Doubly Linked List Structure and Node Design in DSA C

Mental Model

A doubly linked list is a chain of nodes where each node knows both its previous and next neighbor, making it easy to move forward and backward.

Analogy: Imagine a train where each carriage is connected to the one before and after it, so you can walk from the front to the back or back to front easily.

null ← [prev|data|next] -> null

Dry Run Walkthrough

Input: Create a doubly linked list with nodes containing values 1, 2, 3 linked in order

Goal: Build the doubly linked list so each node points correctly to previous and next nodes

Step 1: Create first node with value 1, prev and next point to null

null ← [null|1|null] -> null

Why: First node has no neighbors yet, so both pointers are null

Step 2: Create second node with value 2, link it after first node

null ← [null|1|2] -> [1|2|null] -> null

Why: First node's next points to second node; second node's prev points back to first

Step 3: Create third node with value 3, link it after second node

null ← [null|1|2] -> [1|2|3] -> [2|3|null] -> null

Why: Second node's next points to third node; third node's prev points back to second

Result:

null ← [null|1|2] -> [1|2|3] -> [2|3|null] -> null

Annotated Code

DSA C

#include <stdio.h>
#include <stdlib.h>

// Node structure for doubly linked list
typedef struct Node {
    int data;
    struct Node* prev;
    struct Node* next;
} Node;

// Function to create a new node with given data
Node* createNode(int data) {
    Node* newNode = (Node*)malloc(sizeof(Node));
    newNode->data = data;
    newNode->prev = NULL;
    newNode->next = NULL;
    return newNode;
}

// Function to print the doubly linked list from head to tail
void printList(Node* head) {
    Node* curr = head;
    while (curr != NULL) {
        printf("%d", curr->data);
        if (curr->next != NULL) {
            printf(" <-> ");
        }
        curr = curr->next;
    }
    printf("\n");
}

int main() {
    // Step 1: Create first node
    Node* head = createNode(1);

    // Step 2: Create second node and link
    Node* second = createNode(2);
    head->next = second;          // first node next points to second
    second->prev = head;          // second node prev points to first

    // Step 3: Create third node and link
    Node* third = createNode(3);
    second->next = third;         // second node next points to third
    third->prev = second;         // third node prev points to second

    // Print the list
    printList(head);

    // Free memory
    free(third);
    free(second);
    free(head);

    return 0;
}

Node* newNode = (Node*)malloc(sizeof(Node));

allocate memory for new node

newNode->prev = NULL; newNode->next = NULL;

initialize pointers to null for isolated node

head->next = second; second->prev = head;

link first and second nodes bidirectionally

second->next = third; third->prev = second;

link second and third nodes bidirectionally

while (curr != NULL) { ... curr = curr->next; }

traverse list forward to print all node data

Output

1 <-> 2 <-> 3

Complexity Analysis

Time: O(n) because printing or traversing the list visits each node once

Space: O(n) because each node is stored separately in memory

vs Alternative: Compared to singly linked list, doubly linked list uses extra space for prev pointer but allows backward traversal

Edge Cases

Empty list (no nodes)

Printing shows nothing; no nodes to link

DSA C

while (curr != NULL) { ... }

Single node list

Node prev and next remain NULL; list prints single value

DSA C

newNode->prev = NULL; newNode->next = NULL;

When to Use This Pattern

When you need to move both forward and backward through a list easily, use a doubly linked list structure because each node stores links to both neighbors.

Common Mistakes

Mistake: Forgetting to set the prev pointer when linking nodes
Fix: Always assign both next and prev pointers when connecting nodes

Mistake: Not initializing prev and next pointers to NULL in new nodes
Fix: Set prev and next to NULL in node creation to avoid dangling pointers

Summary

It creates nodes that link forward and backward to form a doubly linked list.

Use it when you want easy two-way navigation through a list of items.

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

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