Insert at Middle Specific Position in DSA C# - Execution Trace

Concept Flow - Insert at Middle Specific Position

Start

↓

Check if position is valid

No↓

Exit with error

Yes↓

Traverse list to position-1

↓

Create new node

↓

Adjust pointers to insert node

↓

Insertion done

↓

End

Check if position is valid, traverse to node before position, create new node, adjust pointers to insert, then finish.

Execution Sample

DSA C#

Node InsertAtPosition(Node head, int data, int pos) {
  if (pos < 1) return head;
  if (pos == 1) {
    Node newNode = new Node(data);
    newNode.Next = head;
    return newNode;
  }
  Node current = head;
  for (int i = 1; i < pos - 1 && current != null; i++) current = current.Next;
  if (current == null) return head;
  Node newNode = new Node(data);
  newNode.Next = current.Next;
  current.Next = newNode;
  return head;
}

Inserts a new node with given data at the specified position in the linked list.

Execution Table

Step	pos	i	current.Data	Action	List State
1	3	-	1 (head)	Check pos valid (3 >= 1)	1 -> 2 -> 4 -> null
2	3	1	1	Traverse to position-1 (i=1)	1 -> 2 -> 4 -> null
3	3	2	2	Traverse to position-1 (i=2)	1 -> 2 -> 4 -> null
4	3	2	2	Create new node with data=3	1 -> 2 -> 4 -> null
5	3	2	2	Set newNode.Next = current.Next (4)	1 -> 2 -> 4 -> null
6	3	2	2	Set current.Next = newNode	1 -> 2 -> 3 -> 4 -> null
7	3	2	2	Return head	1 -> 2 -> 3 -> 4 -> null

💡 Insertion complete at position 3, list updated.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	Final
pos	3	3	3	3	3
i	-	1	2	2	2
current.Data	1	1	2	2	2
List	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 3 -> 4 -> null	1 -> 2 -> 3 -> 4 -> null

Key Moments - 3 Insights

Why do we stop traversing at position-1 and not position?

Because we need to insert the new node after the node at position-1, so we stop traversal there to adjust pointers correctly (see execution_table step 3).

What happens if the position is 1?

We create a new node and set its next to the current head, then return this new node as the new head (not shown in this trace but handled in code).

Why do we check if current is null after traversal?

If current is null, it means position is beyond list length, so we do not insert and return original list (see code line after for-loop).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of current.Data at step 3?

A 1

B 3

C 2

D 4

💡 Hint

Check the 'current.Data' column at step 3 in the execution_table.

At which step is the new node's next pointer set to the next node?

A Step 5

B Step 4

C Step 6

D Step 7

💡 Hint

Look at the 'Action' column describing pointer adjustments in execution_table.

If pos was 1, what would happen differently?

A Traverse loop runs fully and new node inserted at end

B Traverse loop runs zero times and new node becomes new head

C Function returns original list without changes

D New node is inserted after the last node

💡 Hint

Refer to the code snippet's condition for pos == 1 in execution_sample.

Concept Snapshot

Insert at Middle Specific Position in Linked List:
- Check if position is valid (>=1)
- If position is 1, insert at head
- Else traverse to node at position-1
- Create new node
- Adjust pointers: newNode.Next = current.Next; current.Next = newNode
- Return head
- If position > length+1, no insertion

Full Transcript

This concept shows how to insert a new node at a specific middle position in a singly linked list. First, we check if the position is valid. If the position is 1, we insert the new node at the head. Otherwise, we traverse the list to the node just before the desired position. Then we create a new node and adjust the pointers so the new node fits into the list at the correct spot. If the position is beyond the list length, we do not insert and return the original list. The execution table traces each step, showing how variables change and how the list updates after insertion.

Practice

1. What does inserting a node at a specific middle position in a linked list mean?

easy

A. Adding a new element only at the start of the list

B. Adding a new element exactly between two existing nodes at a given position

C. Adding a new element only at the end of the list

D. Removing an element from the middle of the list

Solution

1. Step 1: Understand linked list insertion

   Inserting at a specific position means placing a new node between existing nodes, not just at start or end.

2. Step 2: Define middle position insertion

   Middle insertion targets a position inside the list, not at boundaries.

3. Final Answer:

   Adding a new element exactly between two existing nodes at a given position -> Option B

4. Quick Check:

   Middle insertion = between nodes [OK]

Hint: Middle insertion means placing node between two nodes [OK]

Common Mistakes:

• Confusing middle insertion with adding at start
• Thinking middle insertion removes nodes
• Assuming insertion only happens at end

2. Which of the following C# code snippets correctly inserts a new node at position 3 in a singly linked list?

easy

A. Node current = head; for(int i=1; i<2; i++) current = current.next; newNode.next = current.next; current.next = newNode;

B. Node current = head; for(int i=1; i<3; i++) current = current.next; newNode.next = current.next; current.next = newNode;

C. Node current = head; for(int i=0; i<3; i++) current = current.next; newNode.next = current.next; current.next = newNode;

D. Node current = head; for(int i=1; i<=3; i++) current = current.next; newNode.next = current.next; current.next = newNode;

Solution

1. Step 1: Understand zero-based vs one-based indexing

   Position 3 means after 2 nodes, so loop runs i from 1 to 2 to reach node before position 3.

2. Step 2: Check loop and insertion logic

   Node current = head; for(int i=1; i<2; i++) current = current.next; newNode.next = current.next; current.next = newNode; loops correctly to node 2, then inserts newNode after it.

3. Final Answer:

   Node current = head; for(int i=1; i<2; i++) current = current.next; newNode.next = current.next; current.next = newNode; -> Option A

4. Quick Check:

   Loop to node before position = i < position-1 [OK]

Hint: Loop to node before position: i < position - 1 [OK]

Common Mistakes:

• Looping too many times and skipping target node
• Using wrong loop bounds causing null reference
• Not updating next pointers correctly

3. Given the linked list: 10 -> 20 -> 30 -> 40 -> null, what is the list after inserting 25 at position 3 using this code?

Node newNode = new Node(25);
Node current = head;
for(int i = 1; i < 2; i++) current = current.next;
newNode.next = current.next;
current.next = newNode;

medium

A. 10 -> 25 -> 20 -> 30 -> 40 -> null

B. 25 -> 10 -> 20 -> 30 -> 40 -> null

C. 10 -> 20 -> 30 -> 25 -> 40 -> null

D. 10 -> 20 -> 25 -> 30 -> 40 -> null

Solution

1. Step 1: Trace loop to position before insertion

   Loop runs i=1; i<2, so current moves from 10 to 20 (node at position 2).

2. Step 2: Insert newNode after current

   newNode.next points to current.next (30), current.next points to newNode (25).

3. Final Answer:

   10 -> 20 -> 25 -> 30 -> 40 -> null -> Option D

4. Quick Check:

   Inserted 25 at position 3 correctly [OK]

Hint: Insert after node at position-1 to place at position [OK]

Common Mistakes:

• Inserting at wrong position due to loop bounds
• Not updating newNode.next correctly
• Confusing node values and positions

4. What is the error in this code snippet that tries to insert a node at position 4 in a linked list?

Node current = head;
for(int i = 1; i <= 4; i++)
  current = current.next;
newNode.next = current.next;
current.next = newNode;

medium

A. Loop runs too many times causing current to be null before insertion

B. newNode.next is not assigned before insertion

C. The loop should start from i = 0 instead of 1

D. The newNode is not created before insertion

Solution

1. Step 1: Analyze loop bounds

   Loop runs i=1 to 4 inclusive, moving current 4 times ahead, which may make current null if list length is less than 5.

2. Step 2: Identify null reference risk

   Accessing current.next after current is null causes runtime error.

3. Final Answer:

   Loop runs too many times causing current to be null before insertion -> Option A

4. Quick Check:

   Loop bounds must stop at position-1 [OK]

Hint: Loop to position-1, not position, to avoid null [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Not checking for null before accessing next
• Incorrect loop start index

5. You have a singly linked list: 5 -> 15 -> 25 -> 35 -> 45 -> null. Write the steps to insert a new node with value 30 at position 4, and what will be the final list?

hard

A. Traverse to node at position 2, link new node's next to current's next, then set current's next to new node. Result: 5 -> 15 -> 30 -> 25 -> 35 -> 45 -> null

B. Traverse to node at position 4, link new node's next to current's next, then set current's next to new node. Result: 5 -> 15 -> 25 -> 35 -> 30 -> 45 -> null

C. Traverse to node at position 3, link new node's next to current's next, then set current's next to new node. Result: 5 -> 15 -> 25 -> 30 -> 35 -> 45 -> null

D. Insert new node at head and shift all nodes. Result: 30 -> 5 -> 15 -> 25 -> 35 -> 45 -> null

Solution

1. Step 1: Traverse to node before position 4

   Move current pointer to node at position 3 (value 25).

2. Step 2: Insert new node after current

   Set newNode.next to current.next (35), then current.next to newNode (30).

3. Final Answer:

   5 -> 15 -> 25 -> 30 -> 35 -> 45 -> null -> Option C

4. Quick Check:

   Insert at position 4 means after node 3 [OK]

Hint: Insert after node at position-1 for correct placement [OK]

Common Mistakes:

• Inserting at wrong position by off-by-one error
• Not updating next pointers correctly
• Confusing position with node value