Doubly Linked List Structure and Node Design in DSA Python - Execution Trace

Concept Flow - Doubly Linked List Structure and Node Design

Create Node with data

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Node has prev pointer

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Start with head pointing to first node

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Traverse nodes using next pointer

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Can move backward using prev pointer

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Insert/Delete nodes by updating prev and next pointers

A doubly linked list node holds data and two pointers: one to the previous node and one to the next node, allowing traversal in both directions.

Execution Sample

DSA Python

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

Defines a doubly linked list node with data and two pointers: prev and next.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create node with data=10	Node(10)	prev=None, next=None	None <- [10] -> None
2	Create node with data=20	Node(10), Node(20)	prev=None, next=None for new node	None <- [10] -> None None <- [20] -> None
3	Link node 10 next to node 20; node 20 prev to node 10	Node(10) <-> Node(20)	10.next=20, 20.prev=10	None <- [10] <-> [20] -> None
4	Create node with data=30	Node(10) <-> Node(20), Node(30)	prev=None, next=None for new node	None <- [10] <-> [20] -> None None <- [30] -> None
5	Link node 20 next to node 30; node 30 prev to node 20	Node(10) <-> Node(20) <-> Node(30)	20.next=30, 30.prev=20	None <- [10] <-> [20] <-> [30] -> None
6	Traverse forward from head (node 10)	Node(10) <-> Node(20) <-> Node(30)	No pointer changes	Traverse order: 10 -> 20 -> 30
7	Traverse backward from tail (node 30)	Node(10) <-> Node(20) <-> Node(30)	No pointer changes	Traverse order: 30 -> 20 -> 10
8	End of demonstration	Node(10) <-> Node(20) <-> Node(30)	No pointer changes	List stable with bidirectional links

💡 Demonstration ends after creating and linking three nodes with bidirectional pointers.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	Final
Node(10).prev	None	None	None	None	None	None	None	None	None
Node(10).next	None	None	None	Node(20)	Node(20)	Node(20)	Node(20)	Node(20)	Node(20)
Node(20).prev	None	None	None	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)
Node(20).next	None	None	None	None	None	Node(30)	Node(30)	Node(30)	Node(30)
Node(30).prev	None	None	None	None	None	Node(20)	Node(20)	Node(20)	Node(20)
Node(30).next	None	None	None	None	None	None	None	None	None

Key Moments - 3 Insights

Why does each node have both prev and next pointers?

Each node has prev and next pointers to allow moving forward and backward through the list, as shown in steps 3 and 5 where links are set both ways.

What does it mean when a node's prev or next pointer is None?

A None pointer means there is no node in that direction, like the first node's prev and last node's next in the visual states at steps 1 and 5.

How do we know the list is connected correctly?

By checking pointer changes in steps 3 and 5 and the visual state showing bidirectional arrows, confirming nodes link both ways.

Visual Quiz - 3 Questions

Look at the execution table at step 3. What is Node(10).next pointing to?

A Node(20)

B None

C Node(30)

D Node(10)

💡 Hint

Check the 'Pointer Changes' and 'Visual State' columns at step 3 in the execution table.

At which step does Node(30).prev get assigned to Node(20)?

A Step 2

B Step 4

C Step 5

D Step 1

💡 Hint

Look at the 'Pointer Changes' column for step 5 in the execution table.

If we remove the prev pointer from nodes, what traversal becomes impossible?

A Forward traversal

B Backward traversal

C Both forward and backward traversal

D Traversal is unaffected

💡 Hint

Refer to the concept flow and key moments explaining the role of prev pointers.

Concept Snapshot

Doubly Linked List Node:
- Holds data, prev pointer, next pointer
- prev points to previous node or None
- next points to next node or None
- Allows traversal forward and backward
- Insert/delete by updating prev and next pointers

Full Transcript

A doubly linked list node stores data and two pointers: prev and next. The prev pointer links to the previous node, and the next pointer links to the next node. This structure allows moving forward and backward through the list. Nodes at the ends have None for prev (head) or next (tail). Creating nodes and linking them updates these pointers to connect nodes bidirectionally. Traversal uses next pointers to move forward and prev pointers to move backward. This design supports flexible navigation and easy insertion or deletion by adjusting pointers.