Practice your skills (Linear Data Structures)¶

Use this page to practice linear data structures, such as arrays, lists, stacks, and queues. Write pseudocode first, then implement in Java.

A) Arrays vs ArrayList vs LinkedList (timing experiment)¶

In this experiment you will observe how runtime changes for three classic operations when the data structure changes:
- int[] (classic Java array)
- ArrayList<Integer>
- LinkedList<Integer>

The code is provided (you do not write code), and your job is to analyze the results and connect them to the expected Big‑O behavior.

B) Arrays practice (classic problems)¶

These problems train the core skills you need for arrays: scanning, two pointers, and using an extra data structure when it saves time.

Exercise 1 — Two Sum (fast lookup)¶

int[] twoSum(int[] nums, int target)

Exercise 2 — Remove duplicates from sorted array (two pointers)¶

int removeDuplicates(int[] nums)

Exercise 3 — Reverse an array in-place (two pointers)¶

void reverseArray(int[] nums)

C) Stacks & Queues (Banking Practice)¶

Practice stacks (LIFO) and queues (FIFO) using a banking system context. Write pseudocode first, then implement with proper edge case handling.

Project Files¶

All project files are available in the repository below. Some methods are intentionally left incomplete; your task is to implement them based on the exercises.

🔗 Project Repository Link

Exercise 1: BankAccount Transaction Stack (LIFO)¶

In this exercise you will work with a stack of transactions that records all operations performed on a bank account. The stack follows Last-In-First-Out (LIFO) behavior.

Your task is to implement functionality that allows the program to inspect and undo transactions stored in the stack.

Undo the Last Transaction¶

Implement a method that reverses the most recent transaction in the history.

public void undoLastTransaction()

Requirements

• If the transaction stack is empty, print:
  "No transactions to undo."
• Remove the most recent transaction from the stack.
• Update the account balance depending on the transaction type:
• DEPOSIT → subtract the amount from the balance
• WITHDRAW → add the amount back to the balance
• TRANSFER → add the amount back (only this account’s side)
• Print a message indicating the undone transaction and the new balance.

View the Most Recent Transaction¶

Implement a method that shows the latest transaction without removing it from the stack.

public void getLastTransaction()

Requirements

• If the stack is empty, print:
  "No transactions yet."
• Otherwise, print the most recent transaction.

Print the Transaction History¶

Implement a method that prints all transactions stored in the stack.

public void printTransactionHistory()

Requirements

• If the stack is empty, print:
  "No transaction history available."
• Otherwise, print all transactions belonging to the account in the order stored in the stack.

Exercise 2: BankQueue Customer Service (FIFO)¶

In this exercise you will implement a customer queue for a bank service desk.
The queue follows First-In-First-Out (FIFO) behavior: the first customer to join the queue is the first to be served.

Your task is to implement methods that allow the system to add customers, serve the next customer, and inspect the queue.

Add a Customer to the Queue¶

Implement a method that adds a new customer to the end of the queue.

public void addCustomerToQueue(Customer customer)

Requirements

• If customer is null, print:
  "Invalid customer."
  and do not add anything to the queue.
• Otherwise, add the customer to the end of the queue.

Serve the Next Customer¶

Implement a method that removes and returns the next customer to be served.

public Customer serveCustomer()

Requirements

• If the queue is empty, print:
  "Queue empty."
  and return null.
• Otherwise, remove the first customer in the queue and return it.

Print the Queue¶

Implement a method that prints all customers currently waiting in the queue.

public void printQueue()

Requirements

• If the queue is empty, print:
  "Queue empty."
• Otherwise, print all customers in the queue with their position number (starting from 1).

Example format:

1. Alan Turing
2. Ada Lovalace
3. Dennis Ritchie

Edge cases to consider: empty queue · null customer · adding customers after some have already been served

Exercise 3: BankQueue VIP Priority Service¶

In this exercise you will extend the bank queue system to support VIP priority service.
Customers may have different priority levels, and those with higher priority must be served first.

If two customers have the same priority, they must still follow FIFO order (the one who arrived earlier is served first).

Add a Customer with Priority¶

Implement a method that adds a customer to the queue together with a priority level.

public void addVIPCustomer(Customer customer, int priority)

Requirements

• If customer is null, print:
  "Invalid customer."
  and do not add the customer.
• The priority represents the service level:
• 1 → Normal customer
• 3 → VIP customer
• Customers with higher priority must be served before lower priority customers.
• Customers with the same priority must preserve arrival order (FIFO).

Subtask 2: Serve the Next Customer¶

Implement a method that selects and serves the next appropriate customer based on priority.

public Customer serveNextCustomer()

Requirements

• If the queue is empty, print:
  "VIP queue empty."
  and return null.
• Otherwise:
• Serve the highest priority customer available.
• If multiple customers share the same priority, serve the one who arrived first.
• Remove the served customer from the queue and return it.

Master your data tools: Arrays hold everything together, lists flex when needed, stacks undo your last move, queues keep everything in fair order.

Build reliable systems that handle real chaos!