Balance calculation algorithm in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding balance calculation in ledger systems

In a ledger system, the balance of an account is calculated by summing all credits and debits. Which of the following best describes the correct approach to calculate the balance?

A Sum all debit transactions and subtract all credit transactions for the account.

B Only consider the latest transaction amount as the balance.

C Sum all transactions regardless of type to get the balance.

D Sum all credit transactions and subtract all debit transactions for the account.

💡 Hint

Think about how credits and debits affect the account balance.

Credits increase the balance, debits decrease it. So, summing credits and subtracting debits gives the correct balance.

❓ Architecture

intermediate

Designing a scalable balance calculation service

You need to design a service that calculates user balances in real-time for millions of users. Which architectural choice best supports scalability and low latency?

A Precompute balances periodically and cache them in a distributed cache system.

B Use a centralized database to calculate balances on every request.

C Calculate balances by scanning all transactions on every request.

D Store balances only on user devices and sync occasionally.

💡 Hint

Consider how to reduce computation and improve response time.

Precomputing balances and caching reduces load and latency, making the system scalable.

❓ scaling

advanced

Handling concurrent balance updates

In a system where multiple transactions update the same account balance concurrently, what is the best approach to ensure balance consistency?

A Lock the entire database during balance updates to prevent conflicts.

B Allow all updates without synchronization and resolve conflicts later.

C Use optimistic concurrency control with version checks before updating balances.

D Ignore concurrency issues as they are rare and do not affect balances.

💡 Hint

Think about how to detect and handle conflicting updates efficiently.

Optimistic concurrency control allows concurrent updates but checks versions to avoid conflicts, ensuring consistency without heavy locking.

❓ tradeoff

advanced

Tradeoffs in real-time vs batch balance calculation

Choosing between real-time and batch processing for balance calculation involves tradeoffs. Which statement correctly describes a key tradeoff?

A Real-time processing reduces latency but increases system complexity and resource usage.

B Batch processing provides instant balance updates but requires more storage.

C Real-time processing is simpler to implement than batch processing.

D Batch processing eliminates the need for data consistency checks.

💡 Hint

Consider latency, complexity, and resource consumption.

Real-time systems provide quick updates but need more resources and complex design; batch systems are simpler but have latency.

❓ estimation

expert

Estimating storage for transaction history in balance calculation

You expect 10 million users, each with 1000 transactions per year. Each transaction record is 200 bytes. Estimate the total storage needed for 3 years of transaction history.

A 60 terabytes

B 6 terabytes

C 600 gigabytes

D 600 terabytes

💡 Hint

Calculate total transactions and multiply by record size, then convert to terabytes.

Total transactions = 10M users * 1000 transactions/year * 3 years = 30 billion transactions.
Storage = 30B * 200 bytes = 6 trillion bytes = 6 terabytes.

Practice

1. What is the main purpose of a balance calculation algorithm in a financial system?

easy

A. To add credits and subtract debits from an initial amount

B. To sort transactions by date

C. To encrypt user data

D. To generate random transaction IDs

Solution

1. Step 1: Understand the role of balance calculation

   The balance calculation algorithm updates the current balance by adding credits and subtracting debits.

2. Step 2: Compare with other options

   Sorting transactions, encrypting data, and generating IDs are unrelated to balance calculation.

3. Final Answer:

   To add credits and subtract debits from an initial amount -> Option A

4. Quick Check:

   Balance calculation = add credits - subtract debits [OK]

Hint: Balance means adding credits and subtracting debits [OK]

Common Mistakes:

• Confusing balance calculation with sorting or encryption
• Thinking balance calculation generates IDs
• Ignoring subtraction of debits

2. Which of the following code snippets correctly initializes a balance variable to zero in a balance calculation algorithm?

easy

A. balance := 0

B. balance = 0

C. balance == 0

D. balance = 'zero'

Solution

1. Step 1: Identify correct assignment syntax

   In most programming languages, = assigns a value. So balance = 0 sets balance to zero.

2. Step 2: Check other options for errors

   := is not standard in many languages, == is a comparison, and assigning a string 'zero' is incorrect for numeric balance.

3. Final Answer:

   balance = 0 -> Option B

4. Quick Check:

   Use = for assignment, not == or := [OK]

Hint: Use single = to assign values in most languages [OK]

Common Mistakes:

• Using == instead of = for assignment
• Using := which is not common in many languages
• Assigning string instead of numeric zero

3. Consider this pseudocode for balance calculation:

balance = 100
transactions = [20, -10, 30, -5]
for t in transactions:
    balance += t
print(balance)

What will be the printed balance?

medium

A. 135

B. 145

C. 155

D. 125

Solution

1. Step 1: Calculate sum of transactions

   Sum = 20 + (-10) + 30 + (-5) = 20 - 10 + 30 - 5 = 35

2. Step 2: Add sum to initial balance

   Initial balance 100 + 35 = 135

3. Final Answer:

   135 -> Option A

4. Quick Check:

   100 + (20 -10 +30 -5) = 135 [OK]

Hint: Add all transactions to initial balance [OK]

Common Mistakes:

• Adding absolute values instead of signed values
• Forgetting to add initial balance
• Miscalculating sum of transactions

4. Identify the bug in this balance calculation code snippet:

balance = 50
transactions = [10, -20, 15]
for t in transactions:
    balance = t
print(balance)

medium

A. The initial balance is not set

B. The transactions list is empty

C. The balance is overwritten instead of updated

D. The loop variable is incorrect

Solution

1. Step 1: Analyze the loop operation

   Inside the loop, balance = t overwrites balance each time instead of adding.

2. Step 2: Understand correct update

   It should be balance += t to add each transaction to balance.

3. Final Answer:

   The balance is overwritten instead of updated -> Option C

4. Quick Check:

   Use += to update balance, not = [OK]

Hint: Use += to add transactions, not = [OK]

Common Mistakes:

• Using = instead of += inside loop
• Assuming transactions list is empty
• Ignoring initial balance

5. You need to design a balance calculation system that handles millions of transactions per day with real-time updates. Which design choice best supports scalability and accuracy?

hard

A. Ignore debits and only add credits to simplify calculation

B. Use a single database transaction to update balance after each transaction

C. Store all transactions in memory and recalculate balance on every request

D. Batch transactions and update balance periodically with distributed processing

Solution

1. Step 1: Consider scalability needs

   Millions of transactions require efficient processing; single updates per transaction cause bottlenecks.

2. Step 2: Evaluate batch processing benefits

   Batching with distributed processing reduces load and maintains accuracy by processing groups of transactions.

3. Step 3: Reject other options

   Single DB transactions cause slowdowns, in-memory recalculation is memory-heavy and slow, ignoring debits causes incorrect balances.

4. Final Answer:

   Batch transactions and update balance periodically with distributed processing -> Option D

5. Quick Check:

   Batch + distributed processing = scalable & accurate [OK]

Hint: Batch and distribute processing for large-scale balance updates [OK]

Common Mistakes:

• Updating balance per transaction causing bottlenecks
• Recalculating balance on every request wasting memory
• Ignoring debits leading to wrong balances