Double Ended Queue Deque in DSA Java - Build from Scratch

Double Ended Queue (Deque) Operations

📖 Scenario: Imagine you are managing a line of customers waiting for service at a store. Sometimes customers join the line at the front (priority customers), and sometimes at the back (regular customers). You also serve customers from both ends depending on the situation.

🎯 Goal: You will create a double ended queue (deque) using Java's ArrayDeque. You will add customers to both ends, remove customers from both ends, and finally display the current queue.

📋 What You'll Learn

Create an ArrayDeque of strings called customerQueue.

Add customers to the front and back of the deque.

Remove customers from the front and back of the deque.

Print the final state of the deque showing the order of customers.

💡 Why This Matters

🌍 Real World

Deques are useful in real-world scenarios like managing lines where people can join or leave from both ends, such as customer service, task scheduling, or undo-redo operations.

💼 Career

Understanding deques helps in software development roles that involve data structure manipulation, optimizing performance, and implementing efficient algorithms.

1

Create the Deque and Add Initial Customers

Create an ArrayDeque of strings called customerQueue. Add these customers in order to the back of the deque: "Alice", "Bob", "Charlie".

DSA Java

// Create the deque and add initial customers
// Your code here

Hint

Use Deque<String> customerQueue = new ArrayDeque<>(); to create the deque. Use addLast() to add customers to the back.

2

Add Priority Customers to the Front

Add two priority customers to the front of customerQueue: "Diana" and "Ethan", in that order (Diana first, then Ethan).

DSA Java

import java.util.ArrayDeque;
import java.util.Deque;

public class Main {
    public static void main(String[] args) {
        Deque customerQueue = new ArrayDeque();
        customerQueue.addLast("Alice");
        customerQueue.addLast("Bob");
        customerQueue.addLast("Charlie");
        // Add priority customers to the front
        // Your code here
    }
}

Hint

Use addFirst() to add customers to the front of the deque.

3

Serve Customers from Both Ends

Remove one customer from the front and one customer from the back of customerQueue. Store the removed customers in variables servedFront and servedBack respectively.

DSA Java

import java.util.ArrayDeque;
import java.util.Deque;

public class Main {
    public static void main(String[] args) {
        Deque customerQueue = new ArrayDeque();
        customerQueue.addLast("Alice");
        customerQueue.addLast("Bob");
        customerQueue.addLast("Charlie");
        customerQueue.addFirst("Diana");
        customerQueue.addFirst("Ethan");
        // Remove one customer from front and one from back
        // Your code here
    }
}

Hint

Use removeFirst() to remove from the front and removeLast() to remove from the back.

4

Print the Final State of the Queue

Print the current state of customerQueue to show the order of customers waiting. Use System.out.println(customerQueue);.

DSA Java

import java.util.ArrayDeque;
import java.util.Deque;

public class Main {
    public static void main(String[] args) {
        Deque customerQueue = new ArrayDeque();
        customerQueue.addLast("Alice");
        customerQueue.addLast("Bob");
        customerQueue.addLast("Charlie");
        customerQueue.addFirst("Diana");
        customerQueue.addFirst("Ethan");
        String servedFront = customerQueue.removeFirst();
        String servedBack = customerQueue.removeLast();
        // Print the current queue
        // Your code here
    }
}

Hint

Use System.out.println(customerQueue); to print the deque. The output shows the order from front to back.

Practice

1. What is a Double Ended Queue (Deque)?

easy

A. A queue where elements can only be added at the rear

B. A data structure where elements can be added or removed from both ends

C. A stack where elements can only be added or removed from the top

D. A list that allows random access to elements

Solution

1. Step 1: Understand the definition of Deque

   A Deque allows insertion and deletion from both front and rear ends.

2. Step 2: Compare with other data structures

   Unlike queues or stacks, Deque supports operations at both ends.

3. Final Answer:

   A data structure where elements can be added or removed from both ends -> Option B

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Deque means double-ended queue, both ends usable [OK]

Common Mistakes:

• Confusing Deque with stack or simple queue
• Thinking Deque allows random access like arrays
• Assuming Deque only works from one end

2. Which of the following is the correct way to create a Deque in Java using the standard library?

easy

A. Deque<Integer> dq = new ArrayDeque<>();

B. Deque<Integer> dq = new Deque<>();

C. Deque<Integer> dq = new Stack<>();

D. Deque<Integer> dq = new Queue<>();

Solution

1. Step 1: Identify valid Deque implementations in Java

   ArrayDeque implements the Deque interface and is the preferred implementation.

2. Step 2: Check syntax correctness

   ArrayDeque is instantiated with new ArrayDeque<>(), which is correct syntax.

3. Final Answer:

   Deque<Integer> dq = new ArrayDeque<>(); -> Option A

4. Quick Check:

   ArrayDeque is standard Deque implementation [OK]

Hint: Use ArrayDeque for Deque in Java [OK]

Common Mistakes:

• Using Stack which is not a Deque
• Trying to instantiate interface Queue directly
• Trying to instantiate the Deque interface directly

3. What will be the output of the following Java code?

Deque<String> dq = new ArrayDeque<>();
dq.addFirst("A");
dq.addLast("B");
dq.addFirst("C");
System.out.println(dq);

medium

A. [A, C, B]

B. [A, B, C]

C. [C, A, B]

D. [B, A, C]

Solution

1. Step 1: Trace addFirst and addLast operations

   addFirst("A") adds A at front: [A]
   addLast("B") adds B at rear: [A, B]
   addFirst("C") adds C at front: [C, A, B]

2. Step 2: Print the Deque

   Printing dq shows elements in order: [C, A, B]

3. Final Answer:

   [C, A, B] -> Option C

4. Quick Check:

   addFirst adds front, addLast adds rear [OK]

Hint: addFirst puts element at front, addLast at rear [OK]

Common Mistakes:

• Mixing order of addFirst and addLast
• Assuming addFirst adds at rear
• Confusing print output format

4. Identify the error in the following Java code using Deque:

Deque<Integer> dq = new ArrayDeque<>();
dq.addFirst(10);
dq.addLast(20);
dq.removeLast();
dq.removeFirst();
dq.removeFirst();

medium

A. No error, code runs fine

B. Compilation error due to wrong method names

C. Throws NullPointerException on removeLast()

D. Throws NoSuchElementException on last removeFirst()

Solution

1. Step 1: Trace the Deque operations

   Start: empty
   addFirst(10): [10]
   addLast(20): [10, 20]
   removeLast(): removes 20 -> [10]
   removeFirst(): removes 10 -> []
   removeFirst(): tries to remove from empty Deque

2. Step 2: Identify exception on removing from empty Deque

   removeFirst() on empty Deque throws NoSuchElementException

3. Final Answer:

   Throws NoSuchElementException on last removeFirst() -> Option D

4. Quick Check:

   Removing from empty Deque throws NoSuchElementException [OK]

Hint: Removing from empty Deque throws NoSuchElementException [OK]

Common Mistakes:

• Assuming removeFirst returns null instead of exception
• Confusing method names causing compile error
• Thinking removeLast throws NullPointerException

5. You want to implement a task scheduler that processes tasks in the order they arrive but sometimes needs to prioritize urgent tasks by adding them to the front. Which Deque operation combination is best suited for this scenario?

hard

A. Use addLast() for normal tasks and addFirst() for urgent tasks

B. Use addFirst() for normal tasks and addLast() for urgent tasks

C. Use only addLast() for all tasks

D. Use only addFirst() for all tasks

Solution

1. Step 1: Understand task order requirements

   Normal tasks should be processed in arrival order (queue behavior). Urgent tasks should be processed before normal tasks (front insertion).

2. Step 2: Match Deque operations to requirements

   addLast() adds normal tasks at rear (end), preserving arrival order.
   addFirst() adds urgent tasks at front, prioritizing them.

3. Final Answer:

   Use addLast() for normal tasks and addFirst() for urgent tasks -> Option A

4. Quick Check:

   Normal at rear, urgent at front [OK]

Hint: Normal tasks rear, urgent tasks front with addLast/addFirst [OK]

Common Mistakes:

• Reversing addFirst and addLast roles
• Using only one end for all tasks losing priority
• Confusing queue and stack behavior