Double Ended Queue Deque in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new Deque instance.

DSA Java

Deque<Integer> deque = new [1]<>();

Drag options to blanks, or click blank then click option'

AArrayList

BLinkedList

CHashSet

DStack

Attempts:

3 left

💡 Hint

Common Mistakes

Using ArrayList which does not implement Deque.

Using Stack which is LIFO, not double-ended.

The LinkedList class implements the Deque interface in Java, allowing double-ended queue operations.

2fill in blank

medium

Complete the code to add element 10 to the front of the deque.

DSA Java

deque.[1](10);

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Apush

BaddLast

CaddFirst

DofferLast

Attempts:

3 left

💡 Hint

Common Mistakes

Using addLast which adds to the end.

Using push which is for stack behavior.

addFirst adds an element to the front (head) of the deque.

3fill in blank

hard

Fix the error in the code to remove the last element from the deque.

DSA Java

int last = deque.[1]();

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AremoveLast

BremoveFirst

Cpop

DpollFirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using removeFirst which removes from the front.

Using pop which removes from the front in Deque.

removeLast removes and returns the last element of the deque.

4fill in blank

hard

Fill both blanks to check if the deque is empty and then add element 5 to the end.

DSA Java

if (deque.[1]()) {
    deque.[2](5);
}

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AisEmpty

BaddLast

CaddFirst

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using size() instead of isEmpty() for checking emptiness.

Adding element to front instead of end.

isEmpty() checks if the deque has no elements, and addLast(5) adds 5 to the end.

5fill in blank

hard

Fill all three blanks to create a deque, add 1 to front, add 2 to end, then remove from front.

DSA Java

Deque<Integer> dq = new [1]<>();
dq.[2](1);
dq.[3](2);
dq.removeFirst();

Drag options to blanks, or click blank then click option'

ALinkedList

BaddFirst

CaddLast

DArrayDeque

Attempts:

3 left

💡 Hint

Common Mistakes

Using LinkedList instead of ArrayDeque for better performance.

Mixing addFirst and addLast methods.

ArrayDeque is a fast implementation of Deque. addFirst(1) adds 1 to front, addLast(2) adds 2 to end.

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