Insert at Specific Position in Doubly Linked List in DSA C - Build from Scratch

Insert at Specific Position in Doubly Linked List

📖 Scenario: You are managing a playlist of songs using a doubly linked list. Each song has a unique ID. You want to insert a new song at a specific position in the playlist.

🎯 Goal: Build a program that creates a doubly linked list with initial songs, sets the position to insert a new song, inserts the new song at that position, and then prints the updated playlist.

📋 What You'll Learn

Create a doubly linked list with exactly three nodes having song IDs 101, 102, and 103 in that order.

Create an integer variable called position and set it to 2.

Create a new node with song ID 150 and insert it at the position in the doubly linked list.

Print the updated doubly linked list from head to tail showing song IDs separated by arrows.

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, browsers (history navigation), and many applications where you need to move forward and backward efficiently.

💼 Career

Understanding how to insert nodes in doubly linked lists is important for software engineers working on low-level data structures, embedded systems, or performance-critical applications.

1

Create the initial doubly linked list

Create a doubly linked list with three nodes having song IDs 101, 102, and 103 in that order. Use a struct called Node with int data, Node* prev, and Node* next. Create a pointer called head pointing to the first node.

DSA C

#include 
#include 

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

int main() {
    // Create nodes and link them
    // Your code here
    return 0;
}

Hint

Remember to allocate memory for each node using malloc and link the prev and next pointers correctly.

2

Set the position to insert the new node

Create an integer variable called position and set it to 2. This means the new song will be inserted as the second node in the list.

DSA C

#include 
#include 

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

int main() {
    struct Node* head = (struct Node*)malloc(sizeof(struct Node));
    struct Node* second = (struct Node*)malloc(sizeof(struct Node));
    struct Node* third = (struct Node*)malloc(sizeof(struct Node));

    head->data = 101;
    head->prev = NULL;
    head->next = second;

    second->data = 102;
    second->prev = head;
    second->next = third;

    third->data = 103;
    third->prev = second;
    third->next = NULL;

    // Create position variable
    // Your code here
    return 0;
}

Hint

Just declare an integer variable named position and assign it the value 2.

3

Insert the new node at the specified position

Create a new node with song ID 150 and insert it at the position in the doubly linked list. Update the prev and next pointers of the surrounding nodes accordingly.

DSA C

#include 
#include 

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

int main() {
    struct Node* head = (struct Node*)malloc(sizeof(struct Node));
    struct Node* second = (struct Node*)malloc(sizeof(struct Node));
    struct Node* third = (struct Node*)malloc(sizeof(struct Node));

    head->data = 101;
    head->prev = NULL;
    head->next = second;

    second->data = 102;
    second->prev = head;
    second->next = third;

    third->data = 103;
    third->prev = second;
    third->next = NULL;

    int position = 2;

    // Insert new node at position
    // Your code here
    return 0;
}

Hint

Traverse to the node before the position, then adjust pointers to insert the new node.

4

Print the updated doubly linked list

Print the updated doubly linked list from head to the end. Display the song IDs separated by arrows like 101 <-> 150 <-> 102 <-> 103.

DSA C

#include 
#include 

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

int main() {
    struct Node* head = (struct Node*)malloc(sizeof(struct Node));
    struct Node* second = (struct Node*)malloc(sizeof(struct Node));
    struct Node* third = (struct Node*)malloc(sizeof(struct Node));

    head->data = 101;
    head->prev = NULL;
    head->next = second;

    second->data = 102;
    second->prev = head;
    second->next = third;

    third->data = 103;
    third->prev = second;
    third->next = NULL;

    int position = 2;

    struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
    newNode->data = 150;

    struct Node* temp = head;
    int i;
    for (i = 1; i next;
    }

    newNode->next = temp->next;
    newNode->prev = temp;
    if (temp->next != NULL) {
        temp->next->prev = newNode;
    }
    temp->next = newNode;

    // Print the list
    // Your code here
    return 0;
}

Hint

Use a while loop to traverse from head to the end, printing each node's data. Print arrows between nodes but not after the last node.

Practice

1. What happens when you insert a new node at position 1 in a doubly linked list?

easy

A. The new node becomes the new head of the list.

B. The new node is added at the end of the list.

C. The new node replaces the last node.

D. The list remains unchanged.

Solution

1. Step 1: Understand position 1 insertion

   Inserting at position 1 means adding the node at the start of the list.

2. Step 2: Effect on head pointer

   The new node becomes the head, and its next points to the old head.

3. Final Answer:

   The new node becomes the new head of the list. -> Option A

4. Quick Check:

   Position 1 insertion updates head [OK]

Hint: Position 1 means new head insertion [OK]

Common Mistakes:

• Thinking position 1 means end insertion
• Not updating the head pointer
• Ignoring previous pointer updates

2. Which of the following is the correct way to update pointers when inserting a node at position 3 in a doubly linked list?

easy

A. newNode->next = temp; newNode->prev = temp->prev; temp->prev->next = newNode; temp->prev = newNode;

B. newNode->prev = temp->prev; newNode->next = temp; temp->prev->next = newNode; temp->prev = newNode;

C. newNode->next = temp->next; newNode->prev = temp; temp->next->prev = newNode; temp->next = newNode;

D. newNode->next = temp->next; newNode->prev = temp->prev; temp->next = newNode; temp->prev = newNode;

Solution

1. Step 1: Identify insertion point

   At position 3, newNode is inserted after temp (node at position 2).

2. Step 2: Update pointers correctly

   newNode's next points to temp's next, prev to temp; temp's next's prev updated to newNode; temp's next updated to newNode.

3. Final Answer:

   newNode->next = temp->next; newNode->prev = temp; temp->next->prev = newNode; temp->next = newNode; -> Option C

4. Quick Check:

   Correct pointer updates for middle insertion [OK]

Hint: Update newNode next/prev before changing temp pointers [OK]

Common Mistakes:

• Swapping next and prev pointers
• Not updating temp->next->prev
• Forgetting to update temp->next

3. What will be the list after inserting a node with value 10 at position 2 in the doubly linked list: 5 <-> 15 <-> 20?

medium

A. 5 <-> 15 <-> 10 <-> 20

B. 10 <-> 5 <-> 15 <-> 20

C. 5 <-> 15 <-> 20 <-> 10

D. 5 <-> 10 <-> 15 <-> 20

Solution

1. Step 1: Locate position 2

   Position 2 means new node goes between nodes 1 (5) and 2 (15).

2. Step 2: Insert new node with value 10

   New node's prev points to 5, next points to 15; update 5's next and 15's prev accordingly.

3. Final Answer:

   5 <-> 10 <-> 15 <-> 20 -> Option D

4. Quick Check:

   Inserted node at position 2 between 5 and 15 [OK]

Hint: Position 2 means between first and second nodes [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating both next and prev pointers
• Appending instead of inserting

4. Given the code snippet below, what is the error when inserting at position 4 in a list of 3 nodes?

void insertAtPosition(Node** head, int data, int pos) {
  Node* newNode = createNode(data);
  Node* temp = *head;
  for (int i = 1; i < pos - 1; i++) {
    temp = temp->next;
  }
  newNode->next = temp->next;
  newNode->prev = temp;
  temp->next->prev = newNode;
  temp->next = newNode;
}

medium

A. newNode is not allocated properly.

B. temp->next is NULL, so temp->next->prev causes a runtime error.

C. Loop runs one time too many causing segmentation fault.

D. prev pointer of newNode is not set.

Solution

1. Step 1: Analyze loop for position 4 in 3-node list

   Loop moves temp to node at position 3; temp->next is NULL (end of list).

2. Step 2: Check pointer updates

   Accessing temp->next->prev causes runtime error because temp->next is NULL.

3. Final Answer:

   temp->next is NULL, so temp->next->prev causes a runtime error. -> Option B

4. Quick Check:

   Accessing NULL pointer causes runtime error [OK]

Hint: Check for NULL before accessing next->prev [OK]

Common Mistakes:

• Not checking if temp->next is NULL
• Assuming list is longer than it is
• Ignoring edge case at list end

5. You have a doubly linked list: 1 <-> 3 <-> 5 <-> 7. You want to insert a node with value 4 at position 3. After insertion, what will be the correct list and how do you handle the pointers to maintain list integrity?

hard

A. 1 <-> 3 <-> 4 <-> 5 <-> 7; Set newNode->prev = node at pos 2, newNode->next = node at pos 3, update adjacent nodes' pointers.

B. 1 <-> 3 <-> 5 <-> 4 <-> 7; Insert newNode after node at pos 3 without updating prev pointers.

C. 1 <-> 4 <-> 3 <-> 5 <-> 7; Insert newNode at head and shift others.

D. 1 <-> 3 <-> 5 <-> 7 <-> 4; Append newNode at end regardless of position.

Solution

1. Step 1: Identify insertion position and nodes

   Position 3 means new node goes between nodes with values 3 (pos 2) and 5 (pos 3).

2. Step 2: Update pointers to insert new node

   Set newNode->prev to node with value 3, newNode->next to node with value 5; update node 3's next and node 5's prev to newNode.

3. Step 3: Verify list integrity

   After insertion, list is 1 <-> 3 <-> 4 <-> 5 <-> 7 with all pointers correctly linked.

4. Final Answer:

   1 <-> 3 <-> 4 <-> 5 <-> 7; Set newNode->prev = node at pos 2, newNode->next = node at pos 3, update adjacent nodes' pointers. -> Option A

5. Quick Check:

   Proper pointer updates maintain doubly linked list [OK]

Hint: Insert between nodes, update both next and prev pointers [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating both adjacent nodes' pointers
• Breaking list links causing traversal errors