Insert at Middle Specific Position in DSA Typescript - Execution Trace

Concept Flow - Insert at Middle Specific Position

Start with head node

↓

Check if position is 0

|Yes↓

Insert at head

No↓

Traverse list to position-1

↓

Create new node

↓

Link new node to next

↓

Link previous node to new node

Start from the head, if position is zero insert at head. Otherwise, move to the node before the position, insert new node by adjusting links.

Execution Sample

DSA Typescript

class Node {
  constructor(public val: number, public next: Node | null = null) {}
}

function insertAtPosition(head: Node | null, val: number, pos: number): Node {
  if (pos === 0) {
    return new Node(val, head);
  }
  let current = head;
  let count = 0;
  while (current !== null && count < pos - 1) {
    current = current.next;
    count++;
  }
  if (current === null) {
    // Position is out of bounds, return head unchanged
    return head;
  }
  const newNode = new Node(val, current.next);
  current.next = newNode;
  return head;
}

Insert a new node with value val at position pos in the linked list starting at head.

Execution Table

Step	Current Node Value	Position Counter	Condition	Action	List State
1	Head (1)	0	pos=2? No	Move to next node	1 -> 2 -> 3 -> null
2	Node (2)	1	pos=2? No	Move to next node	1 -> 2 -> 3 -> null
3	Node (3)	2	pos=2? Yes	Insert new node after previous	1 -> 2 -> 99 -> 3 -> null
4	End	-	-	Return updated head	1 -> 2 -> 99 -> 3 -> null

💡 Reached position 2, inserted new node, finished insertion.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
current	Node(1)	Node(2)	Node(3)	Node(3)	Node(3)
posCounter	0	1	2	2	2
head	Node(1)	Node(1)	Node(1)	Node(1)	Node(1)

Key Moments - 2 Insights

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

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

What happens if position is 0?

We insert the new node at the head by making it point to the current head and then update head to this new node (not shown in this trace but important to handle).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of posCounter at step 2?

A 0

B 2

C 1

D 3

💡 Hint

Check the 'Position Counter' column in execution_table row for step 2.

At which step is the new node inserted?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Action' column describing insertion in execution_table.

If we change pos to 0, what would be the first action?

A Traverse to next node

B Insert at head

C Insert after node 1

D Return null

💡 Hint

Refer to concept_flow where position 0 means insert at head.

Concept Snapshot

Insert at Middle Specific Position:
- If pos=0, insert new node at head.
- Else, traverse to node at pos-1.
- Create new node, link it to next node.
- Link previous node to new node.
- Return updated head.

Full Transcript

To insert a node at a specific middle position in a linked list, start from the head. If the position is zero, insert the new node at the head by pointing it to the current head and updating head. Otherwise, move through the list until you reach the node before the desired position. Then create the new node, link it to the next node, and update the previous node's next pointer to the new node. This process ensures the new node is correctly inserted at the specified position. The execution table shows step-by-step traversal and insertion, tracking the current node and position counter. Key moments include understanding why traversal stops at position-1 and handling insertion at position zero.

Practice

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

easy

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

B. Removing a node from the middle of the list

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

D. Adding a new node exactly at the given position counting from the start

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 of the list.

2. Step 2: Differentiate from other insertions

   Insertion at beginning or end is different; here we insert inside the list at a given index.

3. Final Answer:

   Adding a new node exactly at the given position counting from the start -> Option D

4. Quick Check:

   Insert at position = Add node at that index [OK]

Hint: Insert means add node exactly at given index [OK]

Common Mistakes:

• Confusing insertion with deletion
• Thinking insertion only happens at start or end
• Ignoring the position index

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

easy

A. function insertAtPosition(head, val, pos) { let newNode = {value: val, next: null}; let current = head; for(let i=1; i

B. function insertAtPosition(head, val, pos) { let newNode = {value: val, next: null}; let current = head; for(let i=0; i

C. function insertAtPosition(head, val, pos) { let newNode = {value: val, next: null}; newNode.next = head; head = newNode; return head; }

D. function insertAtPosition(head, val, pos) { let newNode = {value: val, next: null}; head.next = newNode; return head; }

Solution

1. Step 1: Analyze loop for position

   The loop runs from 1 to pos-2 to reach node before insertion point (pos-1).

2. Step 2: Insert new node correctly

   New node's next points to current.next, then current.next updated to new node.

3. Final Answer:

   Code in option A correctly inserts at position 2 -> Option A

4. Quick Check:

   Loop to pos-1, link new node [OK]

Hint: Loop to node before position, then insert [OK]

Common Mistakes:

• Looping too far or too short
• Not updating newNode.next correctly
• Inserting at wrong position index

3. What will be the linked list after inserting value 99 at position 3 in the list 1 -> 2 -> 3 -> 4 -> null using the following TypeScript code?

function insertAtPosition(head, val, pos) {
  let newNode = {value: val, next: null};
  if (pos === 1) {
    newNode.next = head;
    return newNode;
  }
  let current = head;
  for (let i = 1; i < pos - 1; i++) {
    current = current.next;
  }
  newNode.next = current.next;
  current.next = newNode;
  return head;
}

// Initial list: 1 -> 2 -> 3 -> 4 -> null
// Insert 99 at position 3
let head = {value:1, next:{value:2, next:{value:3, next:{value:4, next:null}}}};
head = insertAtPosition(head, 99, 3);

medium

A. 1 -> 2 -> 3 -> 4 -> 99 -> null

B. 1 -> 2 -> 99 -> 3 -> 4 -> null

C. 99 -> 1 -> 2 -> 3 -> 4 -> null

D. 1 -> 99 -> 2 -> 3 -> 4 -> null

Solution

1. Step 1: Traverse to node before position 3

   Loop runs i=1 to pos-2=1, so current moves to node with value 2.

2. Step 2: Insert new node after current

   New node 99 points to current.next (node 3), current.next updated to new node.

3. Final Answer:

   1 -> 2 -> 99 -> 3 -> 4 -> null -> Option B

4. Quick Check:

   Insert at pos 3 places 99 after 2 [OK]

Hint: Insert after node at pos-1 [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating links correctly
• Confusing position indexing

4. Identify the error in the following TypeScript function that tries to insert a node at position 2 in a linked list.

function insertAtPosition(head, val, pos) {
  let newNode = {value: val, next: null};
  let current = head;
  for(let i = 0; i < pos - 1; i++) {
    current = current.next;
  }
  newNode.next = current.next;
  current.next = newNode;
  return head;
}

medium

A. The loop runs one step too many, causing current to be null before insertion

B. The newNode.next is not assigned correctly

C. The function does not handle insertion at position 1

D. The function returns newNode instead of head

Solution

1. Step 1: Analyze loop range

   The loop runs from i=0 to pos-2, moving current pos-1 times, which moves current too far.

2. Step 2: Consequence of loop

   Current becomes the node at position pos, so current.next is undefined, causing error on insertion.

3. Final Answer:

   The loop runs one step too many, causing current to be null before insertion -> Option A

4. Quick Check:

   Loop overshoot breaks insertion [OK]

Hint: Loop to node before position, not at position [OK]

Common Mistakes:

• Looping to position instead of position-1
• Not checking for position 1 insertion
• Incorrectly updating links

5. Given a singly linked list and a position that may be greater than the list length, how should you modify the insertAtPosition function to insert the new node at the end if the position is beyond the list length? Choose the correct approach.

hard

A. Insert the new node at the head regardless of position

B. Throw an error if position is greater than list length

C. Traverse until current.next is null or position reached, then insert new node at current.next

D. Ignore the insertion if position is invalid

Solution

1. Step 1: Traverse safely

   Move current node forward until either position-1 steps or current.next is null (end of list).

2. Step 2: Insert at end if position too large

   If position is beyond length, current.next will be null, so insert new node at end by setting current.next to new node.

3. Final Answer:

   Traverse until current.next is null or position reached, then insert new node at current.next -> Option C

4. Quick Check:

   Insert at end if position too large [OK]

Hint: Stop at end or position, then insert [OK]

Common Mistakes:

• Not handling position beyond length
• Throwing error unnecessarily
• Inserting at wrong place when position invalid