Insert at Middle Specific Position in DSA Typescript

Mental Model

We add a new item inside a linked list at a certain spot by moving step-by-step to that spot and linking the new item there.

Analogy: Imagine a line of people holding hands. To add a new friend in the middle, you find the right spot, then everyone nearby holds hands with the new friend instead of directly with each other.

head -> 1 -> 2 -> 3 -> 4 -> null
           ↑
         position 2

Dry Run Walkthrough

Input: list: 1 -> 2 -> 4 -> 5, insert value 3 at position 3

Goal: Insert the value 3 between nodes 2 and 4, so the list becomes 1 -> 2 -> 3 -> 4 -> 5

Step 1: start at head node 1

head -> [curr->1] -> 2 -> 4 -> 5 -> null

Why: We begin from the start to find the correct position

Step 2: move curr to node 2 (position 2)

head -> 1 -> [curr->2] -> 4 -> 5 -> null

Why: We move forward to one before the insertion point

Step 3: create new node 3 and link it after curr

head -> 1 -> 2 -> [new->3] -> 4 -> 5 -> null

Why: Insert new node by adjusting pointers to include it in the list

Step 4: link node 3's next to node 4

head -> 1 -> 2 -> 3 -> 4 -> 5 -> null

Why: Complete insertion by connecting new node to the rest of the list

Result:

head -> 1 -> 2 -> 3 -> 4 -> 5 -> null

Annotated Code

DSA Typescript

class Node {
  value: number;
  next: Node | null;
  constructor(value: number) {
    this.value = value;
    this.next = null;
  }
}

class LinkedList {
  head: Node | null;
  constructor() {
    this.head = null;
  }

  insertAtPosition(value: number, position: number): void {
    const newNode = new Node(value);
    if (position === 1) {
      newNode.next = this.head;
      this.head = newNode;
      return;
    }

    let curr = this.head;
    let count = 1;
    while (curr !== null && count < position - 1) {
      curr = curr.next;
      count++;
    }

    if (curr === null) {
      // position is beyond list length, ignore insertion
      return;
    }

    newNode.next = curr.next; // link new node to next node
    curr.next = newNode;      // link current node to new node
  }

  printList(): void {
    let curr = this.head;
    let result = '';
    while (curr !== null) {
      result += curr.value + ' -> ';
      curr = curr.next;
    }
    result += 'null';
    console.log(result);
  }
}

// Driver code
const list = new LinkedList();
list.head = new Node(1);
list.head.next = new Node(2);
list.head.next.next = new Node(4);
list.head.next.next.next = new Node(5);

list.insertAtPosition(3, 3);
list.printList();

if (position === 1) { newNode.next = this.head; this.head = newNode; return; }

handle insertion at the start by updating head pointer

while (curr !== null && count < position - 1) { curr = curr.next; count++; }

move curr to node before insertion position

if (curr === null) { return; }

stop if position is beyond list length

newNode.next = curr.next; curr.next = newNode;

insert new node by linking pointers

Output

1 -> 2 -> 3 -> 4 -> 5 -> null

Complexity Analysis

Time: O(n) because we may need to move through up to n nodes to reach the insertion point

Space: O(1) because we only create one new node and use a few pointers

vs Alternative: Compared to inserting at the start or end which can be O(1), inserting in the middle requires traversal making it O(n)

Edge Cases

inserting at position 1 (start)

new node becomes the new head

DSA Typescript

if (position === 1) { newNode.next = this.head; this.head = newNode; return; }

inserting at position beyond list length

insertion is ignored to avoid errors

DSA Typescript

if (curr === null) { return; }

empty list insertion at position 1

new node becomes the head of the empty list

DSA Typescript

if (position === 1) { newNode.next = this.head; this.head = newNode; return; }

When to Use This Pattern

When you need to add an item at a specific spot inside a linked list, use the insert at position pattern to carefully link the new node without breaking the list.

Common Mistakes

Mistake: Not moving to the node before the insertion position, causing pointer errors
Fix: Traverse until position - 1 to correctly link the new node

Mistake: Not handling insertion at position 1 separately, causing head pointer errors
Fix: Add a special case for position 1 to update the head pointer

Summary

Inserts a new node at a given position inside a linked list by adjusting pointers.

Use when you want to add an element anywhere except just the start or end of the list.

The key is to stop at the node before the insertion point and link the new node in between.

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