Simple Bank System

Simple Bank System

What is this problem about?

The "Simple Bank System" interview question asks you to design a class-based system that manages bank accounts and transactions. You are typically given an initial array of account balances. The system needs to support three main operations: transferring money between accounts, depositing money into an account, and withdrawing money from an account. Each operation must first validate if the account numbers are valid and if the account has sufficient funds for withdrawals or transfers. This "Simple Bank System coding problem" focuses on object-oriented design and simulation.

Why is this asked in interviews?

This problem is a staple in interviews at companies like Uber, Stripe, and Goldman Sachs because it mimics real-world software engineering tasks. It's not about complex algorithms but about "Design, Simulation, and Array interview pattern". Interviewers are looking for clean code, proper input validation, and the ability to manage state correctly. It also tests your familiarity with basic data structures and how to handle 1-indexed versus 0-indexed logic, which is a common source of bugs in production systems.

Algorithmic pattern used

The pattern here is straightforward simulation and state management using an array or a hash table. Since the account numbers are typically given as a range from 1 to N, an array is the most efficient way to store the balances, where the index corresponds to the account number (with a shift of 1). The core logic involves checking three conditions for every transaction:

1. Does the source account exist?
2. Does the destination account exist (for transfers)?
3. Is there enough balance (for withdrawals and transfers)?

Example explanation

Initial balances: Account 1: $10, Account 2: $20, Account 3: $30.

1. withdraw(2, 5): Check if Account 2 exists (Yes) and has >= $5 (Yes). New balance: Account 2:$15. Return True.
2. transfer(3, 1, 40): Check if Account 3 and 1 exist (Yes). Check if Account 3 has >= $40 (No, it has$30). Return False.
3. deposit(1, 10): Check if Account 1 exists (Yes). New balance: Account 1: $20. Return True. The state of the system is updated incrementally after each successful operation.

Common mistakes candidates make

The most frequent mistake is a "off-by-one error" because bank accounts are usually 1-indexed in the input, while programming arrays are 0-indexed. Another common pitfall is forgetting to check if the account number is within the valid range (1 to N). Some candidates also fail to return a boolean value to indicate the success or failure of the operation, which is a key requirement of the "Simple Bank System interview question".

Interview preparation tip

When designing systems, even simple ones, prioritize readability and validation. Always validate your inputs (like checking if accountID <= 0 or accountID > n) at the very beginning of your functions. This "fail-fast" approach is a highly valued trait in professional software development. Practice implementing this in various languages to become comfortable with how different environments handle class state.