Reverse a Doubly Linked List in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

function reverseDLL(head) {
  let current = head;
  while (current !== null) {
    let temp = current.next;
    current.next = [1];
    current.prev = temp;
    current = temp;
  }
  return head;
}

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

Anull

Bcurrent.next

Ccurrent.prev

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning current.next to current.next instead of current.prev.

Not swapping pointers correctly.

We swap the next and prev pointers by assigning current.prev to current.next.

2fill in blank

medium

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

DSA Javascript

function reverseDLL(head) {
  let current = head;
  let newHead = null;
  while (current !== null) {
    let temp = current.next;
    current.next = current.prev;
    current.prev = temp;
    newHead = current;
    current = temp;
  }
  return [1];
}

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

AnewHead

Bcurrent

Chead

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the old head instead of the new head.

Returning current or temp which may be null.

After reversing, newHead points to the last node processed, which is the new head.

3fill in blank

hard

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

DSA Javascript

function reverseDLL(head) {
  let current = head;
  while ([1]) {
    let temp = current.next;
    current.next = current.prev;
    current.prev = temp;
    current = temp;
  }
  return current;
}

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

Acurrent !== null

Bcurrent === null

Chead !== null

Dtemp !== null

Attempts:

3 left

💡 Hint

Common Mistakes

Using current === null which stops the loop immediately.

Using head or temp in the condition incorrectly.

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 update the current node.

DSA Javascript

function reverseDLL(head) {
  let current = head;
  let newHead = null;
  while (current !== null) {
    let temp = current.[1];
    current.[2] = current.prev;
    current.prev = temp;
    newHead = current;
    current = temp;
  }
  return newHead;
}

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

Anext

Bprev

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Using current.prev instead of current.next for temp.

Assigning current.prev to current.next instead of the other way.

We store 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 with correct pointer swaps and return.

DSA Javascript

function reverseDLL(head) {
  let current = head;
  let newHead = null;
  while (current !== null) {
    let temp = current.[1];
    current.[2] = current.[3];
    current.prev = temp;
    newHead = current;
    current = temp;
  }
  return newHead;
}

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

Anext

Bprev

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up next and prev in assignments.

Not saving current.next before swapping.

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

Practice

1. What does reversing a doubly linked list mean?

easy

A. Adding a new node at the end

B. Deleting all nodes from the list

C. Sorting the nodes in ascending order

D. Swapping the next and previous pointers of each node

Solution

1. Step 1: Understand doubly linked list structure

   Each node has two pointers: one to the next node and one to the previous node.

2. Step 2: Reverse means swapping pointers

   Reversing means changing each node's next pointer to previous and previous pointer to next.

3. Final Answer:

   Swapping the next and previous pointers of each node -> Option D

4. Quick Check:

   Reverse = swap next and prev pointers [OK]

Hint: Reverse means swap next and prev pointers [OK]

Common Mistakes:

• Thinking reverse means deleting nodes
• Confusing reverse with sorting
• Adding nodes instead of reversing pointers

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

easy

A. node.prev = node.next; node.next = node.prev;

B. node.next = node.prev; node.prev = node.next;

C. let temp = node.next; node.next = node.prev; node.prev = temp;

D. node.next = node.next; node.prev = node.prev;

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 = node.prev, then node.prev = temp to complete swap.

3. Final Answer:

   let temp = node.next; node.next = node.prev; node.prev = temp; -> Option C

4. Quick Check:

   Swap with temp variable = correct [OK]

Hint: Use temp variable to swap pointers safely [OK]

Common Mistakes:

• Swapping without temp loses pointer data
• Assigning both pointers to same value
• Not swapping pointers at all

3. What will be the output list after reversing this doubly linked list?

Head -> 1 <-> 2 <-> 3 <-> null

After reversal, what is the new head node's value?

medium

A. 3

B. 1

C. 2

D. null

Solution

1. Step 1: Original list order

   List is 1 <-> 2 <-> 3, head points to 1.

2. Step 2: Reverse swaps pointers, so last node becomes head

   After reversal, 3 becomes the new head, pointing backward to 2 and forward to null.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   New head after reverse = last node = 3 [OK]

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

Common Mistakes:

• Assuming head stays same after reverse
• Confusing node values order
• Thinking null becomes head

4. Identify the bug in this reverse function for a doubly linked list:

function reverse(head) {
  let current = head;
  while (current != null) {
    let temp = current.next;
    current.next = current.prev;
    current.prev = temp;
    current = current.next;
  }
  return head;
}

medium

A. The function does not update the head to the new first node

B. Swapping next and prev pointers is done incorrectly

C. The loop condition should be current.next != null

D. The function should return null instead of head

Solution

1. Step 1: Check pointer swapping

   Swapping next and prev pointers is done correctly using temp variable.

2. Step 2: Check head update after reversal

   Function returns original head, but after reversal head should be last node visited, so head is not updated.

3. Final Answer:

   The function does not update the head to the new first node -> Option A

4. Quick Check:

   Head must update to new first node [OK]

Hint: Update head to last processed node after loop [OK]

Common Mistakes:

• Not updating head after reversal
• Incorrect loop condition causing infinite loop
• Returning wrong node at end

5. Given a doubly linked list with nodes:

Head -> 10 <-> 20 <-> 30 <-> 40 <-> null

After reversing the list, what is the correct sequence of nodes starting from the new head?

hard

A. 10 <-> 20 <-> 30 <-> 40 <-> null

B. 40 <-> 30 <-> 20 <-> 10 <-> null

C. 40 <-> 20 <-> 30 <-> 10 <-> null

D. null <-> 40 <-> 30 <-> 20 <-> 10

Solution

1. Step 1: Original list order

   Nodes are linked as 10 <-> 20 <-> 30 <-> 40, head at 10.

2. Step 2: Reverse swaps pointers and changes head to last node

   After reversal, head is 40, and nodes link backward as 40 <-> 30 <-> 20 <-> 10.

3. Final Answer:

   40 <-> 30 <-> 20 <-> 10 <-> null -> Option B

4. Quick Check:

   Reverse list order = original tail to head [OK]

Hint: Reversed list order is original list backwards [OK]

Common Mistakes:

• Not reversing the order correctly
• Mixing node values in wrong order
• Confusing null placement in list