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.