Why Doubly Linked List Over Singly Linked List in DSA C - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use a doubly linked list instead of a singly linked list?

Which of the following is the main advantage of a doubly linked list over a singly linked list?

A It allows traversal in both forward and backward directions easily.

B It uses less memory per node compared to singly linked list.

C It requires less code to implement than singly linked list.

D It can only be used for storing numbers.

💡 Hint

Think about how you can move through the list in different directions.

Doubly linked lists have two pointers per node, allowing easy movement forward and backward. Singly linked lists only allow forward movement.

❓ Predict Output

intermediate

Output of traversing a doubly linked list backwards

Given the doubly linked list with nodes containing values 1, 2, 3 linked in order, what is the output when traversing backwards from the tail?

DSA C

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

// List: 1 <-> 2 <-> 3
// Tail points to node with data 3

void traverseBackward(struct Node* tail) {
    while (tail != NULL) {
        printf("%d ", tail->data);
        tail = tail->prev;
    }
}

A 1 2 3

B 1 3 2

C 3 1 2

D 3 2 1

💡 Hint

Start from the tail and move using the prev pointer.

Traversing backwards from tail using prev pointer prints nodes in reverse order: 3, 2, 1.

🔧 Debug

advanced

Identify the bug in doubly linked list insertion at head

What is the error in this code snippet for inserting a new node at the head of a doubly linked list?

DSA C

void insertAtHead(struct Node** head, int data) {
    struct Node* newNode = malloc(sizeof(struct Node));
    newNode->data = data;
    newNode->next = *head;
    newNode->prev = NULL;
    if (*head != NULL) {
        (*head)->prev = newNode;
    }
    *head = newNode;
}

A The prev pointer of the old head node is not updated correctly before changing head.

B The line (*head)->prev = newNode; causes a cycle by setting newNode's prev to itself.

C The newNode->next should be set to NULL instead of *head.

D The condition if (*head != NULL) is always true, causing a memory leak.

💡 Hint

Check what happens to prev pointers after changing head.

The prev pointer of the old head node should be updated before changing the head pointer. The original code sets *head = newNode before updating the prev pointer of the old head, causing incorrect behavior.

❓ Predict Output

advanced

Result of deleting a node in doubly linked list

What is the output after deleting the node with value 2 from the doubly linked list 1 <-> 2 <-> 3 and then traversing forward?

DSA C

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

// Initial list: 1 <-> 2 <-> 3

// After deleting node with data 2

void traverseForward(struct Node* head) {
    while (head != NULL) {
        printf("%d ", head->data);
        head = head->next;
    }
}

A 3 1

B 1 2 3

C 1 3

D 2 3

💡 Hint

Deleting node 2 removes it from the chain, so it won't print.

After deleting node 2, the list links 1 directly to 3, so traversal prints 1 and 3 only.

🧠 Conceptual

expert

Why doubly linked lists are preferred for certain operations?

Which scenario best explains why a doubly linked list is preferred over a singly linked list?

A When memory usage must be minimized at all costs.

B When frequent backward traversal or deletion of a node given only its pointer is needed.

C When only forward traversal is required and simplicity is key.

D When the list size is fixed and never changes.

💡 Hint

Think about operations that require moving backwards or removing nodes easily.

Doubly linked lists allow moving backward and deleting nodes without needing access to the previous node pointer externally, which singly linked lists cannot do efficiently.