Doubly Linked List Structure and Node Design 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 two links: one to the next node and one 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?

A node has three parts: the data it holds, a pointer to the next node, and a pointer to the previous node.

beginner

Why do doubly linked lists have two pointers per node?

Two pointers let us move in both directions: forward to the next node and backward to the previous node, making some operations easier.

beginner

Show a simple C++ struct for a doubly linked list node.

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

beginner

What is the difference between singly and doubly linked lists?

Singly linked lists have nodes with one pointer to the next node only. Doubly linked lists have two pointers per node, allowing movement both forward and backward.

What does each node in a doubly linked list contain?

A Data, pointer to next node, pointer to previous node

B Data and pointer to next node only

C Data and pointer to previous node only

D Data only

Each node has data and two pointers: one to the next node and one to the previous node.

Which operation is easier with a doubly linked list compared to a singly linked list?

A Moving backward through the list

B Moving forward through the list

C Accessing the first node

D Storing data

Doubly linked lists allow moving backward easily because nodes have pointers to previous nodes.

In C++, how do you declare a pointer to the previous node in a doubly linked list node struct?

A Node& prev;

B Node* prev;

C Node prev;

D int prev;

Pointers to nodes are declared as Node* prev; to hold the address of the previous node.

What happens if the 'prev' pointer of the first node is not set to nullptr?

A It points to itself

B It points to the last node automatically

C It may point to garbage memory causing errors

D It does not matter

The 'prev' pointer of the first node should be nullptr to avoid pointing to invalid memory.

Which of these is NOT a benefit of doubly linked lists?

A Easy backward traversal

B More memory usage per node

C Efficient deletion of a node given its pointer

D Simpler node structure than singly linked lists

Doubly linked lists have a more complex node structure than singly linked lists because of the extra pointer.

Describe the structure of a node in a doubly linked list and explain why it has two pointers.

Think about how you can move forward and backward in a list.

Explain the difference between singly and doubly linked lists in terms of node design and traversal capabilities.

Focus on how many directions you can move in each list.

Practice

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

easy

A. It can only be traversed forward.

B. It uses less memory than a singly linked list.

C. It requires only one pointer per node.

D. It allows traversal in both forward and backward directions.

Solution

1. Step 1: Understand node pointers in doubly linked list

   Each node has two pointers: one to the next node and one to the previous node.

2. Step 2: Compare traversal capabilities

   This allows moving forward and backward through the list, unlike singly linked lists which only allow forward traversal.

3. Final Answer:

   It allows traversal in both forward and backward directions. -> Option D

4. Quick Check:

   Doubly linked list = two-way traversal [OK]

Hint: Remember: two pointers mean two directions [OK]

Common Mistakes:

• Thinking it uses less memory than singly linked list
• Assuming it has only one pointer per node
• Believing it can only traverse forward

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* next; Node* prev; };

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

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

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

Solution

1. Step 1: Check pointer usage for next and prev

   In a doubly linked list, next and prev must be pointers to Node, so Node* next and Node* prev are correct.

2. Step 2: Identify incorrect declarations

   struct Node { int data; Node next; Node prev; }; uses Node next and Node prev (not pointers), C uses int prev, and D uses int for both next and prev, which is incorrect.

3. Final Answer:

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

4. Quick Check:

   Pointers for next and prev = struct Node { int data; Node* next; Node* prev; }; [OK]

Hint: Pointers to Node must use * symbol [OK]

Common Mistakes:

• Using Node instead of Node* for next and prev
• Using int type for pointers
• Missing pointer symbols

3. Consider the following code snippet for a doubly linked list node insertion at the front:

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

Node* head = nullptr;

Node* newNode = new Node{10, nullptr, nullptr};
newNode->next = head;
if (head != nullptr) {
  head->prev = newNode;
}
head = newNode;

// What is the data in head->next after insertion?

medium

A. nullptr

B. 10

C. Undefined

D. Garbage value

Solution

1. Step 1: Analyze initial head value

   Initially, head is nullptr, meaning the list is empty.

2. Step 2: After insertion, newNode->next points to old head (nullptr)

   Since head was nullptr, newNode->next is set to nullptr, so head->next is nullptr after insertion.

3. Final Answer:

   nullptr -> Option A

4. Quick Check:

   Empty list insertion sets next to nullptr [OK]

Hint: New node next points to old head, which is nullptr here [OK]

Common Mistakes:

• Assuming head->next has data 10
• Thinking head->next is undefined or garbage
• Ignoring initial nullptr head

4. Identify the error in the following code snippet for inserting a node at the end of a doubly linked list:

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

void insertEnd(Node*& head, int val) {
  Node* newNode = new Node{val, nullptr, nullptr};
  if (head == nullptr) {
    head = newNode;
    return;
  }
  Node* temp = head;
  while (temp->next != nullptr) {
    temp = temp->next;
  }
  temp->next = newNode;
  newNode->prev = nullptr;
}

medium

A. temp should be initialized to nullptr.

B. head should be passed by value, not reference.

C. newNode->prev should point to temp, not nullptr.

D. newNode->next should point to head.

Solution

1. Step 1: Check newNode's prev pointer assignment

   After finding the last node (temp), newNode is added after temp, so newNode->prev must point to temp.

2. Step 2: Identify incorrect assignment

   Code sets newNode->prev = nullptr, which breaks backward link; it should be newNode->prev = temp.

3. Final Answer:

   newNode->prev should point to temp, not nullptr. -> Option C

4. Quick Check:

   Prev pointer links new node to last node [OK]

Hint: Prev of new node = last node before insertion [OK]

Common Mistakes:

• Setting newNode->prev to nullptr instead of last node
• Passing head by value losing list updates
• Incorrect initialization of temp pointer

5. You want to design a function to delete a node from a doubly linked list given a pointer to that node (not the head). Which pointer updates are necessary to maintain list integrity?

hard

A. No pointer updates needed; just delete the node.

B. Set node's prev->next to node's next and node's next->prev to node's prev.

C. Set node's prev to nullptr and node's next to nullptr only.

D. Set node's next->next to node's prev and node's prev->prev to node's next.

Solution

1. Step 1: Understand node removal in doubly linked list

   To remove a node, the previous node's next pointer must skip the node and point to node's next.

2. Step 2: Update next node's prev pointer

   Similarly, the next node's prev pointer must skip the node and point to node's prev.

3. Final Answer:

   Set node's prev->next to node's next and node's next->prev to node's prev. -> Option B

4. Quick Check:

   Prev and next pointers bypass deleted node [OK]

Hint: Link prev and next nodes directly to remove node [OK]

Common Mistakes:

• Not updating both prev and next pointers
• Setting wrong pointers to nullptr
• Deleting node without fixing links