Dequeue Operation in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a dequeue (double-ended queue)?

A dequeue is a data structure where elements can be added or removed from both the front and the rear ends.

beginner

Name the two main operations to remove elements in a dequeue.

The two main removal operations are removeFront() and removeRear(), which remove elements from the front and rear ends respectively.

intermediate

What happens if you try to remove an element from an empty dequeue?

Trying to remove an element from an empty dequeue usually results in an error or returns a special value indicating the dequeue is empty.

beginner

Explain the difference between removeFront() and removeRear() in a dequeue.

removeFront() removes the element at the front end, while removeRear() removes the element at the rear end of the dequeue.

intermediate

Why is a dequeue useful compared to a simple queue?

A dequeue is more flexible because it allows insertion and removal from both ends, unlike a simple queue which only allows these operations at fixed ends.

Which operation removes an element from the rear end of a dequeue?

A dequeue()

B removeFront()

C enqueue()

D removeRear()

removeRear() removes an element from the rear end of a dequeue.

What will happen if you call removeFront() on an empty dequeue?

A It throws an error or returns empty

B It swaps front and rear

C It adds a new element

D It removes the last element

Removing from an empty dequeue causes an error or returns a special empty value.

Which of these is NOT a valid dequeue operation?

A peekMiddle()

B removeRear()

C addFront()

D addRear()

peekMiddle() is not a standard dequeue operation.

In a dequeue, where does removeFront() remove an element from?

A Rear end

B Middle

C Front end

D Random position

removeFront() removes the element at the front end.

What is the main advantage of a dequeue over a simple queue?

A Allows insertion/removal only at rear

B Allows insertion/removal at both ends

C Stores elements in sorted order

D Has fixed size

A dequeue allows insertion and removal at both front and rear ends.

Describe the main operations of a dequeue and how they differ from a simple queue.

Think about where elements can be added or removed.

Explain what happens internally when you perform a removeRear() operation on a dequeue.

Imagine removing the last person from a line.

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