Why Doubly Linked List Over Singly Linked List in DSA C - Quick Recap

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a type of linked list where each node has two pointers: one to the next node and one to the previous node. This allows traversal in both directions.

beginner

Why might we prefer a doubly linked list over a singly linked list?

Because doubly linked lists allow easy backward traversal and efficient deletion of nodes without needing to traverse from the head, unlike singly linked lists.

beginner

What is a disadvantage of doubly linked lists compared to singly linked lists?

Doubly linked lists use more memory because each node stores an extra pointer to the previous node, and operations can be slightly slower due to managing two pointers.

intermediate

How does deletion of a node differ between singly and doubly linked lists?

In singly linked lists, to delete a node, you must find the previous node by traversing from the head. In doubly linked lists, you can access the previous node directly, making deletion faster.

beginner

Can doubly linked lists be traversed backwards? Why is this useful?

Yes, doubly linked lists can be traversed backwards because each node has a pointer to the previous node. This is useful for operations like reverse iteration or undo functionality.

What extra pointer does a doubly linked list node have compared to a singly linked list node?

A Pointer to the next next node

B Pointer to the head node

C Pointer to the tail node

D Pointer to the previous node

A doubly linked list node has a pointer to the previous node, allowing backward traversal.

Which operation is faster in a doubly linked list compared to a singly linked list?

A Inserting at the head

B Finding the head node

C Deleting a node when given a pointer to it

D Traversing from head to tail

Deleting a node is faster because the previous node is directly accessible in a doubly linked list.

What is a downside of doubly linked lists?

A Uses more memory per node

B Cannot traverse backwards

C Cannot insert at tail

D No pointer to next node

Doubly linked lists use more memory because each node stores two pointers.

Which traversal is NOT directly supported by a singly linked list?

A Forward traversal

B Backward traversal

C Traversal from head

D Traversal to tail

Singly linked lists do not support backward traversal because nodes only point forward.

Why might you choose a singly linked list over a doubly linked list?

A Want to save memory

B Need faster deletion

C Need backward traversal

D Want to traverse backwards

Singly linked lists use less memory per node, making them better when memory is limited.

Explain the main reasons to use a doubly linked list instead of a singly linked list.

Think about how nodes connect and how operations like deletion work.

Describe the trade-offs between singly and doubly linked lists in terms of memory and operation efficiency.

Consider what each node stores and how it affects traversal and deletion.

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