Create and Initialize Doubly Linked List in DSA Python - Execution Trace

Concept Flow - Create and Initialize Doubly Linked List

Start

↓

Create new node with data

↓

Set new_node.prev = None

↓

Set new_node.next = None

↓

Set head = new_node

↓

Done: List initialized with one node

The flow shows creating a new node, setting its previous and next pointers to None, and assigning it as the head of the list.

Execution Sample

DSA Python

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

head = Node(10)

This code creates a doubly linked list with a single node containing data 10.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with data=10	None	None	List is empty
2	Set new_node.prev = None	Node(10)	new_node.prev -> None	10 (prev: None, next: None)
3	Set new_node.next = None	Node(10)	new_node.next -> None	10 (prev: None, next: None)
4	Set head = new_node	Node(10)	head -> Node(10)	head -> 10 (prev: None, next: None)
5	Initialization complete	Node(10)	No change	head -> 10 (prev: None, next: None)

💡 List initialized with one node; head points to node with data 10

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
head	None	None	None	None	Node(10)	Node(10)
new_node.data	N/A	10	10	10	10	10
new_node.prev	N/A	None	None	None	None	None
new_node.next	N/A	None	None	None	None	None

Key Moments - 2 Insights

Why do we set both prev and next pointers to None when creating the first node?

Because the first node has no neighbors, its prev and next must be None to show it is alone. See execution_table steps 2 and 3.

Why do we assign the new node to head after setting pointers?

We assign head last to ensure the node is fully prepared before the list points to it. See execution_table step 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what does head point to?

A Node with data 10

B None

C Node with data 0

D Node with data 5

💡 Hint

Check the Pointer Changes and Visual State columns at step 4.

At which step is new_node.prev set to None?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the Operation column describing pointer assignments.

If we skip setting new_node.next to None, what would the Visual State show after step 3?

A 10 (prev: None, next: None)

B 10 (prev: None, next: Unknown)

C 10 (prev: None, next: Node)

D Error

💡 Hint

Consider what happens if next pointer is not explicitly set.

Concept Snapshot

Create a node with data.
Set node.prev = None and node.next = None.
Assign node as head of the list.
This initializes a doubly linked list with one node.
Pointers show no neighbors for the first node.

Full Transcript

To create and initialize a doubly linked list, first create a new node with the given data. Then set its previous and next pointers to None because it has no neighbors yet. Finally, assign this node as the head of the list. This process results in a list with one node where head points to it, and both pointers are None indicating no other nodes connected.