Create and Initialize Doubly Linked List in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a doubly linked list node struct.

DSA C++

struct Node {
    int data;
    Node* prev;
    Node* [1];
};

Drag options to blanks, or click blank then click option'

Anext

Bprevious

Clink

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'previous' instead of 'next' for the forward pointer.

Using unrelated names like 'child' or 'link'.

The pointer to the next node in a doubly linked list is commonly named 'next'.

2fill in blank

medium

Complete the constructor to initialize the node's data and pointers.

DSA C++

Node(int val) {
    data = val;
    prev = nullptr;
    [1] = nullptr;
}

Drag options to blanks, or click blank then click option'

Aprev

Blink

Cnext

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing 'prev' twice and forgetting 'next'.

Using incorrect pointer names.

The 'next' pointer should be initialized to nullptr in the constructor.

3fill in blank

hard

Fix the error in the code that creates a new node and sets its data.

DSA C++

Node* newNode = new Node([1]);
newNode->prev = nullptr;
newNode->next = nullptr;

Drag options to blanks, or click blank then click option'

A0

B5

CNULL

Dnullptr

Attempts:

3 left

💡 Hint

Common Mistakes

Passing nullptr or NULL instead of an integer value.

Passing zero which is valid but less clear than a meaningful number.

The constructor expects an integer value; '5' is a valid integer to initialize data.

4fill in blank

hard

Fill both blanks to correctly link two nodes in a doubly linked list.

DSA C++

first->next = [1];
second->prev = [2];

Drag options to blanks, or click blank then click option'

Asecond

Bfirst

Cnullptr

DnewNode

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up 'prev' and 'next' pointers.

Assigning nullptr instead of node pointers.

The first node's next pointer should point to the second node, and the second node's prev pointer should point back to the first node.

5fill in blank

hard

Fill all three blanks to create a doubly linked list with three nodes connected properly.

DSA C++

Node* first = new Node(1);
Node* second = new Node(2);
Node* third = new Node(3);

first->next = [1];
second->prev = [2];
second->next = [3];
third->prev = second;

Drag options to blanks, or click blank then click option'

Asecond

Bfirst

Cthird

Dnullptr

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to link the second node's next pointer.

Mixing up which node pointer to assign.

The first node's next points to second, second's prev points to first, and second's next points to third.

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