Reverse a Doubly Linked List in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to swap the next and prev pointers of the current node.

DSA C

void reverse(struct Node** head_ref) {
    struct Node* temp = NULL;
    struct Node* current = *head_ref;
    while (current != NULL) {
        temp = current->[1];
        current->prev = current->next;
        current->next = temp;
        current = current->prev;
    }
}

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

Aprev

Bhead

Cnext

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using current->prev instead of current->next causes incorrect traversal.

Swapping pointers without saving next node leads to losing the list.

We need to store the next pointer of the current node in temp before swapping pointers.

2fill in blank

medium

Complete the code to update the head pointer after reversing the list.

DSA C

if (temp != NULL) {
    *head_ref = temp->[1];
}

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

Anext

Bprev

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head to temp->next causes wrong head assignment.

Not updating head leads to unchanged list start.

After the loop, temp points to the last processed node, so head should point to temp->prev.

3fill in blank

hard

Fix the error in the while loop condition to correctly traverse the list.

DSA C

while ([1] != NULL) {
    // swapping pointers
}

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

Acurrent

B*head_ref

Ctemp

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using temp instead of current causes infinite loop or no traversal.

Using *head_ref instead of current causes incorrect condition.

The loop should continue while current is not NULL to traverse all nodes.

4fill in blank

hard

Fill both blanks to correctly swap pointers and move to the next node.

DSA C

temp = current->[1];
current->prev = current->[2];

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

Anext

Bprev

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing prev and next pointers causes incorrect reversal.

Using data instead of pointers causes errors.

We save current->next in temp and assign current->prev to current->next to swap pointers.

5fill in blank

hard

Fill all three blanks to complete the reverse function correctly.

DSA C

void reverse(struct Node** head_ref) {
    struct Node* temp = NULL;
    struct Node* current = *head_ref;
    while (current != NULL) {
        temp = current->[1];
        current->prev = current->[2];
        current->next = temp;
        current = current->[3];
    }
    if (temp != NULL) {
        *head_ref = temp->prev;
    }
}

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

Anext

Bprev

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up prev and next pointers in assignments.

Not moving current correctly causes infinite loop.

We save current->next in temp, assign current->prev to current->next, and move current to current->prev.

Practice

1. What is the main purpose of reversing a doubly linked list?

easy

A. To convert it into a singly linked list

B. To delete all nodes from the list

C. To make the last node the first and the first node the last

D. To sort the list in ascending order

Solution

1. Step 1: Understand the structure of a doubly linked list

   A doubly linked list has nodes with pointers to both previous and next nodes.

2. Step 2: Define reversing operation

   Reversing means swapping the direction so the last node becomes the first and vice versa.

3. Final Answer:

   To make the last node the first and the first node the last -> Option C

4. Quick Check:

   Reverse = last becomes first [OK]

Hint: Reversing flips start and end nodes [OK]

Common Mistakes:

• Thinking reversing deletes nodes
• Confusing reversing with sorting
• Assuming it changes list type

2. Which of the following is the correct way to swap the next and prev pointers of a node in C?

easy

A. temp = node->prev; node->prev = node->next; node->next = node->next;

B. node->next = node->prev; node->prev = node->next;

C. node->prev = node->next; node->next = node->prev;

D. temp = node->next; node->next = node->prev; node->prev = temp;

Solution

1. Step 1: Use a temporary variable to hold one pointer

   Store node->next in temp to avoid losing its value.

2. Step 2: Swap pointers correctly

   Assign node->next to node->prev, then node->prev to temp.

3. Final Answer:

   temp = node->next; node->next = node->prev; node->prev = temp; -> Option D

4. Quick Check:

   Swap pointers using temp variable [OK]

Hint: Always use temp variable to swap pointers safely [OK]

Common Mistakes:

• Swapping without temp loses pointer data
• Assigning both pointers to same value
• Mixing order of assignments

3. Given the following doubly linked list nodes and code snippet, what will be the new head node's data after reversal?

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

// List: 1 <-> 2 <-> 3 <-> NULL
struct Node* head = node1; // node1->data = 1

// Reversal code snippet:
struct Node* current = head;
struct Node* temp = NULL;
while (current != NULL) {
  temp = current->prev;
  current->prev = current->next;
  current->next = temp;
  current = current->prev;
}
if (temp != NULL) {
  head = temp->prev;
}

medium

A. 1

B. 3

C. 2

D. NULL

Solution

1. Step 1: Trace pointer swaps in the loop

   Each node swaps its next and prev pointers, moving current backward through original list.

2. Step 2: Update head after loop

   After loop, head is set to temp->prev, which points to original last node (data 3).

3. Final Answer:

   3 -> Option B

4. Quick Check:

   New head = original tail = 3 [OK]

Hint: New head is original tail after reversal [OK]

Common Mistakes:

• Not updating head pointer after reversal
• Confusing prev and next pointers
• Assuming head remains same

4. Identify the error in the following code snippet that attempts to reverse a doubly linked list:

void reverse(struct Node** head_ref) {
  struct Node* current = *head_ref;
  struct Node* temp = NULL;
  while (current != NULL) {
    temp = current->next;
    current->next = current->prev;
    current->prev = temp;
    current = current->prev;
  }
  if (temp != NULL) {
    *head_ref = temp->prev;
  }
}

medium

A. temp should store current->prev instead of current->next

B. current should move to current->next instead of current->prev

C. The head pointer update is incorrect

D. No error, code is correct

Solution

1. Step 1: Verify pointer swaps

   The next/prev swaps correctly reverse directions as current traverses the original next chain (now in prev).

2. Step 2: Analyze temp assignment

   Temp should store current->prev before swapping, not current->next, to avoid losing pointer.

3. Final Answer:

   temp should store current->prev instead of current->next -> Option A

4. Quick Check:

   Correct temp assignment preserves traversal [OK]

Hint: Temp must hold current->prev before swapping pointers [OK]

Common Mistakes:

• Incorrect head pointer update logic
• Moving current incorrectly after swap
• Not updating head pointer properly

5. You have a doubly linked list with nodes containing integer data. After reversing the list using the standard pointer swap method, which of the following statements is TRUE about the list's traversal?

hard

A. Traversing from the new head using next pointers visits nodes in original reverse order

B. Traversing from the new head using prev pointers visits nodes in original order

C. Traversing from the original head using next pointers visits nodes in reversed order

D. Traversing from the new head using next pointers visits nodes in original order

Solution

1. Step 1: Understand reversal effect on pointers

   Reversing swaps next and prev pointers, so next pointers now point backward in original list.

2. Step 2: Analyze traversal from new head

   Starting at new head, following next pointers visits nodes in original reverse order.

3. Final Answer:

   Traversing from the new head using next pointers visits nodes in original reverse order -> Option A

4. Quick Check:

   Next pointers after reverse = original prev [OK]

Hint: Next pointers after reverse follow original prev links [OK]

Common Mistakes:

• Assuming next pointers still go forward after reverse
• Confusing traversal directions
• Thinking original head remains start