Insert at Middle Specific Position in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Insert at Middle Specific Position' mean in a linked list?

It means adding a new node at a given position inside the list, not at the start or end, but somewhere in the middle.

beginner

Why do we need to traverse the list before inserting at a middle position?

Because we must find the node just before the position where we want to insert, so we can link the new node correctly.

beginner

What happens if the position to insert is 1 in a linked list?

Inserting at position 1 means adding the new node at the start (head) of the list.

beginner

In JavaScript, how do you create a new node for a singly linked list?

You create an object with two properties: 'data' to hold the value and 'next' to point to the next node (or null if none).

intermediate

What is the time complexity of inserting a node at a specific middle position in a singly linked list?

It is O(n) because you may need to traverse up to n nodes to reach the position before insertion.

What is the first step to insert a node at position 3 in a singly linked list?

A Traverse to the node at position 2

B Create a new node and set it as head

C Delete the node at position 3

D Traverse to the node at position 4

To insert at position 3, you must reach the node at position 2 to link the new node after it.

If you want to insert at the start of a linked list, which position do you use?

A Middle position

B 0

C Length of list

D 1

Position 1 means the start (head) of the list in 1-based indexing.

What should the 'next' pointer of the new node point to when inserting in the middle?

A The node currently at the insertion position

B The head node

C null

D The last node

The new node's 'next' should point to the node that was previously at the insertion position.

What happens if you try to insert at a position greater than the list length + 1?

A Insert at the end

B Operation fails or no insertion

C Insert in the middle anyway

D Insert at the start

Positions beyond list length + 1 are invalid, so insertion should fail or be ignored.

What is the time complexity to insert at the middle of a singly linked list?

A O(log n)

B O(1)

C O(n)

D O(n^2)

You must traverse the list to the insertion point, which takes O(n) time.

Explain step-by-step how to insert a new node at position 4 in a singly linked list.

Think about how links between nodes change.

Describe how to handle edge cases when inserting at the start or beyond the end of the list.

Consider list length and valid positions.

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