Why Linked List Exists and What Problem It Solves in DSA C++ - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a node pointer named 'head' initialized to nullptr.

DSA C++

Node* [1] = nullptr;

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

Afirst

Bnode

Cstart

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names like 'node' or 'start' which are not standard for the head pointer.

The pointer 'head' is commonly used to mark the start of a linked list. Initializing it to nullptr means the list is empty.

2fill in blank

medium

Complete the code to create a new node dynamically using 'new'.

DSA C++

Node* newNode = [1] Node();

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

Amalloc

Bcreate

Cnew

Dallocate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'malloc' which is not recommended in C++ for object creation.

In C++, 'new' is used to allocate memory dynamically for objects like nodes.

3fill in blank

hard

Fix the error in the code to link the new node to the existing list by setting 'next' pointer.

DSA C++

newNode->next = [1];

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

Anullptr

Bhead

CnewNode

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Setting 'next' to nullptr or newNode itself, breaking the list link.

To insert a new node at the beginning, its 'next' pointer should point to the current head of the list.

4fill in blank

hard

Fill both blanks to complete the insertion of newNode at the beginning of the list.

DSA C++

newNode->next = [1];
[2] = newNode;

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

Ahead

BnewNode

Dnullptr

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the order or assigning wrong variables.

First, the new node's 'next' points to the current head. Then, 'head' is updated to the new node.

5fill in blank

hard

Fill all three blanks to create a simple linked list with two nodes and link them correctly.

DSA C++

Node* first = [1] Node();
Node* second = [2] Node();
first->next = [3];

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

Anew

Csecond

Dnullptr

Attempts:

3 left

💡 Hint

Common Mistakes

Not linking nodes or using wrong keywords for allocation.

Both nodes are created with 'new'. The first node's 'next' points to the second node to link them.

Practice

1. Why do we use a linked list instead of an array in some cases?

easy

A. Because linked lists allow easy insertion and deletion without shifting elements

B. Because linked lists use less memory than arrays always

C. Because linked lists provide faster access to elements by index

D. Because linked lists store elements in contiguous memory locations

Solution

1. Step 1: Understand array limitations

   Arrays store elements in contiguous memory, so inserting or deleting elements requires shifting others, which is slow.

2. Step 2: Understand linked list advantages

   Linked lists store elements in nodes connected by pointers, so insertion and deletion can be done by changing pointers without shifting.

3. Final Answer:

   Because linked lists allow easy insertion and deletion without shifting elements -> Option A

4. Quick Check:

   Insertion/deletion ease = D [OK]

Hint: Linked lists excel at insert/delete without moving all elements [OK]

Common Mistakes:

• Thinking linked lists always use less memory
• Assuming linked lists provide fast random access
• Believing linked lists store data contiguously

2. Which of the following is the correct syntax to declare a node structure for a singly linked list in C++?

easy

A. struct Node { int data; Node& next; };

B. struct Node { int data; Node next; };

C. struct Node { int data; Node* next; };

D. struct Node { int data; Node next*; };

Solution

1. Step 1: Understand node pointer usage

   Each node must have a pointer to the next node, so the type should be Node* next.

2. Step 2: Check syntax correctness

   struct Node { int data; Node* next; }; correctly declares next as a pointer. Options A, B, and D have invalid pointer or reference syntax.

3. Final Answer:

   struct Node { int data; Node* next; }; -> Option C

4. Quick Check:

   Pointer to next node = C [OK]

Hint: Node next must be a pointer type Node* next [OK]

Common Mistakes:

• Using Node next instead of Node* next
• Using reference Node& next incorrectly
• Incorrect pointer syntax like next*

3. What will be the output of the following C++ code that creates a linked list and prints its elements?

struct Node {
  int data;
  Node* next;
};

int main() {
  Node* head = new Node{1, nullptr};
  head->next = new Node{2, nullptr};
  head->next->next = new Node{3, nullptr};

  Node* temp = head;
  while (temp != nullptr) {
    std::cout << temp->data << " -> ";
    temp = temp->next;
  }
  std::cout << "null" << std::endl;
  return 0;
}

medium

A. 1 -> 2 -> 3 -> null

B. 1 -> 2 -> null

C. 3 -> 2 -> 1 -> null

D. Compilation error

Solution

1. Step 1: Trace linked list creation

   Nodes with data 1, 2, and 3 are created and linked in order: 1 -> 2 -> 3 -> nullptr.

2. Step 2: Trace printing loop

   The loop prints each node's data followed by ' -> ' until nullptr, then prints 'null'. Output is "1 -> 2 -> 3 -> null".

3. Final Answer:

   1 -> 2 -> 3 -> null -> Option A

4. Quick Check:

   Print linked list in order = A [OK]

Hint: Follow next pointers from head to null to print list [OK]

Common Mistakes:

• Printing nodes in reverse order
• Missing last 'null' in output
• Confusing pointer traversal causing runtime error

4. The following code attempts to insert a new node with value 5 at the beginning of a linked list. What is the error?

struct Node {
  int data;
  Node* next;
};

void insertAtHead(Node* head, int val) {
  Node* newNode = new Node{val, nullptr};
  newNode->next = head;
  head = newNode;
}

int main() {
  Node* head = nullptr;
  insertAtHead(head, 5);
  std::cout << head->data << std::endl;
  return 0;
}

medium

A. newNode is not allocated properly

B. head is nullptr and cannot be assigned

C. next pointer is not set correctly

D. head is passed by value, so changes don't affect original list

Solution

1. Step 1: Analyze function parameter passing

   The function receives head by value, so modifying head inside does not change the original head pointer in main.

2. Step 2: Understand pointer update requirement

   To update head in main, head must be passed by reference or pointer to pointer; otherwise, main's head remains nullptr.

3. Final Answer:

   head is passed by value, so changes don't affect original list -> Option D

4. Quick Check:

   Pointer update needs reference = B [OK]

Hint: Pass head by reference to update original pointer [OK]

Common Mistakes:

• Assuming head changes inside function affect main
• Ignoring pointer passing semantics
• Thinking newNode allocation is wrong

5. You want to implement a data structure that allows frequent insertions and deletions at both ends efficiently. Why is a linked list a better choice than an array for this problem?

hard

A. Because arrays automatically resize and handle insertions at both ends efficiently

B. Because linked lists can dynamically allocate memory and avoid shifting elements during insertions and deletions

C. Because linked lists store elements contiguously, improving cache performance

D. Because arrays use pointers to link elements making insertions faster

Solution

1. Step 1: Understand array limitations for insert/delete

   Arrays store elements contiguously, so inserting or deleting at ends may require shifting or resizing, which is costly.

2. Step 2: Understand linked list advantages for dynamic operations

   Linked lists allocate nodes dynamically and link them via pointers, allowing insertions and deletions at ends by pointer changes without shifting.

3. Final Answer:

   Because linked lists can dynamically allocate memory and avoid shifting elements during insertions and deletions -> Option B

4. Quick Check:

   Dynamic insert/delete without shifting = A [OK]

Hint: Linked lists avoid shifting by using pointers for dynamic ends [OK]

Common Mistakes:

• Believing arrays resize automatically without cost
• Confusing contiguous storage with faster insertions
• Thinking arrays use pointers like linked lists