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

↓

If list empty?

Yes→Set head and tail to new node

No↓

Link new node after tail

↓

Update tail pointer

↓

Done

This flow shows how a new node is created with data and pointers, then linked into the doubly linked list by updating head, tail, and pointers.

Execution Sample

DSA C

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

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

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with data=10	None	new_node->prev = NULL, new_node->next = NULL	NULL
2	Check if list empty	None	head = NULL, tail = NULL	NULL
3	List empty: set head and tail to new_node	10	head = new_node, tail = new_node	NULL <- [10] -> NULL
4	Create new node with data=20	10	new_node->prev = NULL, new_node->next = NULL	NULL <- [10] -> NULL
5	List not empty: link new_node after tail	10, 20	tail->next = new_node, new_node->prev = tail	NULL <- [10] <-> [20] -> NULL
6	Update tail pointer to new_node	10, 20	tail = new_node	NULL <- [10] <-> [20] -> NULL
7	Create new node with data=30	10, 20	new_node->prev = NULL, new_node->next = NULL	NULL <- [10] <-> [20] -> NULL
8	List not empty: link new_node after tail	10, 20, 30	tail->next = new_node, new_node->prev = tail	NULL <- [10] <-> [20] <-> [30] -> NULL
9	Update tail pointer to new_node	10, 20, 30	tail = new_node	NULL <- [10] <-> [20] <-> [30] -> NULL
10	End of insertion sequence	10, 20, 30	No pointer changes	NULL <- [10] <-> [20] <-> [30] -> NULL

💡 All nodes inserted and linked; list is now 3 nodes long with correct prev and next pointers.

Variable Tracker

Variable	Start	After Step 3	After Step 6	After Step 9	Final
head	NULL	Node(10)	Node(10)	Node(10)	Node(10)
tail	NULL	Node(10)	Node(20)	Node(30)	Node(30)
new_node	NULL	Node(10)	Node(20)	Node(30)	Node(30)

Key Moments - 3 Insights

Why do we set new_node->prev to tail before updating tail?

Because the new node must point back to the current tail before tail moves to the new node, as shown in steps 5 and 8 in the execution_table.

What happens if the list is empty when inserting the first node?

The head and tail pointers are both set to the new node, making it the only node in the list, as shown in step 3.

Why do we initialize new_node->next and new_node->prev to NULL?

To avoid dangling pointers and clearly mark the ends of the list, as shown in steps 1, 4, and 7.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what pointer changes happen?

A head points to new_node and new_node->next points to head

B tail->next points to new_node and new_node->prev points to tail

C new_node->prev and new_node->next are set to NULL

D tail is set to NULL

💡 Hint

Check the 'Pointer Changes' column at step 5 in the execution_table.

At which step does the list first have two nodes linked?

A Step 3

B Step 6

C Step 5

D Step 4

💡 Hint

Look at the 'Nodes in List' and 'Visual State' columns in the execution_table.

If we forget to set new_node->prev to tail, what would happen to the list?

A The list would be singly linked forward only, losing backward links

B The list would have cycles

C The head pointer would be NULL

D The tail pointer would not update

💡 Hint

Refer to the 'Pointer Changes' and 'Visual State' columns showing prev pointers.

Concept Snapshot

Doubly Linked List Node:
struct Node { data, prev, next };

Insert Node:
- Create node with data
- Set prev and next to NULL
- If empty list, head=tail=node
- Else link node after tail
- Update tail pointer

Prev points backward, next points forward.

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 inserting a new node, we first create it and set its prev and next pointers to NULL. If the list is empty, both head and tail point to this new node. If not empty, we link the new node after the current tail by setting the current tail's next pointer to the new node and the new node's prev pointer back to the tail. Then we update the tail pointer to the new node. This process ensures the list can be traversed forward and backward. The execution table shows each step of inserting three nodes with data 10, 20, and 30, updating pointers and the list structure accordingly.

Practice

1. What is the main difference between a doubly linked list node and a singly linked list node?

easy

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

B. A doubly linked list node stores data twice.

C. A doubly linked list node can only point to the next node.

D. A doubly linked list node does not store any data.

Solution

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

   A singly linked list node has only one pointer to the next node.

2. Step 2: Understand node pointers in doubly linked list

   A doubly linked list node has two pointers: one to the next node and one to the previous node.

3. Final Answer:

   A doubly linked list node has pointers to both the previous and next nodes. -> Option A

4. Quick Check:

   Doubly linked list node = two pointers [OK]

Hint: Remember: doubly linked means two directions [OK]

Common Mistakes:

• Thinking doubly linked nodes store data twice
• Confusing singly and doubly linked list pointers
• Assuming doubly linked nodes have only one pointer

2. Which of the following is the correct C struct definition for a doubly linked list node storing an integer?

easy

A. typedef struct Node { int data; struct Node* next; } Node;

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

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

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

Solution

1. Step 1: Identify required pointers in doubly linked list node

   It must have pointers to both previous and next nodes of type struct Node*.

2. Step 2: Check each option for correct pointer types and members

   typedef struct Node { int data; struct Node* prev; struct Node* next; } Node; correctly defines data, prev, and next pointers as struct Node*.

3. Final Answer:

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

4. Quick Check:

   Correct struct has prev and next pointers [OK]

Hint: Doubly linked node needs both prev and next pointers [OK]

Common Mistakes:

• Missing prev pointer in struct
• Using wrong pointer types like int* for nodes
• Defining only one pointer instead of two

3. Consider the following code snippet for a doubly linked list node initialization in C:

Node* createNode(int val) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = val;
    newNode->prev = NULL;
    newNode->next = NULL;
    return newNode;
}

int main() {
    Node* node = createNode(10);
    printf("%d %p %p", node->data, node->prev, node->next);
    return 0;
}

What will be the output of this program?

medium

A. 10 garbage garbage

B. 10 0x0 0x0

C. 10 NULL NULL

D. Compilation error

Solution

1. Step 1: Analyze node initialization

   Node data is set to 10, prev and next pointers are set to NULL (which is 0x0 in memory).

2. Step 2: Understand printf output for pointers

   Printing pointers with %p shows their memory address; NULL prints as 0x0.

3. Final Answer:

   10 0x0 0x0 -> Option B

4. Quick Check:

   NULL pointers print as 0x0 in %p format [OK]

Hint: NULL pointers print as 0x0 with %p in C [OK]

Common Mistakes:

• Expecting 'NULL' string instead of 0x0
• Assuming garbage values for initialized pointers
• Confusing pointer print format specifiers

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

void insertAtHead(Node** head, int val) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = val;
    newNode->prev = NULL;
    newNode->next = *head;
    *head = newNode;
    if (*head != NULL) {
        (*head)->prev = newNode;
    }
}

medium

A. The condition if (*head != NULL) should be before updating *head.

B. newNode->prev should point to *head, not NULL.

C. The malloc size is incorrect.

D. No error; code is correct.

Solution

1. Step 1: Trace pointer updates

   newNode->next is set to current head, then head is updated to newNode.

2. Step 2: Check the if condition usage

   After updating *head, the condition if (*head != NULL) is always true because *head is newNode. The intention is to update old head's prev pointer.

3. Step 3: Correct order

   The if condition should check if old head (newNode->next) is not NULL before setting its prev pointer.

4. Final Answer:

   The condition if (*head != NULL) should be before updating *head. -> Option A

5. Quick Check:

   Check old head before changing pointers [OK]

Hint: Update old head's prev before changing head pointer [OK]

Common Mistakes:

• Checking *head after it points to newNode
• Setting prev pointer incorrectly
• Ignoring NULL checks before pointer updates

5. You want to design a function to delete a node from a doubly linked list given a pointer to that node. Which of the following steps is NOT required in the deletion process?

hard

A. Free the memory allocated to the node.

B. Update the next node's prev pointer to the node's prev pointer.

C. Update the previous node's next pointer to the node's next pointer.

D. Set the node's data to zero before deletion.

Solution

1. Step 1: Understand deletion in doubly linked list

   To delete a node, update the previous node's next and next node's prev pointers to bypass the node.

2. Step 2: Check necessity of setting data to zero

   Setting the node's data to zero is unnecessary because the node will be freed and memory reclaimed.

3. Step 3: Confirm freeing memory

   Freeing the node's memory is essential to avoid memory leaks.

4. Final Answer:

   Set the node's data to zero before deletion. -> Option D

5. Quick Check:

   Data reset before free is unnecessary [OK]

Hint: Free memory; no need to clear data first [OK]

Common Mistakes:

• Clearing data before freeing memory
• Forgetting to update prev or next pointers
• Not freeing the node memory