Insert at Beginning of Doubly Linked List in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after inserting nodes at the beginning

What is the printed state of the doubly linked list after inserting 10, then 20, then 30 at the beginning?

DSA Python

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

class DoublyLinkedList:
    def __init__(self):
        self.head = None

    def insert_at_beginning(self, data):
        new_node = Node(data)
        new_node.next = self.head
        if self.head is not None:
            self.head.prev = new_node
        self.head = new_node

    def print_list(self):
        current = self.head
        result = []
        while current:
            result.append(str(current.data))
            current = current.next
        print(' -> '.join(result) + ' -> null')

dl = DoublyLinkedList()
dl.insert_at_beginning(10)
dl.insert_at_beginning(20)
dl.insert_at_beginning(30)
dl.print_list()

A 30 -> 20 -> 10 -> null

B 10 -> 20 -> 30 -> null

C 20 -> 30 -> 10 -> null

D 30 -> 10 -> 20 -> null

💡 Hint

Remember that inserting at the beginning means the newest node becomes the head.

Each new node is added before the current head, so the list order is reversed from insertion order.

🧠 Conceptual

intermediate

Understanding pointer updates in insertion at beginning

When inserting a new node at the beginning of a doubly linked list, which pointers must be updated to maintain list integrity?

A New node's next and prev both point to old head; head remains unchanged

B New node's next points to old head; old head's prev points to new node; head updated to new node

C New node's prev points to old head; old head's next points to new node; head updated to new node

D Only head pointer is updated to new node; no other pointers change

💡 Hint

Think about how the new node connects forward and backward to the existing list.

The new node's next must point to the old head, and the old head's prev must point back to the new node. Then the head pointer is updated.

🔧 Debug

advanced

Identify the error in this insertion code

What error will this code cause when inserting at the beginning of a doubly linked list?

DSA Python

def insert_at_beginning(self, data):
    new_node = Node(data)
    new_node.prev = self.head
    self.head = new_node
    if new_node.next is not None:
        new_node.next.prev = new_node

A AttributeError because new_node.next is None and accessed without assignment

B No error, code works correctly

C Logical error: new_node.prev should be None, not self.head

D TypeError because new_node.prev is assigned a Node object instead of None

💡 Hint

Check what prev pointer of the new head node should be.

The new node inserted at beginning should have prev = None, but code sets prev = self.head (old head), which is incorrect.

❓ Predict Output

advanced

Output after multiple insertions and one traversal

What is the output of the following code?

DSA Python

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

class DoublyLinkedList:
    def __init__(self):
        self.head = None

    def insert_at_beginning(self, data):
        new_node = Node(data)
        new_node.next = self.head
        if self.head:
            self.head.prev = new_node
        self.head = new_node

    def print_reverse(self):
        current = self.head
        while current and current.next:
            current = current.next
        result = []
        while current:
            result.append(str(current.data))
            current = current.prev
        print(' <- '.join(result) + ' <- null')

dl = DoublyLinkedList()
dl.insert_at_beginning(1)
dl.insert_at_beginning(2)
dl.insert_at_beginning(3)
dl.print_reverse()

A 1 <- 2 <- 3 <- null

B 3 <- 2 <- 1 <- null

C null

D 1 -> 2 -> 3 -> null

💡 Hint

The print_reverse method prints from tail to head.

After inserting 3,2,1 at beginning, list is 3->2->1. Reverse traversal prints 1 <- 2 <- 3 <- null.

🧠 Conceptual

expert

Why is updating the previous pointer important in doubly linked list insertion?

What problem arises if you insert a node at the beginning of a doubly linked list but forget to update the old head's prev pointer to the new node?

A No problem; the list will work fine without updating prev pointer

B Forward traversal will skip the new node, losing data

C The list will become singly linked automatically

D Backward traversal from the old head will be broken, causing incorrect or incomplete reverse iteration

💡 Hint

Think about how prev pointers help in moving backward through the list.

If old head's prev pointer is not updated, backward traversal starting from old head or tail will miss the new node, breaking the list's integrity.