Insert at Specific Position in Doubly Linked List in DSA C++ - Build from Scratch

Insert at Specific Position in Doubly Linked List

📖 Scenario: You are managing a playlist of songs. Each song is linked to the previous and next song, allowing you to move forward or backward easily. You want to add a new song at a specific position in the playlist.

🎯 Goal: Build a doubly linked list of songs and insert a new song at a given position.

📋 What You'll Learn

Create a doubly linked list with three songs: "Song1", "Song2", "Song3"

Add a variable to hold the new song name "NewSong" and the position 2

Write the logic to insert "NewSong" at position 2 in the doubly linked list

Print the playlist from head to tail showing the song names separated by " <-> "

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, browsers, and other apps where you need to move forward and backward through items easily.

💼 Career

Understanding linked lists and pointer manipulation is fundamental for software engineering roles involving data structures and memory management.

1

Create the initial doubly linked list

Create a doubly linked list with nodes containing the song names "Song1", "Song2", and "Song3" in this order. Use a struct called Node with string data, Node* prev, and Node* next. Create pointers head and tail to represent the start and end of the list.

DSA C++

// Define struct Node and create doubly linked list with Song1, Song2, Song3
// Your code here

Hint

Start by defining the Node struct with the required members. Then create three nodes for the songs and link them properly.

2

Add new song name and position variables

Add a variable called newSong and set it to "NewSong". Also add an integer variable called position and set it to 2 to represent where the new song will be inserted.

DSA C++

#include 
#include 
using namespace std;

struct Node {
    string data;
    Node* prev;
    Node* next;
    Node(string val) : data(val), prev(nullptr), next(nullptr) {}
};

int main() {
    Node* head = new Node("Song1");
    Node* second = new Node("Song2");
    Node* third = new Node("Song3");

    head->next = second;
    second->prev = head;
    second->next = third;
    third->prev = second;

    Node* tail = third;

    // Add newSong and position variables here
    // Your code here

    return 0;
}

Hint

Declare newSong as a string and position as an int with the given values.

3

Insert the new song at the given position

Write the logic to insert a new node with data newSong at the position in the doubly linked list. Assume positions start at 1 for the head. Update the links of the surrounding nodes accordingly.

DSA C++

    Node* newNode = new Node(newSong);
    if (position == 1) {
        newNode->next = head;
        head->prev = newNode;
        head = newNode;
    } else {
        Node* current = head;
        int count = 1;
        while (count next != nullptr) {
            current = current->next;
            count++;
        }
        newNode->next = current->next;
        newNode->prev = current;
        if (current->next != nullptr) {
            current->next->prev = newNode;
        } else {
            tail = newNode;
        }
        current->next = newNode;
    }
    // Print the list from head to tail

Hint

Traverse to the node before the insertion position, then adjust the prev and next pointers to insert the new node.

4

Print the doubly linked list from head to tail

Print the song names in the doubly linked list from head to tail. Separate each song name with " <-> ". End with a newline.

DSA C++

Hint

Use a loop starting from head to print each node's data. Print " <-> " between songs but not after the last one.

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