Reverse a Doubly Linked List in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reversing a doubly linked list

What is the printed state of the doubly linked list after reversing it using the given code?

DSA Python

class Node:
    def __init__(self, data):
        self.data = data
        self.prev = None
        self.next = None

def reverse(head):
    current = head
    temp = None
    while current:
        temp = current.prev
        current.prev = current.next
        current.next = temp
        current = current.prev
    if temp:
        head = temp.prev
    return head

# Create doubly linked list: 1 <-> 2 <-> 3 <-> 4
head = Node(1)
node2 = Node(2)
node3 = Node(3)
node4 = Node(4)
head.next = node2
node2.prev = head
node2.next = node3
node3.prev = node2
node3.next = node4
node4.prev = node3

# Reverse the list
head = reverse(head)

# Print list forward
result = []
current = head
while current:
    result.append(current.data)
    current = current.next
print(result)

A [4, 3, 2, 1]

B [1, 2, 3, 4]

C [4, 2, 3, 1]

D [1, 3, 2, 4]

💡 Hint

Think about how the prev and next pointers are swapped for each node.

The reverse function swaps the prev and next pointers for each node, effectively reversing the list. After the loop, the head is updated to the last node processed, which becomes the new head. The printed list shows the nodes in reverse order.

🧠 Conceptual

intermediate

Understanding pointer changes in reversing a doubly linked list

When reversing a doubly linked list, which pointers are swapped for each node?

A Swap the data values of adjacent nodes

B Swap only the prev pointers of each node

C Swap only the next pointers of each node

D Swap the next and prev pointers of each node

💡 Hint

Think about what makes the list reverse direction.

Reversing a doubly linked list requires swapping both the next and prev pointers of each node to reverse the direction of traversal.

🔧 Debug

advanced

Identify the error in this reverse function for doubly linked list

What error will this code produce when reversing a doubly linked list?

DSA Python

def reverse(head):
    current = head
    while current:
        temp = current.next
        current.prev, current.next = current.next, current.prev
        current = temp
    return head

A SyntaxError due to invalid tuple assignment

B The function returns the original head, not the new head after reversal

C Infinite loop because current never becomes None

D TypeError because prev and next are not swappable

💡 Hint

Check what happens to the head pointer after swapping pointers.

The code swaps pointers but does not update the head to the new start of the reversed list, so it returns the old head, which is now the tail.

❓ Predict Output

advanced

Output after reversing a doubly linked list with one node

What is the output after reversing a doubly linked list with a single node containing data 10?

DSA Python

class Node:
    def __init__(self, data):
        self.data = data
        self.prev = None
        self.next = None

def reverse(head):
    current = head
    temp = None
    while current:
        temp = current.prev
        current.prev = current.next
        current.next = temp
        current = current.prev
    if temp:
        head = temp.prev
    return head

head = Node(10)
head = reverse(head)

result = []
current = head
while current:
    result.append(current.data)
    current = current.next
print(result)

A [0]

B []

C [10]

D [None]

💡 Hint

Consider how reversing a single node list affects pointers.

A single node reversed remains the same node with no next or prev nodes, so the output is [10].

🚀 Application

expert

Number of nodes after reversing a doubly linked list

After reversing a doubly linked list with 7 nodes, how many nodes does the reversed list contain?

A 7

B 0

C 1

D 14

💡 Hint

Reversing changes order but not the number of nodes.

Reversing a doubly linked list only changes the direction of links, not the number of nodes.