Pop Using Linked List Node in DSA Python - Execution Trace

Concept Flow - Pop Using Linked List Node

Start with head node

↓

Check if list is empty?

↓

Return None

↓

Move head to next node

↓

Disconnect popped node

↓

Return popped node's value

This flow shows how to remove the first node from a linked list by updating the head pointer and returning the removed node's value.

Execution Sample

DSA Python

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

def pop(head):
    if head is None:
        return None, None
    popped = head
    head = head.next
    popped.next = None
    return popped.val, head

This code removes the first node from a linked list and returns its value.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Start with list: 10 -> 20 -> 30 -> None	10 -> 20 -> 30 -> None	head points to node with val=10	head -> [10] -> [20] -> [30] -> None
2	Check if list is empty	10 -> 20 -> 30 -> None	head is not None	head -> [10] -> [20] -> [30] -> None
3	Save head node to popped	10 -> 20 -> 30 -> None	popped = head (val=10)	popped -> [10] -> [20] -> [30] -> None
4	Move head to next node	10 -> 20 -> 30 -> None	head = head.next (val=20)	head -> [20] -> [30] -> None
5	Disconnect popped node	10 (popped) 20 -> 30 -> None	popped.next = None	popped -> [10] -> None head -> [20] -> [30] -> None
6	Return popped node's value	20 -> 30 -> None	Return 10	head -> [20] -> [30] -> None

💡 After popping, head points to second node; popped node is disconnected and returned.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
head	Node(10)	Node(10)	Node(20)	Node(20)	Node(20)
popped	None	Node(10)	Node(10)	Node(10)	Node(10)
popped.next	N/A	Node(20)	Node(20)	None	None

Key Moments - 3 Insights

Why do we set popped.next = None after moving head?

Setting popped.next = None disconnects the popped node from the list, preventing accidental links. See step 5 in execution_table where this happens.

What happens if the list is empty (head is None)?

The function returns None immediately without changes, as shown in step 2 where the check for empty list occurs.

Why do we update head to head.next before disconnecting popped?

We update head first to keep the list connected starting from the second node, then disconnect popped to isolate it. This order is shown in steps 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of head after step 4?

A Node with value 10

B Node with value 30

C Node with value 20

D None

💡 Hint

Check the 'Pointer Changes' column at step 4 in execution_table.

At which step is the popped node disconnected from the list?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look for 'popped.next = None' in the 'Operation' column of execution_table.

If the list was empty, what would the pop function return?

A The value of the head node

B None

C An error

D The last node's value

💡 Hint

Refer to step 2 in execution_table where the empty list check happens.

Concept Snapshot

Pop Using Linked List Node:
- Check if head is None; if yes, return None
- Save head node as popped
- Move head to next node
- Disconnect popped node by setting popped.next = None
- Return popped node's value
This removes the first node and updates the list head.

Full Transcript

This concept shows how to remove the first node from a linked list. We start by checking if the list is empty. If it is, we return None. Otherwise, we save the current head node as the popped node. Then, we move the head pointer to the next node in the list. After that, we disconnect the popped node by setting its next pointer to None. Finally, we return the value of the popped node. This process updates the linked list by removing its first element.

Practice

1. What does the pop operation do in a linked list when using nodes?

easy

A. Removes the last node and returns its value

B. Adds a new node at the beginning

C. Searches for a node with a specific value

D. Counts the total number of nodes

Solution

1. Step 1: Understand pop operation in linked list

   Pop means removing the last element from the list and returning its value.

2. Step 2: Identify pop behavior with nodes

   In a linked list, pop removes the tail node and returns its data.

3. Final Answer:

   Removes the last node and returns its value -> Option A

4. Quick Check:

   Pop = Remove last node [OK]

Hint: Pop always removes the last node in a linked list [OK]

Common Mistakes:

• Confusing pop with push (adding nodes)
• Thinking pop removes the first node
• Assuming pop searches for a value

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

easy

A. def pop(self):\n if not self.head:\n return None\n current = self.head\n while current.next.next:\n current = current.next\n popped = current.next\n current.next = None\n return popped.data

B. def pop(self):\n popped = self.head\n self.head = self.head.next\n return popped.data

C. def pop(self):\n return self.head.data

D. def pop(self):\n self.head = None

Solution

1. Step 1: Check pop logic for singly linked list

   Pop removes the last node, so we must traverse to the second last node.

2. Step 2: Verify syntax correctness

   def pop(self):\n if not self.head:\n return None\n current = self.head\n while current.next.next:\n current = current.next\n popped = current.next\n current.next = None\n return popped.data correctly traverses until current.next.next is None, then removes last node and returns its data.

3. Final Answer:

   Code that traverses to second last node -> Option A

4. Quick Check:

   Traverse to second last node, remove last [OK]

Hint: Pop needs traversal to second last node before removal [OK]

Common Mistakes:

• Removing head instead of last node
• Not checking for empty list
• Returning node instead of node data

3. What will be the output after popping a node from the linked list 1 -> 2 -> 3 -> null using the pop method below?

def pop(self):
    if not self.head:
        return None
    if not self.head.next:
        val = self.head.data
        self.head = None
        return val
    current = self.head
    while current.next.next:
        current = current.next
    val = current.next.data
    current.next = None
    return val

medium

A. 2 -> 3 -> null

B. 1 -> 3 -> null

C. 1 -> 2 -> null

D. 3 -> null

Solution

1. Step 1: Identify the node popped

   Pop removes the last node, which is 3 in the list 1 -> 2 -> 3 -> null.

2. Step 2: Determine list after pop

   After removing 3, the list becomes 1 -> 2 -> null.

3. Final Answer:

   1 -> 2 -> null -> Option C

4. Quick Check:

   Last node removed, list ends at 2 [OK]

Hint: Pop removes last node, list shortens by one [OK]

Common Mistakes:

• Removing first node instead of last
• Not updating next pointer to None
• Returning wrong list after pop

4. Find the error in this pop method for a singly linked list with at least two nodes:

def pop(self):
    current = self.head
    while current.next:
        current = current.next
    val = current.data
    current = None
    return val

medium

A. It does not handle empty list case

B. It does not update the previous node's next pointer

C. It returns None instead of node data

D. It removes the head instead of last node

Solution

1. Step 1: Analyze traversal and removal

   The code moves current to last node but does not track previous node.

2. Step 2: Check pointer update

   Setting current = None only changes local variable, does not update previous node's next pointer to None, so last node remains linked.

3. Final Answer:

   It does not update the previous node's next pointer -> Option B

4. Quick Check:

   Must update previous.next to None to remove last node [OK]

Hint: Always update previous node's next to None when popping last node [OK]

Common Mistakes:

• Not handling empty list
• Not tracking previous node
• Setting local variable to None instead of pointer

5. Given a linked list with nodes 10 -> 20 -> 30 -> 40 -> null, write the sequence of node values after two consecutive pop operations using a correct pop method that removes the last node each time.

hard

A. 10 -> 30 -> null

B. 10 -> 20 -> 30 -> null

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

D. 10 -> 20 -> null

Solution

1. Step 1: First pop removes last node 40

   List changes from 10 -> 20 -> 30 -> 40 -> null to 10 -> 20 -> 30 -> null.

2. Step 2: Second pop removes new last node 30

   List changes from 10 -> 20 -> 30 -> null to 10 -> 20 -> null.

3. Final Answer:

   10 -> 20 -> null -> Option D

4. Quick Check:

   Two pops remove last two nodes [OK]

Hint: Each pop removes last node, shorten list stepwise [OK]

Common Mistakes:

• Removing nodes from start instead of end
• Not updating list after first pop
• Confusing node values after pops