Doubly Linked List Structure and Node Design in DSA C++ - Execution Trace

Concept Flow - Doubly Linked List Structure and Node Design

Create new node

↓

Set node data

↓

Set node prev pointer to NULL

↓

Set node next pointer to NULL

↓

Node ready to link in list

This flow shows how a new doubly linked list node is created and initialized with data and pointers before linking.

Execution Sample

DSA C++

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

Defines a doubly linked list node with data and two pointers: prev and next.

Execution Table

Step	Operation	Node Data	Prev Pointer	Next Pointer	Visual State
1	Create node	uninitialized	NULL	NULL	NULL <- [ ] -> NULL
2	Set data = 10	10	NULL	NULL	NULL <- [10] -> NULL
3	Set prev pointer	10	NULL	NULL	NULL <- [10] -> NULL
4	Set next pointer	10	NULL	NULL	NULL <- [10] -> NULL
5	Node ready	10	NULL	NULL	NULL <- [10] -> NULL

💡 Node is fully initialized with data and both pointers set to NULL, ready to be linked.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
node.data	uninitialized	uninitialized	10	10	10	10
node.prev	uninitialized	NULL	NULL	NULL	NULL	NULL
node.next	uninitialized	NULL	NULL	NULL	NULL	NULL

Key Moments - 3 Insights

Why do we set both prev and next pointers to NULL when creating a new node?

Setting prev and next to NULL means the node is not linked yet. This is shown in execution_table rows 1 to 4 where pointers remain NULL until linked.

What does the visual state 'NULL <- [10] -> NULL' represent?

It shows the node with data 10 has no previous or next nodes linked, so both sides point to NULL, as seen in all steps of execution_table.

Why is the node data uninitialized at first?

When the node is created (step 1), memory is allocated but data is not set yet. Data is assigned in step 2, as shown in execution_table.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of node.prev after step 3?

A 10

B NULL

C uninitialized

D Points to next node

💡 Hint

Check the 'Prev Pointer' column at step 3 in the execution_table.

At which step does the node data get assigned a value?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Node Data' column in execution_table to see when it changes from uninitialized.

If we skip setting node.next to NULL, what would the visual state likely show?

A NULL <- [10] -> NULL

B [10] with prev and next pointing to itself

C NULL <- [10] -> garbage or random pointer

D Empty list

💡 Hint

Pointers not set explicitly may contain garbage values, check pointer initialization in variable_tracker.

Concept Snapshot

Doubly Linked List Node:
- Contains data, prev pointer, next pointer
- prev points to previous node or NULL
- next points to next node or NULL
- Initialize pointers to NULL before linking
- Visual: NULL <- [data] -> NULL means isolated node

Full Transcript

A doubly linked list node has three parts: data, a pointer to the previous node, and a pointer to the next node. When creating a new node, memory is allocated but data is uninitialized. We then set the data value. Both prev and next pointers are set to NULL to show the node is not linked yet. The visual state shows the node isolated between NULL pointers. This setup prepares the node to be linked into a doubly linked 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