurrently from multiple threads. Make the implementation thread-safe and avoid deadlocks.

Input/Output

This is an in-memory class design question; you do not need to build a database or persistence.