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 node struct for a doubly linked list.

DSA C

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

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

Abefore

BpreviousNode

Cback

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'previousNode' or 'back' which are not standard names.

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

2fill in blank

medium

Complete the code to move forward in a doubly linked list.

DSA C

current = current->[1];

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

Anext

Bprevious

Cprev

Dback

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' to move forward causes moving backward.

To move forward in a doubly linked list, use the next pointer.

3fill in blank

hard

Fix the error in the code to move backward in a doubly linked list.

DSA C

current = current->[1];

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

ApreviousNode

Bnext

Cprev

Dback

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' moves forward, not backward.

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

4fill in blank

hard

Fill both blanks to insert a new node after the current node in a doubly linked list.

DSA C

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

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

Anext

Bprev

Cprevious

Dbefore

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping next and prev pointers.

When inserting after current, newNode's next points to current's next, and newNode's prev points to current.

5fill in blank

hard

Fill all three blanks to update pointers when inserting a new node after current in a doubly linked list.

DSA C

if (current->next != NULL) {
    current->next->[1] = newNode;
}
newNode->[3] = current->next;
current->[2] = newNode;

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

Aprev

Bnext

Attempts:

3 left

💡 Hint

Common Mistakes

Not updating the next node's prev pointer.

When inserting, update the next node's prev to newNode, current's next to newNode, and newNode's next to current's old next.

Practice

1. Why might a doubly linked list be preferred over a singly linked list?

easy

A. Because it cannot be modified after creation

B. Because it uses less memory than singly linked list

C. Because it only stores one pointer per node

D. Because it allows traversal in both forward and backward directions

Solution

1. Step 1: Understand traversal capabilities

   A singly linked list allows traversal only in one direction, from head to tail.

2. Step 2: Compare with doubly linked list

   A doubly linked list stores two pointers per node, allowing traversal forward and backward.

3. Final Answer:

   Because it allows traversal in both forward and backward directions -> Option D

4. Quick Check:

   Traversal direction = Both [OK]

Hint: Doubly linked lists have two pointers per node [OK]

Common Mistakes:

• Thinking doubly linked lists use less memory
• Confusing traversal directions
• Assuming singly linked lists allow backward traversal

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

easy

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

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

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

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

Solution

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

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

2. Step 2: Check each option

   struct Node { int data; struct Node* prev; struct Node* next; }; correctly declares both prev and next pointers as struct Node pointers.

3. Final Answer:

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

4. Quick Check:

   Node pointers = prev and next [OK]

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

Common Mistakes:

• Omitting prev pointer
• Using int instead of pointer for next/prev
• Confusing singly linked list node with doubly linked

3. What will be the output of the following code snippet that uses a doubly linked list?

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

Node* head = NULL;

// Function to add node at end
void append(int val) {
  Node* newNode = (Node*)malloc(sizeof(Node));
  newNode->data = val;
  newNode->next = NULL;
  if (head == NULL) {
    newNode->prev = NULL;
    head = newNode;
    return;
  }
  Node* temp = head;
  while (temp->next != NULL) temp = temp->next;
  temp->next = newNode;
  newNode->prev = temp;
}

// Print list forward
void printForward() {
  Node* temp = head;
  while (temp != NULL) {
    printf("%d ", temp->data);
    temp = temp->next;
  }
}

int main() {
  append(10);
  append(20);
  append(30);
  printForward();
  return 0;
}

medium

A. 10 20 30

B. 30 20 10

C. 10 20

D. Compilation error

Solution

1. Step 1: Trace append function calls

   append(10) creates first node with data 10, head points to it. append(20) adds node with 20 at end. append(30) adds node with 30 at end.

2. Step 2: Trace printForward function

   printForward starts at head and prints data while moving next. It prints 10, then 20, then 30.

3. Final Answer:

   10 20 30 -> Option A

4. Quick Check:

   List forward print = 10 20 30 [OK]

Hint: Forward print shows nodes in insertion order [OK]

Common Mistakes:

• Confusing forward and backward traversal
• Assuming printForward prints in reverse
• Missing NULL checks causing errors

4. Identify the bug in the following code snippet that tries to delete the last node of a doubly linked list:

void deleteLast() {
  if (head == NULL) return;
  Node* temp = head;
  while (temp->next != NULL) {
    temp = temp->next;
  }
  temp->prev->next = NULL;
  free(temp);
}

medium

A. No bug, code works correctly

B. Does not handle case when list has only one node

C. Incorrect pointer used for deletion

D. Memory leak due to missing free

Solution

1. Step 1: Analyze deletion logic

   The code finds last node by moving to node where next is NULL, then sets previous node's next to NULL and frees last node.

2. Step 2: Check edge case for single node list

   If list has only one node, temp->prev is NULL, so temp->prev->next causes a crash or undefined behavior.

3. Final Answer:

   Does not handle case when list has only one node -> Option B

4. Quick Check:

   Single node deletion needs special handling [OK]

Hint: Check for single node before accessing prev pointer [OK]

Common Mistakes:

• Ignoring single node edge case
• Assuming prev pointer always exists
• Not freeing memory after deletion

5. You want to implement a feature that allows moving backward through a list of recently opened files in a text editor. Which linked list type is best and why?

hard

A. Doubly linked list, because it allows moving backward and forward easily

B. Singly linked list, because it uses less memory and is simpler

C. Circular singly linked list, because it loops infinitely

D. Array, because linked lists cannot move backward

Solution

1. Step 1: Understand feature requirements

   The feature requires moving backward and forward through the list of files.

2. Step 2: Match data structure capabilities

   Singly linked lists allow only forward traversal, arrays allow random access but not efficient insertion/deletion, doubly linked lists allow easy backward and forward traversal.

3. Step 3: Evaluate options

   Doubly linked list fits best because it supports moving backward and forward efficiently.

4. Final Answer:

   Doubly linked list, because it allows moving backward and forward easily -> Option A

5. Quick Check:

   Backward traversal = Doubly linked list [OK]

Hint: Backward navigation needs doubly linked list [OK]

Common Mistakes:

• Choosing singly linked list for backward moves
• Ignoring traversal needs
• Assuming arrays handle backward traversal efficiently