Dequeue Operation in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element at the front of the dequeue.

DSA Java

deque.[1](10);

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

AaddFirst

BremoveLast

CremoveFirst

DaddLast

Attempts:

3 left

💡 Hint

Common Mistakes

Using addLast instead of addFirst adds element at the end, not front.

Using remove methods instead of add methods.

The method addFirst adds an element at the front of the dequeue.

2fill in blank

medium

Complete the code to remove an element from the rear of the dequeue.

DSA Java

int removed = deque.[1]();

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

AaddLast

BremoveFirst

CaddFirst

DremoveLast

Attempts:

3 left

💡 Hint

Common Mistakes

Using removeFirst removes from front, not rear.

Using add methods instead of remove methods.

The method removeLast removes and returns the element from the rear of the dequeue.

3fill in blank

hard

Fix the error in the code to peek at the front element without removing it.

DSA Java

int front = deque.[1]();

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

ApollLast

BpeekFirst

CpeekLast

DpollFirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using poll methods removes the element.

Using peekLast looks at the rear, not front.

peekFirst returns the front element without removing it. pollFirst removes it.

4fill in blank

hard

Fill both blanks to add 20 at the rear and remove from the front.

DSA Java

deque.[1](20);
int removed = deque.[2]();

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

AaddLast

BremoveFirst

CaddFirst

DremoveLast

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing addFirst with removeFirst or addLast with removeLast incorrectly.

Using removeLast instead of removeFirst.

addLast adds at rear, removeFirst removes from front.

5fill in blank

hard

Fill all three blanks to add 5 at front, 15 at rear, and then remove from rear.

DSA Java

deque.[1](5);
deque.[2](15);
int removed = deque.[3]();

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

AaddFirst

BaddLast

CremoveLast

DremoveFirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using removeFirst instead of removeLast for rear removal.

Mixing addFirst and addLast positions.

addFirst adds at front, addLast adds at rear, removeLast removes from rear.

Practice

1. What is the main difference between a deque and a regular queue?

easy

A. Deque is slower than queue in all operations

B. Deque only allows insertion, queue only allows deletion

C. Deque allows insertion and deletion from both ends, queue only from one end

D. Deque stores elements in sorted order, queue does not

Solution

1. Step 1: Understand queue operations

   A regular queue allows insertion at the rear and deletion from the front only.

2. Step 2: Understand deque operations

   A deque (double-ended queue) allows insertion and deletion at both front and rear ends.

3. Final Answer:

   Deque allows insertion and deletion from both ends, queue only from one end -> Option C

4. Quick Check:

   Deque = Both ends operations [OK]

Hint: Deque = double-ended operations, queue = single-ended [OK]

Common Mistakes:

• Thinking deque only allows insertion
• Confusing deque with stack
• Assuming deque stores sorted data

2. Which of the following is the correct way to add an element to the front of a Java Deque named dq?

easy

A. dq.addLast(element);

B. dq.addFirst(element);

C. dq.pushBack(element);

D. dq.enqueueFront(element);

Solution

1. Step 1: Recall Java Deque methods

   Java Deque interface provides addFirst() to add element at the front.

2. Step 2: Check options for correct method

   Only dq.addFirst(element); correctly adds at front.

3. Final Answer:

   dq.addFirst(element); -> Option B

4. Quick Check:

   addFirst() adds front [OK]

Hint: Use addFirst() to add at front in Java Deque [OK]

Common Mistakes:

• Using addLast() to add at front
• Using non-existent methods like pushBack()
• Confusing enqueue with Deque methods

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

Deque<Integer> dq = new ArrayDeque<>();
dq.addLast(10);
dq.addFirst(20);
dq.addLast(30);
System.out.println(dq);

medium

A. [20, 30, 10]

B. [10, 20, 30]

C. [30, 10, 20]

D. [20, 10, 30]

Solution

1. Step 1: Trace additions to deque

   First addLast(10) adds 10 at rear: [10]. Then addFirst(20) adds 20 at front: [20, 10]. Then addLast(30) adds 30 at rear: [20, 10, 30].

2. Step 2: Print deque state

   Printing dq shows elements in order: [20, 10, 30].

3. Final Answer:

   [20, 10, 30] -> Option D

4. Quick Check:

   addFirst adds front, addLast adds rear [OK]

Hint: Track addFirst at front, addLast at rear [OK]

Common Mistakes:

• Mixing order of addFirst and addLast
• Assuming addLast adds at front
• Confusing printed order with insertion order

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

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

medium

A. removeFront() method does not exist in Deque

B. addFirst() cannot be used with LinkedList

C. Deque cannot store String elements

D. dq should be initialized with ArrayDeque, not LinkedList

Solution

1. Step 1: Check Deque methods

   Deque interface does not have a method named removeFront(). The correct method to remove from front is removeFirst().

2. Step 2: Verify other statements

   addFirst() works with LinkedList, Deque can store Strings, and LinkedList is a valid Deque implementation.

3. Final Answer:

   removeFront() method does not exist in Deque -> Option A

4. Quick Check:

   Use removeFirst() to remove front [OK]

Hint: Use removeFirst(), not removeFront() [OK]

Common Mistakes:

• Using removeFront() instead of removeFirst()
• Thinking LinkedList can't implement Deque
• Confusing method names for removal

5. Given a Deque dq initially empty, which sequence of operations will result in the deque containing [5, 10, 15] from front to rear?

hard

A. dq.addFirst(5); dq.addLast(10); dq.addLast(15);

B. dq.addLast(15); dq.addLast(10); dq.addLast(5);

C. dq.addFirst(5); dq.addFirst(10); dq.addFirst(15);

D. dq.addLast(5); dq.addLast(15); dq.addLast(10);

Solution

1. Step 1: Analyze dq.addFirst(5); dq.addLast(10); dq.addLast(15); operations

   Start empty: []. addFirst(5) -> [5]. addLast(10) -> [5, 10]. addLast(15) -> [5, 10, 15].

2. Step 2: Verify final deque order

   Deque after operations is [5, 10, 15], matching the required order.

3. Final Answer:

   dq.addFirst(5); dq.addLast(10); dq.addLast(15); -> Option A

4. Quick Check:

   addFirst front, addLast rear = correct order [OK]

Hint: Add 5 front, then 10 and 15 at rear for correct order [OK]

Common Mistakes:

• Adding all elements at front reverses order
• Adding all elements at rear but starting with wrong element
• Mixing addFirst and addLast order