Insert at Middle Specific Position in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a new node at the given position in a singly linked list.

DSA Typescript

function insertAtPosition(head: ListNode | null, val: number, pos: number): ListNode | null {
  const newNode = new ListNode(val);
  if (pos === 0) {
    newNode.next = head;
    return [1];
  }
  let current = head;
  let count = 0;
  while (current !== null && count < pos - 1) {
    current = current.next;
    count++;
  }
  if (current === null) return head;
  newNode.next = current.next;
  current.next = newNode;
  return head;
}

Drag options to blanks, or click blank then click option'

AnewNode

Bhead

Ccurrent

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the old head instead of the new node when inserting at position 0.

Not updating the new node's next pointer.

When inserting at position 0, the new node becomes the new head, so we return newNode.

2fill in blank

medium

Complete the code to traverse the list until the node before the insertion position.

DSA Typescript

let current = head;
let count = 0;
while (current !== null && [1] < pos - 1) {
  current = current.next;
  count++;
}

Drag options to blanks, or click blank then click option'

Acurrent

Bpos

Ccount

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'current' instead of 'count' in the loop condition.

Using 'pos' directly in the loop condition.

We use 'count' to track how many nodes we have traversed until reaching the node before the insertion position.

3fill in blank

hard

Fix the error in the code that inserts the new node after the current node.

DSA Typescript

newNode.next = [1].next;
[2].next = newNode;

Drag options to blanks, or click blank then click option'

Acurrent

Bhead

CnewNode

Dpos

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'head' or 'newNode' instead of 'current' in these assignments.

Not updating both pointers correctly.

We must use 'current' to access the next node and update its next pointer to the new node.

4fill in blank

hard

Fill both blanks to handle the case when the insertion position is beyond the end of the list (do not insert).

DSA Typescript

if (current === null) {
  [1] = newNode;
  return [2];
}

Drag options to blanks, or click blank then click option'

Ahead

BnewNode

Ccurrent

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Returning newNode instead of head.

Assigning newNode to head instead of current.

If current is null, it means the position is beyond the list length. Assigning newNode to current has no effect on the list (local variable), and we return head unchanged without inserting.

5fill in blank

hard

Fill all three blanks to complete the function that inserts a node at a specific position.

DSA Typescript

function insertAtPosition(head: ListNode | null, val: number, pos: number): ListNode | null {
  const newNode = new ListNode(val);
  if (pos === 0) {
    newNode.next = head;
    return [1];
  }
  let current = head;
  let count = 0;
  while (current !== null && count < [2]) {
    current = current.next;
    count++;
  }
  if (current === null) return head;
  newNode.next = current.[3];
  current.next = newNode;
  return head;
}

Drag options to blanks, or click blank then click option'

AnewNode

Bpos - 1

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Incorrect loop condition.

Not updating newNode.next correctly.

Returning wrong node at position 0.

Return newNode when inserting at position 0; loop until count < pos - 1; set newNode.next to current.next.

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