Recently, a client approached CSOAHELP while preparing for a technical interview with Coinbase. Initially, they believed that regular algorithm preparation would suffice, but the real interview content turned out to be far more complex. On-the-spot deduction and flexible problem-solving were the true skills being assessed.

Today, we share this real interview experience to show how an ordinary-looking transaction processing question actually hides many assessment points. Without our real-time remote assistance, it would have been nearly impossible for the candidate to succeed.

On the day of the interview, the candidate logged in and was immediately presented with the problem without any small talk.

Here is the original problem statement:

Introduce an ability to delete a transaction given the user and transaction's number with respect to user. User Y wants to delete their nth transaction. What is the balance of user X?

The input consists of initial balances and a series of transactions, each representing a payer-to-payee relationship where the amount transferred is a percentage of the payer's current balance. The interview required the candidate to delete the nth transaction of a given user, recalculate all users' final balances, and report the balance of a specified user.

At first, the candidate was nervous. Although they understood the basic requirement, they quickly became stuck: how to delete a transaction? Would subsequent transactions be affected? Is full recalculation necessary? How to ensure efficiency if there are millions of transactions? Faced with these questions, the candidate was momentarily silent.

At this critical moment, CSOAHELP provided real-time textual guidance:

Start by re-simulating the transaction flow from the initial state, deleting the specified transaction, and executing the remaining transactions in order to finally calculate each user's balance.

We also reminded the candidate: after deleting a transaction, the basis for subsequent transactions would change, so a complete re-simulation was necessary.

With this clear direction, the candidate quickly stabilized, confirmed the approach with the interviewer, and received approval to proceed with the detailed implementation.

Soon after, the interviewer raised a follow-up question: if there are millions of transactions, is re-simulating each time reasonable? Are there optimization methods?

The candidate hesitated again, and CSOAHELP immediately pushed supplementary ideas:

Use lazy recalculation strategies, triggering recomputation only when necessary, while maintaining caching for normal queries to improve efficiency. Alternatively, periodically save transaction results to reduce redundant calculations.

The candidate successfully paraphrased this approach, which the interviewer acknowledged as a good direction.

Next came the implementation phase. Since the candidate had limited experience with complex logic, we provided structured prompts throughout:

First, build a transaction list where each record includes payer, payee, and percentage. After deleting the nth transaction for the specified user, re-execute all transactions from the initial balance. Each transfer dynamically calculates the amount based on the payer's current balance and updates the payer's and payee's balances accordingly.

The candidate simply needed to naturally organize their language according to the prompts, and write down the key parts based on the guidance. The whole process was smooth, with no major stumbles.

Just as the implementation was nearing completion, the interviewer added another layer of inquiry: what if the initial balance is negative, or if a payer's balance is insufficient, resulting in a zero-dollar transfer? How should the system handle this?

CSOAHELP quickly pushed a coping strategy:

Perform balance validation during data initialization. If a negative balance is detected, immediately throw an exception and prohibit further transactions. During execution, if a payer's balance is insufficient, the current transfer amount should be treated as zero, an exception should be logged, but the process should not be interrupted.

The candidate followed the prompts to restate the handling plan and proactively added thoughts about logging exceptions and ensuring system robustness. The interviewer appreciated this consideration and further asked how to maintain consistency in a concurrent environment with multiple users operating simultaneously.

We promptly supplemented the candidate's response idea: use pessimistic locking to ensure atomic operations, or apply optimistic concurrency control based on version numbers to balance consistency and performance under high concurrency.

The candidate fully articulated the solution following our guidance, and the interviewer was clearly impressed with the candidate's systematic thinking.

After nearly forty minutes of detailed reasoning and intensive questioning, the candidate successfully completed all tasks. In the closing remarks, the interviewer stated that the candidate's thinking was clear, reasoning was solid, and their ability to adapt under pressure was very close to their expectations for an excellent engineer.

In reality, the candidate was well aware that without CSOAHELP providing comprehensive support—whether guiding ideas, suggesting key logic, or supplementing answers to follow-up questions—this interview would hardly have gone so smoothly.

Many people think that big tech interviews are only about solving problems, but the real challenge is: can you quickly and steadily build solutions in uncertain scenarios? Can you maintain clarity when being constantly questioned and requirements keep changing? Can you propose practical engineering optimizations under large-scale system settings?

These capabilities are precisely what CSOAHELP's real-time remote interview assistance helps you achieve.

We are far more than just a question coaching service. During your real interviews, we:

Observe your responses in real time Promptly provide complete and systematic textual guidance at key moments Supplement code structure hints if needed to prevent detail-related mistakes Quickly support your logical expansion during follow-up questioning Allow you to focus solely on expressing yourself without being overwhelmed by panic.

If you also have important interviews with companies like Coinbase, Stripe, Google, Apple, Meta, or Amazon, and you want to perform at your best during these crucial moments, CSOAHELP is your most reliable choice.

True experts know how to choose the most trustworthy support at critical times.

经过csoahelp的面试辅助，候选人获取了良好的面试表现。如果您需要面试辅助或面试代面服务，帮助您进入梦想中的大厂，请随时联系我。

If you need more interview support or interview proxy practice, feel free to contact us. We offer comprehensive interview support services to help you successfully land a job at your dream company.