Double Ended Queue Deque in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Deque Operations Sequence

What is the printed state of the deque after executing the following Java code?

DSA Java

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

public class Main {
    public static void main(String[] args) {
        Deque<Integer> deque = new ArrayDeque<>();
        deque.addFirst(10);
        deque.addLast(20);
        deque.addFirst(5);
        deque.removeLast();
        deque.addLast(15);
        System.out.println(deque);
    }
}

A [10, 20, 15]

B [10, 5, 15]

C [5, 10, 15]

D [5, 10, 20]

💡 Hint

Remember addFirst adds to the front, addLast adds to the end, and removeLast removes from the end.

The operations addFirst(10), addLast(20), addFirst(5) result in deque: 5, 10, 20.

Then removeLast() removes 20, leaving 5, 10.

Finally addLast(15) adds 15 at the end, resulting in 5, 10, 15.

🧠 Conceptual

intermediate

Deque Characteristics

Which of the following statements about a double ended queue (deque) is true?

A Elements can be added or removed from both front and rear ends.

B Elements can only be added or removed from the front.

C Deque only supports FIFO (First In First Out) behavior.

D Deque is a fixed size data structure and cannot grow dynamically.

💡 Hint

Think about what 'double ended' means in deque.

A deque allows insertion and removal of elements from both ends, front and rear.

It supports both FIFO and LIFO behaviors depending on usage.

🔧 Debug

advanced

Identify the Runtime Error in Deque Usage

What error will this Java code produce when run?

DSA Java

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

public class Main {
    public static void main(String[] args) {
        Deque<Integer> deque = new ArrayDeque<>();
        deque.addFirst(1);
        deque.addLast(2);
        deque.removeFirst();
        deque.removeFirst();
        deque.removeFirst();
    }
}

A No error, program runs successfully.

B java.util.NoSuchElementException at removeFirst() when deque is empty.

C NullPointerException at removeFirst() when deque is empty.

D ArrayIndexOutOfBoundsException at removeFirst() when deque is empty.

💡 Hint

What happens if you remove an element from an empty deque using removeFirst()?

Calling removeFirst() on an empty deque throws NoSuchElementException.

Here, after removing two elements, the third removeFirst() tries to remove from empty deque causing the exception.

❓ Predict Output

advanced

Deque State After Mixed Operations

What is the output of the following Java code snippet?

DSA Java

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

public class Main {
    public static void main(String[] args) {
        Deque<String> deque = new ArrayDeque<>();
        deque.offerLast("A");
        deque.offerFirst("B");
        deque.offerLast("C");
        deque.pollFirst();
        deque.offerFirst("D");
        System.out.println(deque);
    }
}

A [D, B, C]

B [B, A, C]

C [A, C, D]

D [D, A, C]

💡 Hint

Remember offerFirst adds to front, offerLast adds to end, pollFirst removes from front.

Initial deque after offerLast("A"): [A]

After offerFirst("B"): [B, A]

After offerLast("C"): [B, A, C]

pollFirst() removes "B": [A, C]

offerFirst("D"): [D, A, C]

🧠 Conceptual

expert

Deque Implementation Choice

Which data structure is best suited for implementing a deque to achieve O(1) time complexity for insertions and deletions at both ends?

A Doubly linked list with head and tail pointers

B Singly linked list with head pointer only

C Array with shifting elements on insertions and deletions

D Binary search tree

💡 Hint

Think about how to access both ends efficiently without shifting elements.

A doubly linked list with head and tail pointers allows O(1) insertions and deletions at both ends.

Singly linked list or array require shifting or traversal, causing O(n) operations.

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