Create and Initialize Doubly Linked List in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows moving forward and backward through the list.

beginner

What are the main parts of a node in a doubly linked list?

Each node has: 1) data (the value stored), 2) a pointer to the next node, and 3) a pointer to the previous node.

beginner

How do you initialize an empty doubly linked list in C++?

You create a pointer to the head node and set it to nullptr, meaning the list has no nodes yet.

intermediate

Why do we need both next and previous pointers in a doubly linked list?

Next pointer lets us move forward, previous pointer lets us move backward. This makes it easier to traverse and modify the list from both ends.

beginner

Show the basic C++ struct for a doubly linked list node.

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

What does the 'prev' pointer in a doubly linked list node point to?

A The previous node in the list

B The next node in the list

C The head of the list

D The tail of the list

The 'prev' pointer always points to the previous node, allowing backward traversal.

How do you represent an empty doubly linked list in C++?

A Node* head = nullptr;

B Node head = 0;

C Node* head = new Node();

D Node head = NULL;

Setting the head pointer to nullptr means the list has no nodes.

Which of these is NOT a part of a doubly linked list node?

A Pointer to previous node

B Pointer to next node

C Pointer to random node

D Data

A standard doubly linked list node does not have a pointer to a random node.

What is the main advantage of a doubly linked list over a singly linked list?

A Simpler to implement

B Can traverse both forward and backward

C Uses less memory

D Faster insertion at head only

Doubly linked lists allow traversal in both directions, unlike singly linked lists.

When creating a new node in a doubly linked list, what should its next and prev pointers be set to initially?

A random address

B head

C tail

D nullptr

New nodes start with next and prev pointers set to nullptr until linked.

Explain how to create and initialize an empty doubly linked list in C++.

Think about the node structure and how to represent no nodes.

Describe the role of next and prev pointers in a doubly linked list node.

Imagine walking forward and backward through a chain.

Practice

1. What is the main purpose of a doubly linked list in data structures?

easy

A. To allow traversal in both forward and backward directions

B. To store data in a fixed-size array

C. To sort elements automatically

D. To store data only at the beginning

Solution

1. Step 1: Understand doubly linked list structure

   A doubly linked list has nodes with pointers to both previous and next nodes.

2. Step 2: Identify traversal capability

   This structure allows moving forward and backward through the list easily.

3. Final Answer:

   To allow traversal in both forward and backward directions -> Option A

4. Quick Check:

   Doubly linked list = forward and backward traversal [OK]

Hint: Remember: doubly means two directions [OK]

Common Mistakes:

• Confusing doubly linked list with arrays
• Thinking it sorts data automatically
• Assuming it only allows forward traversal

2. Which of the following is the correct way to declare a node structure for a doubly linked list in C++?

easy

A. struct Node { int data; Node prev; Node next; };

B. struct Node { int data; Node next; Node prev; };

C. struct Node { int data; Node* next; };

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

Solution

1. Step 1: Check pointer usage in node structure

   Each node must have pointers to next and previous nodes, so use Node* for both.

2. Step 2: Verify syntax correctness

   struct Node { int data; Node* next; Node* prev; }; correctly uses pointers (Node*) for next and prev, while others use incorrect types.

3. Final Answer:

   struct Node { int data; Node* next; Node* prev; }; -> Option D

4. Quick Check:

   Node pointers use * for next and prev [OK]

Hint: Use pointers (*) for next and prev nodes [OK]

Common Mistakes:

• Using Node instead of Node* for pointers
• Missing prev pointer in node
• Incorrect order of members

3. What will be the output of the following code snippet that creates a doubly linked list with two nodes and prints their data?

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

int main() {
  Node* head = new Node{10, nullptr, nullptr};
  Node* second = new Node{20, nullptr, head};
  head->next = second;
  std::cout << head->data << " -> " << second->data << " -> null";
  return 0;
}

medium

A. 20 -> 10 -> null

B. 10 -> 20 -> null

C. 10 -> null

D. Compilation error

Solution

1. Step 1: Analyze node creation and linking

   First node has data 10, second node has data 20 and points back to first node. Head's next points to second.

2. Step 2: Trace output statement

   Prints head data (10), then second data (20), then "-> null" string.

3. Final Answer:

   10 -> 20 -> null -> Option B

4. Quick Check:

   Print order matches linked list nodes [OK]

Hint: Follow pointers to print nodes in order [OK]

Common Mistakes:

• Confusing prev and next pointers
• Printing second node first
• Forgetting to set head->next

4. Identify the error in the following code that attempts to create a doubly linked list with one node:

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

int main() {
  Node* head = new Node;
  head->data = 5;
  head->next = head->prev = nullptr;
  head->prev = head;
  return 0;
}

medium

A. head->prev should not point to head itself

B. Missing initialization of head->data

C. head->next should point to head

D. Node structure is incomplete

Solution

1. Step 1: Review pointer assignments

   head->next and head->prev are set to nullptr, then head->prev is set to head itself.

2. Step 2: Identify logical error

   head->prev pointing to head creates a circular reference which is incorrect for a single node list.

3. Final Answer:

   head->prev should not point to head itself -> Option A

4. Quick Check:

   Single node prev = nullptr, not self [OK]

Hint: Single node prev and next must be nullptr [OK]

Common Mistakes:

• Setting prev or next to self causing loops
• Forgetting to initialize data
• Confusing next and prev pointers

5. You want to create a doubly linked list with three nodes containing values 1, 2, and 3 respectively. Which of the following code snippets correctly initializes the list and links all nodes properly?

hard

A. Node* first = new Node{1, nullptr, nullptr};
Node* second = new Node{2, nullptr, first};
first->prev = second;
Node* third = new Node{3, nullptr, second};
second->prev = third;

B. Node* first = new Node{1, nullptr, nullptr};
Node* second = new Node{2, first, nullptr};
first->next = second;
Node* third = new Node{3, second, nullptr};
second->next = third;

C. Node* first = new Node{1, nullptr, nullptr};
Node* second = new Node{2, nullptr, first};
first->next = second;
Node* third = new Node{3, nullptr, second};
second->next = third;

D. Node* first = new Node{1, nullptr, nullptr};
Node* second = new Node{2, first, nullptr};
first->prev = second;
Node* third = new Node{3, second, nullptr};
second->prev = third;

Solution

1. Step 1: Check node creation and pointer assignments

   Node* first = new Node{1, nullptr, nullptr}; Node* second = new Node{2, nullptr, first}; first->next = second; Node* third = new Node{3, nullptr, second}; second->next = third; creates nodes with correct data and sets next and prev pointers properly: second's prev points to first, first's next points to second, third's prev points to second, second's next points to third.

2. Step 2: Verify linkage correctness

   All nodes are linked forward and backward correctly, forming a proper doubly linked list.

3. Final Answer:

   Node* first = new Node{1, nullptr, nullptr}; Node* second = new Node{2, nullptr, first}; first->next = second; Node* third = new Node{3, nullptr, second}; second->next = third; -> Option C

4. Quick Check:

   Next and prev pointers set correctly in Node* first = new Node{1, nullptr, nullptr}; Node* second = new Node{2, nullptr, first}; first->next = second; Node* third = new Node{3, nullptr, second}; second->next = third; [OK]

Hint: Prev points backward, next points forward in order [OK]

Common Mistakes:

• Swapping next and prev pointers
• Not linking nodes both ways
• Setting prev or next to nullptr incorrectly