Enqueue Using Linked List in DSA Python - Execution Trace

Concept Flow - Enqueue Using Linked List

Create new node with value

↓

Is queue empty?

Yes→Set head and tail to new node

No↓

Set tail.next to new node

↓

Update tail to new node

↓

Done

This flow shows how a new node is created and added at the end of the linked list queue, updating pointers accordingly.

Execution Sample

DSA Python

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

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

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

This code adds a new value to the end of a queue implemented with a linked list.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with value 10	None	None	null
2	Queue empty? Yes	None	head -> new_node(10), tail -> new_node(10)	10 -> null
3	Create new node with value 20	10	None	10 -> null
4	Queue empty? No	10	tail.next -> new_node(20)	10 -> 20 -> null
5	Update tail to new_node(20)	10 -> 20	tail -> new_node(20)	10 -> 20 -> null
6	Create new node with value 30	10 -> 20	None	10 -> 20 -> null
7	Queue empty? No	10 -> 20	tail.next -> new_node(30)	10 -> 20 -> 30 -> null
8	Update tail to new_node(30)	10 -> 20 -> 30	tail -> new_node(30)	10 -> 20 -> 30 -> null
9	End of enqueue operations	10 -> 20 -> 30	No changes	10 -> 20 -> 30 -> null

💡 All enqueue operations completed, queue updated with new nodes at tail.

Variable Tracker

Variable	Start	After Step 2	After Step 5	After Step 8	Final
head	None	Node(10)	Node(10)	Node(10)	Node(10)
tail	None	Node(10)	Node(20)	Node(30)	Node(30)
new_node	None	Node(10)	Node(20)	Node(30)	Node(30)

Key Moments - 3 Insights

Why do we check if the queue is empty before adding a new node?

Because if the queue is empty (head is None), both head and tail must point to the new node. This is shown in step 2 where head and tail are set to the new node.

Why do we update tail.next before moving tail to the new node?

Updating tail.next links the current last node to the new node, maintaining the chain. Then tail moves to the new node to mark the new end. This is shown in steps 4 and 5.

What happens if we forget to update the tail pointer after enqueue?

The tail pointer would still point to the old last node, breaking the queue's structure and causing incorrect enqueue behavior. Step 8 shows the correct tail update.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of the queue after step 5?

A 10 -> null

B 10 -> 20 -> null

C 20 -> null

D null

💡 Hint

Check the Visual State column at step 5 in the execution_table.

At which step does the tail pointer first change from Node(10) to Node(20)?

A Step 2

B Step 4

C Step 5

D Step 7

💡 Hint

Look at the Pointer Changes and variable_tracker for tail pointer updates.

If the queue was initially empty, what pointers are set when the first node is enqueued?

A Both head and tail are set to new node

B Only head is set to new node

C Only tail is set to new node

D Neither head nor tail is set

💡 Hint

Refer to step 2 in execution_table where queue is empty and pointers are set.

Concept Snapshot

Enqueue Using Linked List:
- Create a new node with the value.
- If queue is empty, set head and tail to new node.
- Else, set tail.next to new node and update tail.
- This adds elements at the end efficiently.
- Maintains FIFO order in queue.

Full Transcript

Enqueue operation in a linked list queue involves creating a new node with the given value. If the queue is empty, both head and tail pointers are set to this new node. Otherwise, the current tail's next pointer is linked to the new node, and then the tail pointer is updated to the new node. This process ensures new elements are added at the end, maintaining the queue's FIFO order. The execution table shows step-by-step how nodes are created, pointers updated, and the visual state of the queue after each operation. Key moments clarify why checking for empty queue and updating tail pointer are essential. The visual quiz tests understanding of pointer updates and queue state during enqueue.

Practice

1. What does the enqueue operation do in a linked list implementation of a queue?

easy

A. Adds a new element at the end (tail) of the linked list

B. Removes the first element from the linked list

C. Adds a new element at the beginning (head) of the linked list

D. Deletes the last element from the linked list

Solution

1. Step 1: Understand enqueue operation in queue

   Enqueue means adding an element to the queue, which is done at the end (tail) to maintain FIFO order.

2. Step 2: Linked list enqueue behavior

   In a linked list, adding at the tail means creating a new node and linking it as the last node.

3. Final Answer:

   Adds a new element at the end (tail) of the linked list -> Option A

4. Quick Check:

   Enqueue = Add at tail [OK]

Hint: Enqueue always adds at the tail in linked list queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue operation
• Adding new element at head instead of tail
• Thinking enqueue removes elements

2. Which of the following is the correct syntax to create a new node and enqueue it at the tail in Python linked list?

easy

A. new_node = Node(data); new_node.next = tail; tail = new_node

B. new_node = Node(data); tail.next = new_node; tail = new_node

C. new_node = Node(data); new_node.next = head; head = new_node

D. new_node = Node(data); head.next = new_node; head = new_node

Solution

1. Step 1: Create new node with data

   We create a new node object with the given data.

2. Step 2: Link new node at tail and update tail pointer

   Set current tail's next to new node, then update tail to new node.

3. Final Answer:

   new_node = Node(data); tail.next = new_node; tail = new_node -> Option B

4. Quick Check:

   New node linked at tail [OK]

Hint: Tail.next points to new node, then tail moves to new node [OK]

Common Mistakes:

• Linking new node to head instead of tail
• Not updating tail pointer after enqueue
• Setting new_node.next to tail (wrong direction)

3. What will be the state of the queue after executing this code?

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

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

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

q = Queue()
q.enqueue(10)
q.enqueue(20)
q.enqueue(30)
current = q.head
result = []
while current:
    result.append(current.data)
    current = current.next
print(result)

medium

A. []

B. [30, 20, 10]

C. [10, 30, 20]

D. [10, 20, 30]

Solution

1. Step 1: Enqueue 10, 20, 30 in order

   First enqueue sets head and tail to node with 10. Next enqueues add nodes at tail with 20 and 30.

2. Step 2: Traverse from head to tail collecting data

   Traversal collects 10, then 20, then 30 in order.

3. Final Answer:

   [10, 20, 30] -> Option D

4. Quick Check:

   Queue order = insertion order [OK]

Hint: Enqueue adds at tail, so order stays insertion order [OK]

Common Mistakes:

• Assuming enqueue adds at head reversing order
• Not initializing head and tail properly
• Confusing output list order

4. Find the error in this enqueue method for a linked list queue:

def enqueue(self, data):
    new_node = Node(data)
    if self.head is None:
        self.head = new_node
    else:
        self.tail.next = new_node
        self.tail = new_node

medium

A. Does not set new_node.next to None

B. Does not update head when queue is empty

C. Does not update tail when queue is empty

D. Does not link new_node to tail.next

Solution

1. Step 1: Check initialization when queue is empty

   When head is None, head is set to new_node but tail is not updated.

2. Step 2: Tail must also point to new_node when queue is empty

   Without updating tail, tail remains None causing errors on next enqueue.

3. Final Answer:

   Does not update tail when queue is empty -> Option C

4. Quick Check:

   Empty queue needs both head and tail set [OK]

Hint: Set both head and tail on empty queue enqueue [OK]

Common Mistakes:

• Updating only head or only tail on empty queue
• Forgetting to set tail.next on non-empty queue
• Assuming new_node.next needs manual None assignment

5. You want to implement a queue using a linked list that supports enqueue and dequeue in O(1) time. Which of the following changes is necessary to the enqueue method to handle the edge case when the queue is empty?

hard

A. Set both head and tail to the new node when queue is empty

B. Set only tail to the new node when queue is empty

C. Set new node's next to head when queue is empty

D. Set tail.next to head when queue is empty

Solution

1. Step 1: Understand queue empty state

   When queue is empty, both head and tail pointers are None.

2. Step 2: Enqueue first element must update both pointers

   To maintain O(1) enqueue and dequeue, head and tail must both point to the new node.

3. Step 3: This ensures correct linking and prevents errors

   Without setting both, dequeue or next enqueue operations may fail or cause errors.

4. Final Answer:

   Set both head and tail to the new node when queue is empty -> Option A

5. Quick Check:

   Empty queue enqueue sets head and tail [OK]

Hint: Empty queue needs head and tail set to new node [OK]

Common Mistakes:

• Setting only tail or only head on empty enqueue
• Linking new node incorrectly to head or tail
• Not handling empty queue edge case