Insert at Middle Specific Position in DSA Python - Execution Trace

Concept Flow - Insert at Middle Specific Position

Start with Linked List

↓

Check if position is valid

Yes↓

Traverse to node before position

↓

Create new node

↓

Adjust pointers to insert new node

↓

New node inserted

↓

End

This flow shows how to insert a new node at a specific middle position in a linked list by traversing to the correct spot and adjusting pointers.

Execution Sample

DSA Python

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

def insert_middle(head, data, position):
    new_node = Node(data)
    if position == 0:
        new_node.next = head
        return new_node
    current = head
    for _ in range(position - 1):
        current = current.next
    new_node.next = current.next
    current.next = new_node
    return head

This code inserts a new node with given data at the specified position in a singly linked list.

Execution Table

Step	Action	Current Node Data	Position Counter	Pointers Changed	List State
1	Create new node with data=99	-	-	None	1 -> 2 -> 3 -> 4 -> null
2	Check if position=2 is 0	-	-	No	1 -> 2 -> 3 -> 4 -> null
3	Set current to head (data=1)	1	0	None	1 -> 2 -> 3 -> 4 -> null
4	Traverse to position-1 (1)	1	1	None	1 -> 2 -> 3 -> 4 -> null
5	Set new_node.next = current.next (2)	1	1	new_node.next points to 2	1 -> 2 -> 3 -> 4 -> null
6	Set current.next = new_node (99)	1	1	current.next points to new_node	1 -> 99 -> 2 -> 3 -> 4 -> null
7	Return head	-	-	None	1 -> 99 -> 2 -> 3 -> 4 -> null
8	End	-	-	-	Insertion complete

💡 Position reached and new node inserted, traversal stops

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 6	Final
head	Node(1)	Node(1)	Node(1)	Node(1)	Node(1)	Node(1)
current	None	Node(1)	Node(1)	Node(1)	Node(1)	Node(1)
new_node	None	Node(99)	Node(99)	Node(99)	Node(99)	Node(99)
position	2	2	2	2	2	2

Key Moments - 3 Insights

Why do we traverse to position-1 instead of position?

We stop at position-1 (Step 4) because we need to change the next pointer of the node before the insertion point to point to the new node (Step 6).

What happens if position is 0?

If position is 0 (Step 2), the new node becomes the new head by pointing its next to the current head, then returning itself as the new head.

How do pointers change to insert the new node?

First, new_node.next points to current.next (Step 5), then current.next points to new_node (Step 6), linking the new node into the list.

Visual Quiz - 3 Questions

Look at the execution table, what is the data of the current node at Step 4?

A 2

B 1

C 3

D 99

💡 Hint

Check the 'Current Node Data' column at Step 4 in the execution_table.

At which step does the new node's next pointer get assigned?

A Step 3

B Step 6

C Step 5

D Step 7

💡 Hint

Look for 'new_node.next points to' in the 'Pointers Changed' column.

If position was 0, what would happen according to the code?

A The new node becomes the new head

B The new node is inserted at the end

C The list remains unchanged

D An error occurs

💡 Hint

Refer to Step 2 where position == 0 is checked.

Concept Snapshot

Insert at Middle Specific Position in Linked List:
- Traverse to node before target position (position-1).
- Create new node.
- Set new_node.next to current.next.
- Set current.next to new_node.
- If position is 0, new node becomes new head.
- Adjust pointers carefully to avoid breaking the list.

Full Transcript

This lesson shows how to insert a new node at a specific middle position in a singly linked list. We start by checking if the position is zero, which means inserting at the head. Otherwise, we traverse the list to the node just before the desired position. Then we create a new node and adjust pointers: the new node's next points to the current node's next, and the current node's next points to the new node. This links the new node into the list without losing any nodes. The execution table traces each step, showing how variables and pointers change. Key moments clarify why we stop at position-1 and how pointers are updated. The visual quiz tests understanding of these steps. This method keeps the list connected and inserts the new node exactly where needed.

Practice

1. What does it mean to insert an element at a specific middle position in a list?

easy

A. Add the element exactly at the given index, shifting others right

B. Replace the element at the given index with the new element

C. Add the element only if the list is empty

D. Add the element at the end of the list regardless of position

Solution

1. Step 1: Understand list insertion

   Inserting at a specific position means placing the new element at that index and moving existing elements to the right.

2. Step 2: Differentiate from replacement

   Replacing overwrites the element, but insertion shifts elements to keep all.

3. Final Answer:

   Add the element exactly at the given index, shifting others right -> Option A

4. Quick Check:

   Insert means add and shift, not replace or append [OK]

Hint: Insert shifts elements right at the given index [OK]

Common Mistakes:

• Confusing insert with replace
• Thinking insert always adds at end
• Assuming insert only works on empty lists

2. Which of the following is the correct syntax to insert the value 10 at index 2 in a Python list named lst?

easy

A. lst.append(2, 10)

B. lst.insert(2, 10)

C. lst.add(2, 10)

D. lst[2] = 10

Solution

1. Step 1: Recall Python list insertion method

   Python lists use the insert(index, value) method to add an element at a specific position.

2. Step 2: Check other options

   add is not a list method, append adds only at the end, and lst[2] = 10 replaces the element at index 2.

3. Final Answer:

   lst.insert(2, 10) -> Option B

4. Quick Check:

   Use insert(index, value) to add at position [OK]

Hint: Use insert(index, value) to add at any position [OK]

Common Mistakes:

• Using append to insert in middle
• Trying to assign with lst[2] = 10 instead of insert
• Using non-existent add method

3. What will be the output of the following code?

lst = [1, 2, 4, 5]
lst.insert(2, 3)
print(lst)

medium

A. [1, 2, 3, 4, 5]

B. [1, 2, 4, 3, 5]

C. [3, 1, 2, 4, 5]

D. [1, 2, 4, 5, 3]

Solution

1. Step 1: Understand insert at index 2

   Index 2 means the third position (0-based). Inserting 3 here shifts 4 and 5 right.

2. Step 2: Visualize the list after insertion

   List becomes [1, 2, 3, 4, 5] after insertion.

3. Final Answer:

   [1, 2, 3, 4, 5] -> Option A

4. Quick Check:

   Insert at index 2 places 3 before 4 [OK]

Hint: Index 2 means third position, insert shifts right [OK]

Common Mistakes:

• Inserting after index instead of at index
• Confusing insert with replace
• Miscounting zero-based index

4. Find the error in the following code that tries to insert 99 at index 3:

lst = [10, 20, 30]
lst.insert(4, 99)
print(lst)

medium

A. Index 4 is out of range, causes error

B. insert method requires two arguments

C. No error; 99 is added at the end

D. lst should be converted to a set before insert

Solution

1. Step 1: Check index validity for insert

   Python's insert allows index beyond list length; it adds element at the end.

2. Step 2: Confirm behavior of insert with large index

   Since index 4 > length 3, 99 is appended at the end without error.

3. Final Answer:

   No error; 99 is added at the end -> Option C

4. Quick Check:

   Insert with large index appends at end [OK]

Hint: Insert with index > length appends at end [OK]

Common Mistakes:

• Assuming index out of range causes error
• Confusing insert with assignment
• Thinking list must be converted to set

5. Given a list lst = [5, 10, 15, 20, 25], write code to insert the value 12 exactly in the middle of the list. What will be the list after insertion?

hard

A. [5, 10, 15, 12, 20, 25]

B. [12, 5, 10, 15, 20, 25]

C. [5, 12, 10, 15, 20, 25]

D. [5, 10, 12, 15, 20, 25]

Solution

1. Step 1: Calculate middle index of the list

   List length is 5, middle index is length // 2 = 2.

2. Step 2: Insert 12 at index 2

   Using lst.insert(2, 12) places 12 between 10 and 15.

3. Final Answer:

   [5, 10, 12, 15, 20, 25] -> Option D

4. Quick Check:

   Middle index = 2, insert shifts elements right [OK]

Hint: Middle index = length // 2, insert there [OK]

Common Mistakes:

• Inserting at wrong index (like 3)
• Replacing element instead of inserting
• Not calculating middle index correctly