Delete Node at End in DSA Python - Execution Trace

Concept Flow - Delete Node at End

Start at head

↓

Check if list empty?

Yes→Nothing to delete, EXIT

No↓

Traverse to second last node

↓

Set second last node's next to null

↓

Delete last node

↓

End

Start from the head, check if list is empty, then move to the second last node and remove the last node by updating pointers.

Execution Sample

DSA Python

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

def delete_end(head):
    if not head:
        return None
    if not head.next:
        return None
    temp = head
    while temp.next.next:
        temp = temp.next
    temp.next = None
    return head

Deletes the last node from a singly linked list and returns the updated head.

Execution Table

Step	Action	Current Node Data	Next Node Data	Pointer Update	List State
1	Check if head is None	1	2	No	1 -> 2 -> 3 -> None
2	Check if head.next is None	1	2	No	1 -> 2 -> 3 -> None
3	Set temp = head	1	2	No	1 -> 2 -> 3 -> None
4	Check temp.next.next exists	1	2	No	1 -> 2 -> 3 -> None
5	Move temp to temp.next	2	3	No	1 -> 2 -> 3 -> None
6	Check temp.next.next exists	2	3	No	1 -> 2 -> 3 -> None
7	temp.next.next is None, stop loop	2	3	No	1 -> 2 -> 3 -> None
8	Set temp.next = None (delete last node)	2	None	Yes	1 -> 2 -> None
9	Return head	1	2	No	1 -> 2 -> None

💡 Last node deleted by setting second last node's next to None

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 8	Final
head	Node(1)	Node(1)	Node(1)	Node(1)	Node(1)
temp	None	Node(1)	Node(2)	Node(2)	Node(2)

Key Moments - 3 Insights

Why do we check if head.next is None before traversing?

Because if head.next is None, it means there is only one node. We delete it by returning None. This is shown in execution_table row 2.

Why do we use temp.next.next in the while loop condition?

To stop at the second last node, so we can remove the last node by setting temp.next to None. This is shown in execution_table rows 4-7.

What happens if the list is empty at the start?

The function returns None immediately, meaning no deletion happens. This is shown in execution_table row 1.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of temp after step 5?

A Node with data 2

B Node with data 1

C Node with data 3

D None

💡 Hint

Check the 'Current Node Data' column at step 5 in execution_table

At which step does the last node get deleted?

A Step 4

B Step 7

C Step 8

D Step 9

💡 Hint

Look for the step where 'Pointer Update' is 'Yes' in execution_table

If the list had only one node, what would the function return?

A The same single node

B None

C A new node

D An error

💡 Hint

Refer to execution_table row 2 where head.next is None

Concept Snapshot

Delete Node at End in singly linked list:
- Check if list empty or single node
- Traverse to second last node
- Set second last node's next to None
- Last node is removed
- Return updated head

Full Transcript

This visual execution shows how to delete the last node in a singly linked list. We start by checking if the list is empty or has only one node. If empty, we return None. If only one node, we delete it by returning None. Otherwise, we traverse the list to find the second last node by checking temp.next.next. Once found, we set temp.next to None, removing the last node. The updated list is then returned. The execution table traces each step, showing variable values and list state. Key moments clarify why we check conditions and how traversal works. The quiz tests understanding of variable states and deletion step.

Practice

1. What is the main purpose of the Delete Node at End operation in a singly linked list?

easy

A. Remove the first node from the linked list

B. Add a new node at the end of the linked list

C. Remove the last node from the linked list

D. Reverse the linked list

Solution

1. Step 1: Understand the operation

   The operation aims to remove a node from the end of the singly linked list.

2. Step 2: Identify the correct description

   Removing the last node means deleting the tail node, not adding or reversing nodes.

3. Final Answer:

   Remove the last node from the linked list -> Option C

4. Quick Check:

   Delete Node at End = Remove last node [OK]

Hint: Deleting at end means removing the last node [OK]

Common Mistakes:

• Confusing delete at end with delete at beginning
• Thinking it adds a node instead of removing
• Mixing up with reversing the list

2. Which of the following is the correct syntax to delete the last node in a singly linked list in Python?

easy

A. Set last node's next to head

B. Set head = null directly

C. Create a new node and assign it to head

D. Traverse to second last node and set its next to null

Solution

1. Step 1: Understand deletion at end

   To delete the last node, we must find the second last node and make its next pointer null.

2. Step 2: Check each option

   Setting head = null deletes the whole list, not just last node. Setting last node's next to head creates a cycle. Creating a new node and assigning to head adds a node, not deletes.

3. Final Answer:

   Traverse to second last node and set its next to null -> Option D

4. Quick Check:

   Delete last node = second last node next = null [OK]

Hint: Set second last node's next to null to delete last node [OK]

Common Mistakes:

• Setting head to null deletes entire list
• Creating cycles by wrong pointer assignments
• Adding nodes instead of deleting

3. What will be the output linked list after deleting the last node from the list: 1 -> 2 -> 3 -> 4 -> null?

medium

A. 1 -> 2 -> 3 -> null

B. 1 -> 2 -> 3 -> 4 -> null

C. 2 -> 3 -> 4 -> null

D. 1 -> 2 -> null

Solution

1. Step 1: Identify the last node

   The last node in the list is the node with value 4.

2. Step 2: Delete the last node

   Removing node 4 means the new last node is 3, so the list ends at 3 with next as null.

3. Final Answer:

   1 -> 2 -> 3 -> null -> Option A

4. Quick Check:

   Last node 4 removed = 1 -> 2 -> 3 [OK]

Hint: Last node removed means list ends one node earlier [OK]

Common Mistakes:

• Not updating second last node's next to null
• Removing wrong node (like first node)
• Confusing output with original list

4. Consider this code snippet to delete the last node of a singly linked list:

def delete_end(head):
    if head is None:
        return None
    if head.next is None:
        return None
    temp = head
    while temp.next is not None:
        temp = temp.next
    temp.next = None
    return head

What is the error in this code?

medium

A. The loop never stops because condition is wrong

B. temp.next is null check misses second last node

C. It deletes the first node instead of last

D. No error, code works correctly

Solution

1. Step 1: Analyze the loop condition

   The loop runs while temp.next is not null, so temp moves to the last node.

2. Step 2: Identify pointer update mistake

   After loop, temp points to last node, but to delete last node, we need second last node to set its next to null. Here, temp.next = null sets last node's next to null, which is already null, so no deletion happens.

3. Final Answer:

   temp.next is null check misses second last node -> Option B

4. Quick Check:

   Must stop at second last node to delete last [OK]

Hint: Stop at second last node, not last, to delete end node [OK]

Common Mistakes:

• Stopping loop at last node instead of second last
• Not handling single node list correctly
• Setting wrong pointers causing no deletion

5. Given a singly linked list with nodes 10 -> 20 -> 30 -> 40 -> 50 -> null, write the steps to delete the last node and then add a new node with value 60 at the end. What will be the final list?

hard

A. 10 -> 20 -> 30 -> 40 -> 60 -> null

B. 10 -> 20 -> 30 -> 40 -> 50 -> 60 -> null

C. 10 -> 20 -> 30 -> 40 -> null

D. 10 -> 20 -> 30 -> 40 -> 50 -> null

Solution

1. Step 1: Delete the last node (50)

   Traverse to second last node (40) and set its next to null, removing 50.

2. Step 2: Add new node with value 60 at end

   Traverse to last node (40) and set its next to new node with value 60, new node's next is null.

3. Final Answer:

   10 -> 20 -> 30 -> 40 -> 60 -> null -> Option A

4. Quick Check:

   Delete last 50, add 60 at end = 10->20->30->40->60 [OK]

Hint: Delete last node, then link new node at end [OK]

Common Mistakes:

• Not updating second last node's next to null before adding
• Appending new node without deleting last node
• Confusing node values or order