Double Ended Queue Deque in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C#

deque.[1](10);

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

AAddLast

BAddFirst

CRemoveFirst

DRemoveLast

Attempts:

3 left

💡 Hint

Common Mistakes

Using AddLast adds to the end, not the front.

Using Remove methods instead of Add.

Use AddFirst to add an element at the front of the deque.

2fill in blank

medium

Complete the code to remove an element from the end of the deque.

DSA C#

int last = deque.[1]();

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

ARemoveFirst

BAddFirst

CRemoveLast

DAddLast

Attempts:

3 left

💡 Hint

Common Mistakes

Using Add methods instead of Remove.

Using RemoveFirst removes from the front, not the end.

Use RemoveLast to remove and return the element at the end of the deque.

3fill in blank

hard

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

DSA C#

int first = deque.[1]();

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

APeekFirst

BRemoveFirst

CPeek

DFirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using RemoveFirst removes the element instead of peeking.

Using Peek alone is not a method in C# deque.

PeekFirst returns the first element without removing it from the deque.

4fill in blank

hard

Fill both blanks to create a deque and add elements at both ends.

DSA C#

var deque = new [1]<int>();
deque.[2](5);

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

ADeque

BAddFirst

CLinkedList

DAddLast

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use a non-existent Deque class.

Using AddFirst when the instruction says to add at the end.

Use LinkedList<int> to create a deque-like structure in C#. Use AddLast to add at the end.

5fill in blank

hard

Fill all three blanks to remove the first element, add a new element at the front, and peek at the last element.

DSA C#

int removed = deque.[1]();
deque.[2](20);
int last = deque.[3]();

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

ARemoveFirst

BAddFirst

CPeekLast

DRemoveLast

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up AddFirst and AddLast.

Using RemoveLast instead of RemoveFirst.

Using PeekFirst instead of PeekLast.

First, remove the first element with RemoveFirst. Then add 20 at the front with AddFirst. Finally, peek at the last element with PeekLast.

Practice

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

easy

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

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

C. A stack where elements are added and removed from the top only

D. A list that allows random access to elements

Solution

1. Step 1: Understand the definition of Deque

   A Deque allows insertion and deletion at both front and rear ends, unlike a queue or stack.

2. Step 2: Compare with other data structures

   Queue restricts insertion to rear and deletion from front; stack restricts both to one end.

3. Final Answer:

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

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 queue
• Thinking deque allows random access
• Assuming insertion only at one end

2. Which of the following is the correct way to declare a Deque of integers in C# using System.Collections.Generic?

easy

A. LinkedList<int> dq = new LinkedList<int>();

B. Queue<int> dq = new Queue<int>();

C. Deque<int> dq = new Deque<int>();

D. List<int> dq = new List<int>();

Solution

1. Step 1: Recall C# standard library support for Deque

   C# does not have a built-in Deque class, but LinkedList can be used as a deque.

2. Step 2: Analyze options

   Deque<int> dq = new Deque<int>(); is invalid as Deque class does not exist; Queue<int> dq = new Queue<int>(); is a queue, not deque; List<int> dq = new List<int>(); is a list, not deque.

3. Final Answer:

   LinkedList<int> dq = new LinkedList<int>(); -> Option A

4. Quick Check:

   LinkedList supports deque operations [OK]

Hint: Use LinkedList for deque in C# as no built-in Deque [OK]

Common Mistakes:

• Trying to use non-existent Deque class
• Using Queue which is single-ended
• Using List which lacks efficient front operations

3. What will be the output of the following C# code snippet?

var dq = new LinkedList<int>();
dq.AddLast(10);
dq.AddFirst(20);
dq.AddLast(30);
dq.RemoveFirst();
foreach(var item in dq) Console.Write(item + " ");

medium

A. 20 10 30

B. 30 10

C. 20 30

D. 10 30

Solution

1. Step 1: Trace the additions to the deque

   Added 10 at last: [10]; Added 20 at first: [20, 10]; Added 30 at last: [20, 10, 30]

2. Step 2: Remove first element

   RemoveFirst removes 20; deque becomes [10, 30]

3. Step 3: Print elements

   Output is "10 30 "

4. Final Answer:

   10 30 -> Option D

5. Quick Check:

   After RemoveFirst, deque = 10 30 [OK]

Hint: AddFirst adds front, RemoveFirst removes front [OK]

Common Mistakes:

• Confusing AddFirst and AddLast positions
• Forgetting RemoveFirst removes front element
• Assuming RemoveFirst removes last element

4. Identify the error in the following C# code using LinkedList as a deque:

var dq = new LinkedList<int>();
dq.AddFirst(5);
dq.RemoveLast();
dq.RemoveLast();
Console.WriteLine(dq.Count);

medium

A. No error, output is 0

B. InvalidOperationException at second RemoveLast call

C. Compile-time error due to RemoveLast misuse

D. Output is 1

Solution

1. Step 1: Trace the operations on deque

   Added 5 at front: deque = [5]; RemoveLast removes 5: deque = []

2. Step 2: Second RemoveLast on empty deque

   Calling RemoveLast on empty LinkedList throws InvalidOperationException

3. Final Answer:

   InvalidOperationException at second RemoveLast call -> Option B

4. Quick Check:

   Removing from empty deque throws exception [OK]

Hint: Removing from empty deque causes exception [OK]

Common Mistakes:

• Assuming RemoveLast on empty deque does nothing
• Expecting output 0 instead of exception
• Confusing compile-time and runtime errors

5. You want to implement a task scheduler using a deque where urgent tasks are added to the front and normal tasks to the rear. Which sequence of operations correctly adds urgent task 1, normal task 2, urgent task 3, then removes one task from the front?

var dq = new LinkedList<int>();
// Fill in operations below

hard

A. dq.AddFirst(1); dq.AddFirst(2); dq.AddLast(3); dq.RemoveLast();

B. dq.AddLast(1); dq.AddFirst(2); dq.AddLast(3); dq.RemoveLast();

C. dq.AddFirst(1); dq.AddLast(2); dq.AddFirst(3); dq.RemoveFirst();

D. dq.AddLast(1); dq.AddLast(2); dq.AddFirst(3); dq.RemoveFirst();

Solution

1. Step 1: Add urgent tasks to front and normal tasks to rear

   Urgent task 1: AddFirst(1); Normal task 2: AddLast(2); Urgent task 3: AddFirst(3)

2. Step 2: Remove one task from front

   RemoveFirst removes the front urgent task 3

3. Final Answer:

   dq.AddFirst(1); dq.AddLast(2); dq.AddFirst(3); dq.RemoveFirst(); -> Option C

4. Quick Check:

   Urgent front, normal rear, remove front [OK]

Hint: Urgent = AddFirst, Normal = AddLast, Remove from front [OK]

Common Mistakes:

• Mixing AddFirst and AddLast for task priority
• Removing from wrong end
• Incorrect order of operations