Why Doubly Linked List Over Singly Linked List in DSA C++ - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a pointer to the previous node in a doubly linked list node.

DSA C++

struct Node {
    int data;
    Node* next;
    Node* [1];
};

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

Alast

BpreviousNode

Cback

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of the previous pointer.

Using a non-standard name that confuses readers.

The pointer to the previous node in a doubly linked list is commonly named prev.

2fill in blank

medium

Complete the code to move backward in a doubly linked list using the previous pointer.

DSA C++

Node* current = tail;
while (current != nullptr) {
    std::cout << current->data << " ";
    current = current->[1];
}

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

Aprev

Bprevious

Cnext

Dback

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' which moves forward.

Using incorrect pointer names like 'back'.

To move backward in a doubly linked list, use the prev pointer.

3fill in blank

hard

Fix the error in the code to insert a new node after a given node in a doubly linked list.

DSA C++

void insertAfter(Node* prev_node, int new_data) {
    if (prev_node == nullptr) return;
    Node* new_node = new Node();
    new_node->data = new_data;
    new_node->next = prev_node->next;
    prev_node->next = [1];
    new_node->prev = prev_node;
    if (new_node->next != nullptr)
        new_node->next->prev = new_node;
}

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

Aprev_node

Bnew_data

Cnew_node

Dnullptr

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning prev_node->next to itself or to nullptr.

Using wrong variable names.

We must set prev_node->next to point to the newly created node new_node.

4fill in blank

hard

Fill both blanks to correctly remove a node from a doubly linked list.

DSA C++

void deleteNode(Node** head_ref, Node* del) {
    if (*head_ref == nullptr || del == nullptr) return;
    if (*head_ref == del) *head_ref = del->[1];
    if (del->next != nullptr) del->next->[2] = del->prev;
    if (del->prev != nullptr) del->prev->next = del->next;
    delete del;
}

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

Anext

Bprev

Cprevious

Dlast

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up next and prev pointers.

Not updating the head pointer when deleting the first node.

When deleting a node, update the head if needed by moving it to del->next. Then update the next node's prev pointer to skip the deleted node.

5fill in blank

hard

Fill all three blanks to traverse a doubly linked list forward and backward.

DSA C++

void printList(Node* node) {
    Node* last = nullptr;
    std::cout << "Forward: ";
    while (node != nullptr) {
        std::cout << node->[1] << " ";
        last = node;
        node = node->[2];
    }
    std::cout << "\nBackward: ";
    while (last != nullptr) {
        std::cout << last->[3] << " ";
        last = last->prev;
    }
}

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

Adata

Bnext

Cprev

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' instead of 'data'.

Mixing up 'next' and 'prev' pointers.

To print the list, access the data field for values. Move forward using next pointer, and backward using prev pointer.

Practice

1. Why is a doubly linked list preferred over a singly linked list when you need to traverse the list backwards?

easy

A. Because doubly linked lists use less memory than singly linked lists.

B. Because singly linked lists have pointers to previous nodes only.

C. Because doubly linked lists have pointers to both next and previous nodes, allowing backward traversal.

D. Because singly linked lists can only store integer data.

Solution

1. Step 1: Understand pointer directions in linked lists

   Singly linked lists have pointers only to the next node, so you cannot move backwards easily. Doubly linked lists have pointers to both next and previous nodes.

2. Step 2: Identify traversal capability

   Because doubly linked lists have previous pointers, they allow traversal in both directions, including backwards.

3. Final Answer:

   Because doubly linked lists have pointers to both next and previous nodes, allowing backward traversal. -> Option C

4. Quick Check:

   Backward traversal needs previous pointers = A [OK]

Hint: Backward traversal needs previous pointers in nodes [OK]

Common Mistakes:

• Thinking singly linked lists have previous pointers
• Confusing memory usage with traversal ability
• Assuming singly linked lists can traverse backwards

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

easy

A. struct Node { int data; Node* prev; Node* next; };

B. struct Node { int data; Node* prev; };

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

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

Solution

1. Step 1: Identify required pointers for doubly linked list node

   A doubly linked list node must have pointers to both the previous and next nodes.

2. Step 2: Match the structure declaration

   struct Node { int data; Node* prev; Node* next; }; declares both Node* prev and Node* next, which is correct. Others miss one pointer or have duplicate pointers.

3. Final Answer:

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

4. Quick Check:

   Doubly linked list node needs prev and next pointers = C [OK]

Hint: Doubly linked list nodes have prev and next pointers [OK]

Common Mistakes:

• Missing prev pointer in node structure
• Duplicating next pointer instead of prev
• Confusing singly linked list node with doubly linked list node

3. You need to implement a feature that allows moving both forward and backward through a list of songs in a music player. Why is a doubly linked list a better choice than a singly linked list for this feature?

easy

A. Because singly linked lists use less memory and are faster for this feature.

B. Because singly linked lists automatically reverse the list when needed.

C. Because doubly linked lists can store song metadata directly in nodes.

D. Because doubly linked lists allow efficient backward and forward traversal.

Solution

1. Step 1: Understand traversal needs in music player

   The feature requires moving forward and backward through songs, so traversal in both directions is needed.

2. Step 2: Compare singly and doubly linked lists for traversal

   Singly linked lists only allow forward traversal easily, while doubly linked lists have pointers to previous nodes, enabling backward traversal efficiently.

3. Final Answer:

   Because doubly linked lists allow efficient backward and forward traversal. -> Option D

4. Quick Check:

   Bidirectional traversal needs doubly linked list = B [OK]

Hint: Bidirectional traversal needs doubly linked list [OK]

Common Mistakes:

• Assuming singly linked lists can traverse backward easily
• Confusing memory usage with traversal capability
• Thinking doubly linked lists store metadata differently

4. Consider the following C++ code snippet for a doubly linked list insertion at the end. What will be the output after inserting nodes with values 10 and 20?

medium

A. 10 <-> 20 <-> null

B. 20 <-> 10 <-> null

C. 10 -> 20 -> null

D. null <- 10 <- 20

Solution

1. Step 1: Understand insertion at end in doubly linked list

   Inserting 10 first creates a node with data 10. Inserting 20 next adds it after 10, linking 10's next to 20 and 20's prev to 10.

2. Step 2: Visualize the list after insertions

   The list looks like: 10 <-> 20 <-> null, showing bidirectional links and null at the end.

3. Final Answer:

   10 <-> 20 <-> null -> Option A

4. Quick Check:

   Insertion at end keeps order 10 then 20 = D [OK]

Hint: Insertion at end appends nodes in order [OK]

Common Mistakes:

• Reversing node order after insertion
• Using singly linked list arrow notation
• Ignoring prev pointer links

5. The following code attempts to delete a node from a doubly linked list but causes a runtime error. What is the main issue?

void deleteNode(Node* del) {
  if (del == nullptr) return;
  if (del->prev != nullptr) del->prev->next = del->next;
  if (del->next != nullptr) del->next->prev = del->prev;
  delete del;
}

medium

A. It deletes the node before updating pointers.

B. It does not update the head pointer if the deleted node is the head.

C. It does not check if del is the tail node.

D. It uses incorrect pointer syntax.

Solution

1. Step 1: Analyze pointer updates in deletion

   The code updates previous and next nodes' pointers correctly but does not handle the case when the node to delete is the head (no prev).

2. Step 2: Identify missing head update

   If the node is head, the external head pointer must be updated to del->next, which is missing, causing dangling head pointer and runtime errors.

3. Final Answer:

   It does not update the head pointer if the deleted node is the head. -> Option B

4. Quick Check:

   Head node deletion needs head pointer update = A [OK]

Hint: Always update head if deleting the first node [OK]

Common Mistakes:

• Deleting node before fixing pointers
• Ignoring head pointer update
• Confusing tail node handling with head node