Insert at Middle Specific Position in DSA Javascript - 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 Javascript

function insertAtPosition(head, data, position) {
  let newNode = { value: data, next: null };
  if (position === 0) {
    newNode.next = head;
    return newNode;
  }
  let current = head;
  let count = 0;
  while (current !== null && count < position - 1) {
    current = current.[1];
    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'

Ahead

Bprev

Cvalue

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which does not exist in singly linked lists.

Trying to access 'value' to move forward.

Using 'head' inside the loop incorrectly.

To move forward in a singly linked list, we use the 'next' pointer of the current node.

2fill in blank

medium

Complete the code to correctly insert a new node at the middle position of the linked list.

DSA Javascript

function insertMiddle(head, data) {
  if (head === null) return { value: data, next: null };
  let slow = head;
  let fast = head.next;
  while (fast !== null && fast.next !== null) {
    slow = slow.[1];
    fast = fast.next.next;
  }
  let newNode = { value: data, next: slow.next };
  slow.next = newNode;
  return head;
}

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

Anext

Bprev

Cvalue

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which is not defined in singly linked lists.

Trying to move slow pointer using 'value'.

Using 'head' instead of 'next' to move forward.

To move the slow pointer forward, we use the 'next' property of the current node.

3fill in blank

hard

Fix the error in the code to insert a node at a specific position in the linked list.

DSA Javascript

function insertAtPos(head, data, pos) {
  let newNode = { value: data, next: null };
  if (pos === 0) {
    newNode.next = head;
    return newNode;
  }
  let current = head;
  let count = 0;
  while (current !== null && count < pos - 1) {
    current = current.next;
    count++;
  }
  if (current === null) return head;
  newNode.next = current.[1];
  current.next = newNode;
  return head;
}

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

Avalue

Bprev

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which does not exist.

Using 'value' instead of 'next'.

Using 'head' incorrectly.

To insert the new node, we set its 'next' to the current node's 'next'.

4fill in blank

hard

Fill both blanks to create a function that inserts a node at a given position in a linked list.

DSA Javascript

function insertNode(head, data, pos) {
  let newNode = { value: data, next: null };
  if (pos === 0) {
    newNode.next = head;
    return newNode;
  }
  let current = head;
  let count = 0;
  while (current !== null && count < pos - 1) {
    current = current.[1];
    count[2];
  }
  if (current === null) return head;
  newNode.next = current.next;
  current.next = newNode;
  return head;
}

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

Anext

Bprev

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which is not defined.

Using '--' which decreases count incorrectly.

To move forward, use 'next'. To increase count, use '++'.

5fill in blank

hard

Fill all three blanks to insert a node at a specific position in a linked list.

DSA Javascript

function insertAt(head, data, pos) {
  let newNode = { value: data, next: null };
  if (pos === 0) {
    newNode.next = head;
    return newNode;
  }
  let current = head;
  let count = 0;
  while (current !== null && count < pos - 1) {
    current = current.[1];
    count[2];
  }
  if (current === null) return head;
  newNode.next = current.[3];
  current.next = newNode;
  return head;
}

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

Anext

B++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--' to decrease count.

Using 'prev' which does not exist.

Setting newNode.next incorrectly.

Move current with 'next', increment count with '++', and set newNode.next to current.next.

Practice

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

easy

A. Removing a node from the middle of the list

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

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

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

Solution

1. Step 1: Understand insertion position

   Inserting at a specific middle position means placing the new node exactly at that index, shifting others if needed.

2. Step 2: Differentiate from other operations

   Insertion at start or end is different; here we insert inside the list at the given position.

3. Final Answer:

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

4. Quick Check:

   Insert at position = Add node there [OK]

Hint: Insert means add new node at exact position [OK]

Common Mistakes:

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

2. Which of the following is the correct syntax to insert a node at position 3 in a singly linked list in JavaScript?

easy

A. list.insert(newNode, 3);

B. list.addNode(3, newNode);

C. list.pushAt(3, newNode);

D. list.insertAt(3, newNode);

Solution

1. Step 1: Identify common method naming

   Common method to insert at a position is named insertAt(position, node).

2. Step 2: Match parameters order

   Position comes first, then the node to insert, matching list.insertAt(3, newNode).

3. Final Answer:

   list.insertAt(3, newNode); -> Option D

4. Quick Check:

   insertAt(position, node) syntax [OK]

Hint: insertAt(position, node) is standard [OK]

Common Mistakes:

• Swapping parameter order
• Using wrong method names
• Confusing insertAt with push or addNode

3. What will be the linked list after inserting value 99 at position 2 in the list: 10 -> 20 -> 30 -> null?

function insertAtPosition(head, value, position) {
  let newNode = { value: value, next: null };
  if (position === 1) {
    newNode.next = head;
    return newNode;
  }
  let current = head;
  let count = 1;
  while (count < position - 1 && current != null) {
    current = current.next;
    count++;
  }
  if (current == null) return head;
  newNode.next = current.next;
  current.next = newNode;
  return head;
}

let head = { value: 10, next: { value: 20, next: { value: 30, next: null } } };
head = insertAtPosition(head, 99, 2);

// Print list
let result = [];
let temp = head;
while(temp) {
  result.push(temp.value);
  temp = temp.next;
}
console.log(result);

medium

A. [99, 10, 20, 30]

B. [10, 99, 20, 30]

C. [10, 20, 99, 30]

D. [10, 20, 30, 99]

Solution

1. Step 1: Locate position 2 in the list

   Position 2 means after first node (10), before second node (20).

2. Step 2: Insert new node with value 99 at position 2

   New node points to 20, first node points to new node, resulting list: 10 -> 99 -> 20 -> 30.

3. Final Answer:

   [10, 99, 20, 30] -> Option B

4. Quick Check:

   Insert 99 at pos 2 shifts 20 right [OK]

Hint: Insert shifts nodes right at position [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating next pointers correctly
• Confusing position counting starting at 0

4. The following code tries to insert a node at position 4 in a list with 3 nodes. What is the error?

function insertAtPosition(head, value, position) {
  let newNode = { value: value, next: null };
  if (position === 1) {
    newNode.next = head;
    return newNode;
  }
  let current = head;
  let count = 1;
  while (count < position - 1) {
    current = current.next;
    count++;
  }
  newNode.next = current.next;
  current.next = newNode;
  return head;
}

medium

A. The while loop can cause an error if position is greater than list length + 1

B. The newNode is not created properly

C. The function does not return the head

D. The position check for 1 is incorrect

Solution

1. Step 1: Analyze while loop condition

   The loop runs without checking if current is null, so if position > list length + 1, current becomes null and next access fails.

2. Step 2: Identify missing boundary check

   There is no check to stop loop if current is null, causing runtime error when accessing current.next.

3. Final Answer:

   The while loop can cause an error if position is greater than list length + 1 -> Option A

4. Quick Check:

   Loop needs null check to avoid error [OK]

Hint: Always check current != null in loops [OK]

Common Mistakes:

• Ignoring null checks in loops
• Assuming position is always valid
• Not returning head after insertion

5. You have a singly linked list representing a queue: 5 -> 10 -> 15 -> 20 -> null. You want to insert a new value 12 exactly in the middle position (between 10 and 15). Which code snippet correctly inserts 12 at position 3?

function insertAtPosition(head, value, position) {
  let newNode = { value: value, next: null };
  if (position === 1) {
    newNode.next = head;
    return newNode;
  }
  let current = head;
  let count = 1;
  while (count < position - 1 && current != null) {
    current = current.next;
    count++;
  }
  if (current == null) return head;
  newNode.next = current.next;
  current.next = newNode;
  return head;
}

let head = { value: 5, next: { value: 10, next: { value: 15, next: { value: 20, next: null } } } };
head = insertAtPosition(head, 12, 3);

// Print list
let result = [];
let temp = head;
while(temp) {
  result.push(temp.value);
  temp = temp.next;
}
console.log(result);

What is the output?

hard

A. [5, 10, 12, 15, 20]

B. [5, 10, 15, 12, 20]

C. [12, 5, 10, 15, 20]

D. [5, 12, 10, 15, 20]

Solution

1. Step 1: Identify position 3 in the list

   Position 3 means after nodes 5 (pos 1) and 10 (pos 2), so between 10 and 15.

2. Step 2: Insert new node with value 12 at position 3

   New node points to 15, node 10 points to new node, resulting list: 5 -> 10 -> 12 -> 15 -> 20.

3. Final Answer:

   [5, 10, 12, 15, 20] -> Option A

4. Quick Check:

   Insert 12 at pos 3 shifts 15 right [OK]

Hint: Insert at position shifts next nodes right [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating next pointers correctly
• Confusing position indexing starting at 0