Why Insert at Specific Position in Doubly Linked List in DSA C++?

The Big Idea

What if you could add anything anywhere in your list instantly, without rewriting everything?

The Scenario

Imagine you have a paper list of your favorite songs. You want to add a new song exactly at position 3, but you only have a pen and no eraser. You have to rewrite the whole list from scratch to insert the new song in the right place.

The Problem

Manually rewriting the list every time you want to add a song in the middle is slow and tiring. You might make mistakes, lose track of the order, or accidentally skip songs. It's hard to keep the list neat and correct.

The Solution

A doubly linked list lets you insert a new item exactly where you want without rewriting everything. You just change a few links before and after the position, and the list updates itself smoothly.

Before vs After

✗ Before

vector<string> songs = {"Song1", "Song2", "Song3", "Song4"};
// To insert at position 3, shift all songs from position 3 onward
songs.resize(5);
for (int i = 4; i > 3; i--) {
  songs[i] = songs[i-1];
}
songs[3] = "NewSong";

✓ After

struct Node {
  string data;
  Node* prev;
  Node* next;
};

void insertAtPosition(Node*& head, string newData, int position) {
  // Create new node and adjust links to insert at position
}

What It Enables

You can quickly and safely add items anywhere in your list without disturbing the rest.

Real Life Example

Adding a new contact in the middle of your phone's contact list without scrolling through all contacts or rewriting the entire list.

Key Takeaways

Manual insertion in the middle is slow and error-prone.

Doubly linked lists let you insert by changing just a few links.

This makes list updates fast, safe, and easy.

Practice

1. What happens when you insert a new node at position 1 in a doubly linked list?

easy

A. The list remains unchanged.

B. The new node becomes the new head of the list.

C. The new node replaces the last node.

D. The new node is added at the end of the list.

Solution

1. Step 1: Understand position 1 in doubly linked list

   Position 1 means the start of the list, so the new node should be the first node.

2. Step 2: Insert new node at head

   The new node's next pointer points to the old head, and the old head's previous pointer points back to the new node.

3. Final Answer:

   The new node becomes the new head of the list. -> Option B

4. Quick Check:

   Insert at position 1 = new head [OK]

Hint: Position 1 means insert at the start [OK]

Common Mistakes:

• Inserting at position 1 but not updating head pointer
• Not updating previous pointer of old head
• Confusing position 1 with end of list

2. Which of the following is the correct syntax to insert a new node at position pos in a doubly linked list in C++?

easy

A. newNode->next = temp->next; temp->next->prev = newNode; temp->next = newNode; newNode->prev = temp;

B. newNode->prev = temp->prev; temp->prev->next = newNode; temp->prev = newNode; newNode->next = temp;

C. newNode->next = temp->prev; temp->prev->prev = newNode; temp->prev = newNode; newNode->prev = temp;

D. newNode->prev = temp->next; temp->next->next = newNode; temp->next = newNode; newNode->next = temp;

Solution

1. Step 1: Identify correct pointer updates for insertion

   When inserting after temp, newNode's next points to temp's next, and newNode's prev points to temp.

2. Step 2: Update surrounding nodes' pointers

   temp's next's prev must point to newNode, and temp's next must point to newNode.

3. Final Answer:

   newNode->next = temp->next; temp->next->prev = newNode; temp->next = newNode; newNode->prev = temp; -> Option A

4. Quick Check:

   Correct pointer updates = newNode->next = temp->next; temp->next->prev = newNode; temp->next = newNode; newNode->prev = temp; [OK]

Hint: Update next and prev pointers carefully [OK]

Common Mistakes:

• Mixing up next and prev pointers
• Not updating temp->next->prev pointer
• Assigning pointers in wrong order causing broken links

3. Given the doubly linked list: 10 <-> 20 <-> 30 <-> null, what will be the list after inserting 15 at position 2?

void insertAtPosition(Node*& head, int data, int pos) {
  Node* newNode = new Node(data);
  if (pos == 1) {
    newNode->next = head;
    if (head) head->prev = newNode;
    head = newNode;
    return;
  }
  Node* temp = head;
  for (int i = 1; i < pos - 1 && temp != nullptr; i++) {
    temp = temp->next;
  }
  if (temp == nullptr) return;
  newNode->next = temp->next;
  if (temp->next) temp->next->prev = newNode;
  temp->next = newNode;
  newNode->prev = temp;
}

medium

A. 10 <-> 15 <-> 20 <-> 30 <-> null

B. 15 <-> 10 <-> 20 <-> 30 <-> null

C. 10 <-> 20 <-> 15 <-> 30 <-> null

D. 10 <-> 20 <-> 30 <-> 15 <-> null

Solution

1. Step 1: Locate position 2 in the list

   Position 2 means after the first node (10). So temp points to node with data 10.

2. Step 2: Insert new node with data 15 after temp

   New node's next points to 20, and prev points to 10. Update 10's next to new node and 20's prev to new node.

3. Final Answer:

   10 <-> 15 <-> 20 <-> 30 <-> null -> Option A

4. Quick Check:

   Insert at position 2 places new node after first node [OK]

Hint: Position 2 means insert after first node [OK]

Common Mistakes:

• Inserting before first node instead of after
• Not updating prev pointer of next node
• Skipping position count in loop

4. Identify the error in the following code snippet for inserting at position pos in a doubly linked list:

void insertAtPosition(Node*& head, int data, int pos) {
  Node* newNode = new Node(data);
  if (pos == 1) {
    newNode->next = head;
    if (head) head->prev = newNode;
    head = newNode;
    return;
  }
  Node* temp = head;
  for (int i = 1; i < pos - 1; i++) {
    temp = temp->next;
  }
  newNode->next = temp->next;
  temp->next = newNode;
  newNode->prev = temp;
  if (newNode->next) newNode->next->prev = newNode;
}

medium

A. The newNode->next pointer is assigned after temp->next is changed.

B. The newNode->prev pointer is not set correctly.

C. The head pointer is not updated when inserting at position 1.

D. The loop does not check if temp is nullptr, causing possible crash.

Solution

1. Step 1: Analyze the for loop traversal

   The loop moves temp forward but does not check if temp becomes nullptr, risking null pointer dereference.

2. Step 2: Identify missing safety check

   Without checking temp != nullptr, accessing temp->next can cause a crash if pos is invalid or list is short.

3. Final Answer:

   The loop does not check if temp is nullptr, causing possible crash. -> Option D

4. Quick Check:

   Always check temp != nullptr in traversal loops [OK]

Hint: Always check for nullptr when traversing [OK]

Common Mistakes:

• Ignoring nullptr checks in loops
• Not updating head for position 1 insert
• Incorrect order of pointer assignments

5. You have a doubly linked list: 5 <-> 10 <-> 15 <-> 20 <-> null. You want to insert a new node with value 12 at position 3. After insertion, what will be the correct sequence of the list? Also, explain how to handle insertion at the last position if the position is greater than the list length.

hard

A. 5 <-> 12 <-> 10 <-> 15 <-> 20 <-> null; Insert at last by replacing last node.

B. 5 <-> 10 <-> 15 <-> 12 <-> 20 <-> null; Insert at last by adding node before head.

C. 5 <-> 10 <-> 12 <-> 15 <-> 20 <-> null; Insert at last by traversing till last node and updating pointers.

D. 5 <-> 10 <-> 15 <-> 20 <-> 12 <-> null; Insert at last by ignoring position.

Solution

1. Step 1: Insert 12 at position 3

   Position 3 means after second node (10). Insert new node between 10 and 15, updating next and prev pointers accordingly.

2. Step 2: Handle insertion at last position if position > length

   Traverse to last node, then insert new node after it, updating last node's next and new node's prev pointers.

3. Final Answer:

   5 <-> 10 <-> 12 <-> 15 <-> 20 <-> null; Insert at last by traversing till last node and updating pointers. -> Option C

4. Quick Check:

   Insert at position 3 places node between 10 and 15; last insert by traversal [OK]

Hint: Insert at position or append if position too large [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating prev pointer of next node
• Ignoring last position insertion logic