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

Concept Flow - Insert at Middle Specific Position

Start with Linked List

↓

Check if position is valid

Yes↓

Traverse to (pos-1)th node

↓

Create new node

↓

Adjust pointers to insert new node

↓

Insertion complete

↓

End

Start with the list, check position validity, move to the node before insertion point, create and link new node, then finish.

Execution Sample

DSA C++

struct Node {
  int data;
  Node* next;
};

void insertAtPosition(Node*& head, int data, int pos) {
  // Insert node at pos
}

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

Execution Table

Step	Current Node	Position Counter	Action	List State
1	head (data=1)	1	Check if pos=1 (insert at head)	1 -> 2 -> 3 -> null
2	head (data=1)	1	pos=3, move to next node	1 -> 2 -> 3 -> null
3	node with data=2	2	Reached (pos-1)th node, create new node with data=99	1 -> 2 -> 3 -> null
4	node with data=2	2	Adjust pointers: new node next=3, node 2 next=new node	1 -> 2 -> 99 -> 3 -> null
5	new node with data=99	3	Insertion done	1 -> 2 -> 99 -> 3 -> null
6	N/A	N/A	Exit	Insertion complete

💡 Insertion complete after adjusting pointers at position 3

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
current	head (data=1)	node with data=2	node with data=2	node with data=2
pos_counter	1	2	2	2
new_node	null	null	node with data=99	node with data=99
head	1 -> 2 -> 3 -> null	1 -> 2 -> 3 -> null	1 -> 2 -> 99 -> 3 -> null	1 -> 2 -> 99 -> 3 -> null

Key Moments - 3 Insights

Why do we stop at (pos-1)th node before inserting?

Because we need to change the next pointer of the node before the insertion point to link to the new node, as shown in step 3 and 4 of the execution_table.

What happens if position is 1?

We insert the new node at the head by making it point to the current head and updating head to new node. This is checked in step 1 of the execution_table.

Why do we adjust pointers in two steps?

First, new node's next points to the next node of current, then current's next points to new node. This order prevents losing the rest of the list, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the list state after step 4?

A 1 -> 2 -> 3 -> null

B 1 -> 99 -> 2 -> 3 -> null

C 1 -> 2 -> 99 -> 3 -> null

D 99 -> 1 -> 2 -> 3 -> null

💡 Hint

Check the 'List State' column in row for step 4 in execution_table.

At which step do we create the new node?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column in execution_table for when new node is created.

If we want to insert at position 1, which step's action would change?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Refer to the check for position 1 in execution_table step 1.

Concept Snapshot

Insert at Middle Position in Linked List:
- Traverse to (pos-1)th node
- Create new node
- Set new node's next to current's next
- Set current's next to new node
- If pos=1, insert at head
- Adjust pointers carefully to avoid losing list

Full Transcript

This visual execution 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 1 to insert at head. Otherwise, we move to the node before the desired position. Then we create a new node and adjust pointers so the new node links into the list without losing any nodes. The execution table traces each step, showing current node, position counter, actions, and the list state after each change. Variable tracker shows how pointers and counters change. Key moments clarify why we stop at (pos-1)th node and the order of pointer changes. The quiz tests understanding of list state and steps. This method ensures safe insertion at any valid middle position.

Practice

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

easy

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

B. Adding a new node exactly at the given position counting from the head

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

D. Replacing the node at the given position with a new node

Solution

1. Step 1: Understand insertion position

   Inserting at a specific middle position means placing the new node exactly at that position counting from the start (head) of the list.

2. Step 2: Differentiate from other insertions

   Insertion at start or end is different; here we insert in between nodes without replacing any node.

3. Final Answer:

   Adding a new node exactly at the given position counting from the head -> Option B

4. Quick Check:

   Insert at position means place new node there [OK]

Hint: Insert means add new node at exact position, not replace [OK]

Common Mistakes:

• Confusing insertion with replacement
• Thinking insertion only happens at start or end
• Ignoring position counting from head

2. Which of the following C++ code snippets correctly inserts a new node with value 10 at position 3 in a singly linked list?

easy

A. Node* temp = head; for(int i=1; i<2; i++) temp = temp->next; Node* newNode = new Node(10); newNode->next = temp->next; temp->next = newNode;

B. Node* newNode = new Node(10); newNode->next = head; head = newNode;

C. Node* temp = head; for(int i=1; i<3; i++) temp = temp->next; Node* newNode = new Node(10); newNode->next = temp->next; temp->next = newNode;

D. Node* newNode = new Node(10); head->next = newNode; newNode->next = NULL;

Solution

1. Step 1: Identify correct loop for position 3 insertion

   To insert at position 3, we stop at node 2 (position 2), so loop runs i=1 to i<2.

2. Step 2: Check pointer updates

   New node's next points to temp->next, then temp->next updated to new node. This correctly inserts at position 3.

3. Final Answer:

   Code snippet in option A correctly inserts at position 3 -> Option A

4. Quick Check:

   Loop stops at position before insertion [OK]

Hint: Loop till position-1, then insert new node [OK]

Common Mistakes:

• Looping one extra time causing wrong position
• Not updating newNode->next before temp->next
• Inserting at wrong position due to loop error

3. What will be the linked list after inserting value 15 at position 2 in the list: 5 -> 10 -> 20 -> null?

Node* head = new Node(5);
head->next = new Node(10);
head->next->next = new Node(20);

// Insert 15 at position 2
Node* temp = head;
for(int i=1; i<1; i++) temp = temp->next;
Node* newNode = new Node(15);
newNode->next = temp->next;
temp->next = newNode;

medium

A. 5 -> 15 -> 10 -> 20 -> null

B. 15 -> 5 -> 10 -> 20 -> null

C. 5 -> 10 -> 15 -> 20 -> null

D. 5 -> 10 -> 20 -> 15 -> null

Solution

1. Step 1: Trace loop to find insertion point

   Loop runs i=1 to i<1, so no iteration. temp points to head (value 5).

2. Step 2: Insert new node after temp

   New node 15 points to temp->next (10), then temp->next points to new node. List becomes 5 -> 15 -> 10 -> 20 -> null.

3. Final Answer:

   5 -> 15 -> 10 -> 20 -> null -> Option A

4. Quick Check:

   Insert after node 1 gives new node at position 2 [OK]

Hint: Loop zero times means insert after head [OK]

Common Mistakes:

• Miscounting loop iterations
• Inserting at wrong position
• Not updating pointers correctly

4. Identify the error in the following code that tries to insert a node with value 25 at position 4 in a linked list:

Node* temp = head;
for(int i=1; i<=4; i++) {
  temp = temp->next;
}
Node* newNode = new Node(25);
newNode->next = temp->next;
temp->next = newNode;

medium

A. No error, code works fine

B. New node's next pointer is not set correctly

C. New node is not created properly

D. Loop runs too many times causing temp to become NULL

Solution

1. Step 1: Analyze loop iterations

   Loop runs i=1 to i<=4 (4 times), moving temp 4 nodes ahead. For insertion at position 4, temp should stop at position 3.

2. Step 2: Consequence of loop error

   Temp moves too far, possibly NULL, causing runtime error when accessing temp->next.

3. Final Answer:

   Loop runs too many times causing temp to become NULL -> Option D

4. Quick Check:

   Loop must run position-2 times, not position times [OK]

Hint: Loop till position-2 to avoid NULL pointer [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Not checking for NULL before accessing next
• Incorrect loop range for insertion

5. Given a singly linked list with nodes: 1 -> 3 -> 5 -> 7 -> null, write the steps to insert a new node with value 4 at position 3. What will be the final list?

hard

A. 1 -> 4 -> 3 -> 5 -> 7 -> null

B. 4 -> 1 -> 3 -> 5 -> 7 -> null

C. 1 -> 3 -> 4 -> 5 -> 7 -> null

D. 1 -> 3 -> 5 -> 7 -> 4 -> null

Solution

1. Step 1: Traverse to node before position 3

   Start at head (1), move to next node (3) which is position 2. Stop here to insert after this node.

2. Step 2: Insert new node with value 4

   Create new node 4, set its next to current node's next (5), then update current node's next to new node.

3. Final Answer:

   1 -> 3 -> 4 -> 5 -> 7 -> null -> Option C

4. Quick Check:

   Insert at position 3 places new node between 3 and 5 [OK]

Hint: Stop at position-1 node, insert new node after it [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating pointers correctly
• Confusing position counting from 0 or 1