Linked List vs Array When to Choose Which in DSA C# - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an array of integers with 5 elements.

DSA C#

int[] numbers = new int[1];

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

A(5)

B[5]

C{5}

D<5>

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets.

Using curly braces which are for initialization, not size.

In C#, arrays are declared using square brackets []. So to create an array of 5 integers, use new int[5].

2fill in blank

medium

Complete the code to add a new node with value 10 at the start of a linked list.

DSA C#

Node newNode = new Node(10);
newNode.[1] = head;
head = newNode;

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

Aprev

Bchild

Cparent

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which is for doubly linked lists.

Using unrelated property names.

In a singly linked list, each node points to the next node using the 'next' property.

3fill in blank

hard

Fix the error in the code to correctly traverse a linked list and print values.

DSA C#

Node current = head;
while (current != null) {
    Console.WriteLine(current.Value);
    current = current.[1];
}

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

Anext

Bchild

Cprev

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which goes backward in doubly linked lists.

Using unrelated properties.

To move through a singly linked list, update current to current.next each time.

4fill in blank

hard

Fill both blanks to create a list of squares for numbers 1 to 5 using an array.

DSA C#

int[] squares = new int[5];
for (int i = 0; i [1] 5; i++) {
    squares[i] = (i + 1)[2]2;
}

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

A<

B<=

C**

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' causes index out of range error.

Using '**' which is not valid in C# for power.

The loop runs while i is less than 5 (i < 5). To square a number, multiply it by itself using '*'.

5fill in blank

hard

Fill all three blanks to create a dictionary from words to their lengths, filtering words longer than 3 characters.

DSA C#

var result = new Dictionary<string, int>();
foreach (var word in words) {
    if (word.[1] > [2]) {
        result[word] = word.[3];
    }
}

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

ALength

B3

DCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Count' which is not a string property.

Comparing to wrong number.

Use 'Length' property to get word length. Filter words with length > 3. Assign word length to dictionary.

Practice

1. Which data structure is better when you need fast random access to elements by index?

easy

A. Array

B. Linked List

C. Stack

D. Queue

Solution

1. Step 1: Understand random access in arrays

   Arrays store elements in contiguous memory, so accessing any element by index is fast and direct.

2. Step 2: Compare with linked list access

   Linked lists require traversal from the head to reach an element, making random access slower.

3. Final Answer:

   Array -> Option A

4. Quick Check:

   Fast random access = Array [OK]

Hint: For fast index access, choose Array [OK]

Common Mistakes:

• Thinking linked list supports fast random access
• Confusing stack or queue with random access
• Assuming linked list nodes are contiguous

2. Which of the following is the correct way to declare a linked list of integers in C#?

easy

A. List<int> list = new LinkedList<int>();

B. LinkedList<int> list = new LinkedList<int>();

C. LinkedList list = new List<int>();

D. List list = new LinkedList();

Solution

1. Step 1: Recall C# syntax for generic linked list

   The correct syntax uses LinkedList<T> with type parameter and new LinkedList<T>() to create an instance.

2. Step 2: Check each option for correctness

   LinkedList<int> list = new LinkedList<int>(); correctly declares and initializes a LinkedList<int>. Others mix types or omit generics.

3. Final Answer:

   LinkedList<int> list = new LinkedList<int>(); -> Option B

4. Quick Check:

   Correct generic linked list syntax = LinkedList<int> list = new LinkedList<int>(); [OK]

Hint: Use LinkedList<T> with new LinkedList<T>() [OK]

Common Mistakes:

• Mixing List and LinkedList types
• Omitting generic type parameter
• Using non-generic LinkedList without type

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

var list = new LinkedList<int>();
list.AddLast(10);
list.AddLast(20);
list.AddFirst(5);
foreach(var item in list) { Console.Write(item + " "); }

medium

A. 5 10 20

B. 10 20 5

C. 20 10 5

D. 5 20 10

Solution

1. Step 1: Trace linked list insertions

   First, 10 is added at the end, then 20 at the end, so list is [10, 20]. Then 5 is added at the front, making list [5, 10, 20].

2. Step 2: Output the list elements in order

   The foreach prints elements in order: 5, 10, 20 separated by spaces.

3. Final Answer:

   5 10 20 -> Option A

4. Quick Check:

   LinkedList order after AddFirst and AddLast = 5 10 20 [OK]

Hint: AddFirst puts element at start, AddLast at end [OK]

Common Mistakes:

• Assuming AddLast adds at front
• Confusing order of AddFirst and AddLast
• Forgetting foreach prints in list order

4. Identify the error in this code snippet that tries to add an element at index 2 in a linked list:

var list = new LinkedList<int>();
list.AddLast(1);
list.AddLast(2);
list.AddLast(3);
list.Add(2, 99);

medium

A. AddLast requires index parameter

B. AddLast cannot be called multiple times

C. List must be initialized with size

D. LinkedList does not have Add method with index parameter

Solution

1. Step 1: Check LinkedList methods in C#

   LinkedList<T> does not support Add with index; it only supports AddFirst, AddLast, or AddBefore/After nodes.

2. Step 2: Understand why Add(2, 99) causes error

   Calling Add with two parameters is invalid for LinkedList; this method exists in List<T> but not LinkedList.

3. Final Answer:

   LinkedList does not have Add method with index parameter -> Option D

4. Quick Check:

   LinkedList Add with index = invalid [OK]

Hint: LinkedList uses AddFirst/AddLast, no index Add [OK]

Common Mistakes:

• Using List methods on LinkedList
• Assuming Add can take index in LinkedList
• Confusing List<T> and LinkedList<T> APIs

5. You need to frequently insert and delete elements in the middle of a large collection. Which data structure is more efficient and why?

hard

A. Array, because it uses less memory

B. Array, because it allows fast random access

C. Linked List, because it allows fast insertions and deletions without shifting elements

D. Linked List, because it stores elements contiguously

Solution

1. Step 1: Analyze insertion and deletion in arrays

   Arrays require shifting elements after insertion or deletion in the middle, which is slow for large collections.

2. Step 2: Analyze linked list behavior

   Linked lists can insert or delete nodes by changing pointers without shifting, making these operations faster in the middle.

3. Final Answer:

   Linked List, because it allows fast insertions and deletions without shifting elements -> Option C

4. Quick Check:

   Fast middle insert/delete = Linked List [OK]

Hint: For frequent middle changes, choose Linked List [OK]

Common Mistakes:

• Choosing array for fast middle insert/delete
• Thinking linked list stores elements contiguously
• Ignoring cost of shifting in arrays