Delete from Beginning of Doubly Linked List in DSA C++ - Build from Scratch

Delete from Beginning of Doubly Linked List

📖 Scenario: You are managing a playlist of songs using a doubly linked list. Each song is a node with a title. You want to remove the first song from the playlist when you no longer want to listen to it.

🎯 Goal: Build a doubly linked list with three songs, then delete the first song from the list, and finally print the updated playlist from start to end.

📋 What You'll Learn

Create a doubly linked list node structure with string title, Node* next, and Node* prev.

Create a doubly linked list with exactly three nodes with titles: "Song1", "Song2", and "Song3" in this order.

Write a function deleteFromBeginning that deletes the first node of the doubly linked list.

Print the titles of the songs in the list from beginning to end after deletion.

💡 Why This Matters

🌍 Real World

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

💼 Career

Understanding linked list operations like deletion is fundamental for software engineering roles involving data structures and memory management.

1

Create the doubly linked list with three songs

Create a struct called Node with string title, Node* next, and Node* prev. Then create three nodes with titles "Song1", "Song2", and "Song3" linked in order. Store the head of the list in a variable called head.

DSA C++

// Define Node struct and create three linked nodes with titles "Song1", "Song2", "Song3"
// Link them in order and assign head
// Your code here

Hint

Remember to set both next and prev pointers for each node to link them properly.

2

Add a function to delete the first node

Add a function called deleteFromBeginning that takes a reference to Node*& head and deletes the first node of the doubly linked list, updating head to the next node and fixing pointers.

DSA C++

#include 
#include 
using namespace std;

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

// Write deleteFromBeginning function here
// Your code here

int main() {
    Node* head = new Node{"Song1", nullptr, nullptr};
    Node* second = new Node{"Song2", nullptr, head};
    Node* third = new Node{"Song3", nullptr, second};
    head->next = second;
    second->next = third;

    return 0;
}

Hint

Remember to check if the list is empty before deleting. Update head and fix the prev pointer of the new head.

3

Call the delete function to remove the first song

Call the function deleteFromBeginning(head) in main to delete the first node from the doubly linked list.

DSA C++

#include 
#include 
using namespace std;

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

void deleteFromBeginning(Node*& head) {
    if (head == nullptr) return;
    Node* temp = head;
    head = head->next;
    if (head != nullptr) {
        head->prev = nullptr;
    }
    delete temp;
}

int main() {
    Node* head = new Node{"Song1", nullptr, nullptr};
    Node* second = new Node{"Song2", nullptr, head};
    Node* third = new Node{"Song3", nullptr, second};
    head->next = second;
    second->next = third;

    // Call deleteFromBeginning here
    // Your code here

    return 0;
}

Hint

Simply call deleteFromBeginning(head); inside main.

4

Print the updated playlist from beginning to end

Use a for loop with a variable current starting at head to traverse the doubly linked list. Print each node's title followed by " -> ". After the loop, print "null".

DSA C++

#include 
#include 
using namespace std;

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

void deleteFromBeginning(Node*& head) {
    if (head == nullptr) return;
    Node* temp = head;
    head = head->next;
    if (head != nullptr) {
        head->prev = nullptr;
    }
    delete temp;
}

int main() {
    Node* head = new Node{"Song1", nullptr, nullptr};
    Node* second = new Node{"Song2", nullptr, head};
    Node* third = new Node{"Song3", nullptr, second};
    head->next = second;
    second->next = third;

    deleteFromBeginning(head);

    for (Node* current = head; current != nullptr; current = current->next) {
        // Print title and arrow
        // Your code here
    }
    cout << "null" << endl;

    return 0;
}

Hint

Use cout << current->title << " -> " inside the loop and print null after the loop.

Practice

1. What happens to the head pointer when you delete the first node from a doubly linked list?

easy

A. It points to the last node

B. It becomes null immediately

C. It moves to the second node in the list

D. It remains pointing to the deleted node

Solution

1. Step 1: Understand the head pointer role

   The head points to the first node in the doubly linked list.

2. Step 2: After deletion, update the head pointer

   When the first node is deleted, the head must point to the next node, which becomes the new first node.

3. Final Answer:

   It moves to the second node in the list -> Option C

4. Quick Check:

   Head points to new first node after deletion [OK]

Hint: Head always moves to next node after deleting first [OK]

Common Mistakes:

• Not updating head pointer after deletion
• Setting head to null without checking list size
• Leaving head pointing to deleted node

2. Which of the following is the correct syntax to delete the first node in a doubly linked list in C++?

easy

A. head = head->next; delete head->prev; head->prev = nullptr;

B. Node* temp = head; head = head->next; head->prev = nullptr; delete temp;

C. head->prev = nullptr; delete head; head = head->next;

D. delete head; head = head->next; head->prev = nullptr;

Solution

1. Step 1: Store current head in temp

   We save the current first node in a temporary pointer to delete it safely after updating head.

2. Step 2: Update head and fix previous pointer

   Set head to the next node, then set head->prev to nullptr to detach the old first node.

3. Step 3: Delete the old first node

   Finally, delete the node stored in temp to free memory.

4. Final Answer:

   Node* temp = head; head = head->next; head->prev = nullptr; delete temp; -> Option B

5. Quick Check:

   Proper order: save, update, fix pointers, delete [OK]

Hint: Save head first, update pointers, then delete saved node [OK]

Common Mistakes:

• Deleting head before updating pointers
• Not setting head->prev to nullptr
• Accessing head->prev when head is nullptr

3. What will be the output of the following code snippet after deleting the first node?

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

Node* head = new Node{1, nullptr, nullptr};
Node* second = new Node{2, head, nullptr};
head->next = second;

// Delete from beginning
Node* temp = head;
head = head->next;
head->prev = nullptr;
delete temp;

// Print list
Node* curr = head;
while(curr != nullptr) {
  std::cout << curr->data << " ";
  curr = curr->next;
}

medium

A. 1 2

B. No output (runtime error)

C. 1

D. 2

Solution

1. Step 1: Initial list setup

   List has two nodes: 1 (head) -> 2. Head points to node with data 1.

2. Step 2: Delete first node and update pointers

   Head moves to second node (data 2), and its prev is set to nullptr. The first node is deleted.

3. Step 3: Print remaining list

   Only node with data 2 remains, so output is "2".

4. Final Answer:

   2 -> Option D

5. Quick Check:

   After deletion, list starts at 2 [OK]

Hint: After deleting head, print starts from new head [OK]

Common Mistakes:

• Printing deleted node's data
• Not updating head->prev to nullptr
• Assuming both nodes print after deletion

4. Identify the error in the following code that deletes the first node of a doubly linked list:

void deleteBeginning(Node*& head) {
  if (head == nullptr) return;
  delete head;
  head = head->next;
  if (head != nullptr) head->prev = nullptr;
}

medium

A. Deleting head before updating head pointer causes undefined behavior

B. Missing check if head->next is nullptr

C. head->prev should be set to head->next

D. No error, code works fine

Solution

1. Step 1: Analyze deletion order

   The code deletes head first, then tries to access head->next, which is invalid after deletion.

2. Step 2: Correct order to avoid undefined behavior

   First save head->next in a temp pointer, update head, then delete old node.

3. Final Answer:

   Deleting head before updating head pointer causes undefined behavior -> Option A

4. Quick Check:

   Accessing deleted memory causes error [OK]

Hint: Always update pointers before deleting nodes [OK]

Common Mistakes:

• Deleting node before updating pointers
• Accessing deleted node's members
• Not checking for empty list

5. You have a doubly linked list with nodes: 10 <-> 20 <-> 30 <-> 40. After deleting the first node twice from the beginning, what will be the printed list?

hard

A. 30 <-> 40

B. 20 <-> 30 <-> 40

C. 10 <-> 20 <-> 30 <-> 40

D. 40

Solution

1. Step 1: Initial list nodes

   List is 10 <-> 20 <-> 30 <-> 40, head points to 10.

2. Step 2: First deletion

   Delete node 10, head moves to 20, set head->prev = nullptr. List now: 20 <-> 30 <-> 40.

3. Step 3: Second deletion

   Delete node 20, head moves to 30, set head->prev = nullptr. List now: 30 <-> 40.

4. Final Answer:

   30 <-> 40 -> Option A

5. Quick Check:

   Two deletions remove first two nodes [OK]

Hint: Each deletion moves head to next node [OK]

Common Mistakes:

• Deleting only once instead of twice
• Not updating head->prev after second deletion
• Printing old list without deletions