Dequeue Using Linked List in DSA Python - Execution Trace

Concept Flow - Dequeue Using Linked List

Create new node

↓

Is dequeue empty?

Yes→Set head and tail to new node

No↓

Add at front or rear?

↓

Insert at head

↓

Update pointers

↓

Done

↓

Is dequeue empty?

Yes→Underflow error

No↓

Remove from front or rear?

↓

Delete head

↓

Update pointers

↓

Done

Shows how nodes are added or removed from front or rear in a linked list based dequeue, updating pointers accordingly.

Execution Sample

DSA Python

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

class Dequeue:
    def __init__(self):
        self.head = None
        self.tail = None

    def insert_rear(self, val):
        new_node = Node(val)
        if not self.tail:
            self.head = self.tail = new_node
        else:
            self.tail.next = new_node
            self.tail = new_node

    def delete_front(self):
        if not self.head:
            return None
        val = self.head.val
        self.head = self.head.next
        if not self.head:
            self.tail = None
        return val

This code inserts at the rear and deletes from the front of a dequeue implemented with a linked list.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Insert 10 at rear	10	head -> 10, tail -> 10	10 -> null
2	Insert 20 at rear	10 -> 20	tail.next -> 20, tail -> 20	10 -> 20 -> null
3	Insert 30 at rear	10 -> 20 -> 30	tail.next -> 30, tail -> 30	10 -> 20 -> 30 -> null
4	Delete front	20 -> 30	head -> 20	20 -> 30 -> null
5	Delete front	30	head -> 30	30 -> null
6	Delete front	empty	head -> None, tail -> None	null
7	Delete front (empty)	empty	No change	null

💡 Stops after deleting all nodes; next delete returns None as dequeue is empty.

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
head	None	Node(10)	Node(10)	Node(10)	Node(20)	Node(30)	None	None
tail	None	Node(10)	Node(20)	Node(30)	Node(30)	Node(30)	None	None
List Size	0	1	2	3	2	1	0	0

Key Moments - 3 Insights

Why does the tail pointer update only when inserting at rear?

Because tail always points to the last node. When inserting at rear (see Step 2 and 3), tail moves to new node. In deletion from front (Step 4-6), tail changes only if list becomes empty (Step 6).

What happens when deleting from an empty dequeue?

No pointers change and operation returns None (Step 7). This prevents errors by checking if head is None before deletion.

Why does the head pointer move forward on deletion?

Deleting front removes the first node, so head moves to next node (Step 4 and 5). This effectively removes the old front node from the list.

Visual Quiz - 3 Questions

Look at the execution table, what is the visual state after Step 3?

A 20 -> 30 -> null

B 10 -> 20 -> 30 -> null

C 30 -> null

D null

💡 Hint

Check the 'Visual State' column in Step 3 of the execution table.

At which step does the dequeue become empty?

A Step 5

B Step 4

C Step 6

D Step 7

💡 Hint

Look at the 'Nodes in List' and 'Pointer Changes' columns to see when head and tail become None.

If we insert 40 at rear after Step 6, what will be the new visual state?

A 40 -> null

B 30 -> 40 -> null

C null

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

💡 Hint

After Step 6, list is empty. Inserting at rear sets head and tail to new node only.

Concept Snapshot

Dequeue using linked list:
- Insert at front or rear by adding nodes and updating head/tail pointers.
- Delete from front or rear by removing nodes and updating pointers.
- If list empty, head and tail are None.
- Tail updates only on rear insert or when list becomes empty.
- Head moves forward on front deletion.

Full Transcript

This visual trace shows how a dequeue works using a linked list. We add nodes at the rear by creating a new node and linking it after the tail, then moving the tail pointer. Deletion from the front removes the head node and moves the head pointer forward. When the list becomes empty, both head and tail become None. Trying to delete from an empty dequeue returns None safely. The execution table tracks each step's operation, the nodes present, pointer changes, and the visual linked list state. Variable tracker shows how head, tail, and list size change after each step. Key moments clarify why pointers update as they do and how empty conditions are handled. The quiz tests understanding of list state at key steps and effects of operations.