Delete Node at Specific Position in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the first step when deleting a node at a specific position in a singly linked list?

Check if the list is empty or if the position is valid (greater than 0). If invalid, no deletion should occur.

beginner

How do you delete the head node (position 1) in a singly linked list?

Update the head pointer to point to the second node, then free the original head node's memory.

intermediate

Why do we need to keep track of the previous node when deleting a node at a specific position (other than head)?

Because we must update the previous node's next pointer to skip the node being deleted and link to the next node after it.

beginner

What happens if the position to delete is greater than the length of the linked list?

No deletion occurs because the position is invalid; the list remains unchanged.

beginner

In C, why is it important to free the memory of the deleted node?

To avoid memory leaks by releasing the allocated memory back to the system.

What should you do first when deleting a node at position 3 in a singly linked list?

A Traverse to the node at position 2

B Delete the head node

C Free the memory of the node at position 3 immediately

D Set head to NULL

You must reach the node before the one to delete (position 2) to update its next pointer.

If you want to delete the first node in a linked list, what pointer do you update?

A The last node's next pointer

B The previous node's next pointer

C The head pointer

D The deleted node's next pointer

The head pointer must be updated to point to the second node.

What happens if you try to delete a node at position 10 but the list has only 5 nodes?

A The last node is deleted

B The list becomes empty

C The first node is deleted

D No deletion occurs

Position 10 is invalid, so no deletion happens.

Why do you need to free memory after deleting a node in C?

A To avoid memory leaks

B To increase the list size

C To copy the node data

D To print the list

Freeing memory prevents memory leaks by releasing unused memory.

Which pointer is updated to remove a node from the middle of a linked list?

A The deleted node's next pointer

B The previous node's next pointer

C The head pointer

D The last node's next pointer

The previous node's next pointer is updated to skip the deleted node.

Explain the steps to delete a node at a specific position in a singly linked list.

Think about handling the head node separately and updating pointers carefully.

Why is it important to handle boundary cases when deleting nodes in a linked list?

Consider what happens if you try to delete a node that does not exist.

Practice

1. What happens when you delete a node at position 1 in a singly linked list?

easy

A. The list remains unchanged

B. The last node is removed

C. The head node is removed and the next node becomes the new head

D. All nodes are deleted

Solution

1. Step 1: Understand position 1 in linked list

   Position 1 refers to the head node of the list.

2. Step 2: Deleting the head node

   Removing the head means the next node becomes the new head.

3. Final Answer:

   The head node is removed and the next node becomes the new head -> Option C

4. Quick Check:

   Deleting position 1 removes head [OK]

Hint: Deleting position 1 always removes the head node [OK]

Common Mistakes:

• Thinking position 1 deletes the last node
• Assuming list stays same after deletion
• Deleting all nodes instead of one

2. Which of the following is the correct syntax to delete a node at position pos in a singly linked list in C?

easy

A. void deleteNode(struct Node* head, int pos);

B. void deleteNode(struct Node** head_ref, int pos);

C. void deleteNode(int pos, struct Node* head);

D. void deleteNode(int pos);

Solution

1. Step 1: Understand pointer usage for head modification

   To delete a node, the function must modify the head pointer if position 1 is deleted, so it needs a pointer to pointer.

2. Step 2: Check function parameters

   Only void deleteNode(struct Node** head_ref, int pos); uses struct Node** head_ref allowing head modification.

3. Final Answer:

   void deleteNode(struct Node** head_ref, int pos); -> Option B

4. Quick Check:

   Use pointer to pointer for head changes [OK]

Hint: Use double pointer to head to delete at any position [OK]

Common Mistakes:

• Using single pointer and expecting head to change
• Wrong parameter order
• Missing head pointer in parameters

3. Given the linked list: 10 -> 20 -> 30 -> 40 -> 50 -> NULL, what is the list after deleting the node at position 3?

medium

A. 10 -> 20 -> 40 -> 50 -> NULL

B. 10 -> 20 -> 30 -> 50 -> NULL

C. 10 -> 30 -> 40 -> 50 -> NULL

D. 20 -> 30 -> 40 -> 50 -> NULL

Solution

1. Step 1: Identify node at position 3

   Position 3 node is 30 in the list.

2. Step 2: Remove node 30 and link previous to next

   Link node 20 to node 40, skipping 30.

3. Final Answer:

   10 -> 20 -> 40 -> 50 -> NULL -> Option A

4. Quick Check:

   Deleted 3rd node (30) correctly [OK]

Hint: Skip the node at position by linking previous to next [OK]

Common Mistakes:

• Deleting wrong node position
• Not updating links correctly
• Removing head instead of position 3

4. Consider this code snippet to delete a node at position pos in a linked list:

void deleteNode(struct Node** head_ref, int pos) {
  struct Node* temp = *head_ref;
  if (pos == 1) {
    *head_ref = temp->next;
    free(temp);
    return;
  }
  for (int i = 1; temp != NULL && i < pos - 1; i++) {
    temp = temp->next;
  }
  if (temp == NULL || temp->next == NULL) return;
  struct Node* next = temp->next->next;
  free(temp->next);
  temp->next = next;
}

What is the bug in this code?

medium

A. It does not handle empty list when pos is 1

B. It frees the wrong node

C. It does not update head pointer correctly

D. It does not check if pos is valid

Solution

1. Step 1: Check deletion at position 1

   If list is empty (*head_ref is NULL), temp is NULL, so temp->next causes error.

2. Step 2: Identify missing empty list check

   Code should check if temp is NULL before accessing temp->next when pos == 1.

3. Final Answer:

   It does not handle empty list when pos is 1 -> Option A

4. Quick Check:

   Empty list check missing at head deletion [OK]

Hint: Always check if list is empty before deleting [OK]

Common Mistakes:

• Ignoring empty list case
• Assuming pos is always valid
• Not freeing the deleted node

5. You have a singly linked list: 5 -> 10 -> 15 -> 20 -> 25 -> NULL. You want to delete the node at position 6. What should happen after calling the delete function?

hard

A. The last node (25) is deleted

B. The function crashes due to invalid position

C. The head node (5) is deleted

D. The list remains unchanged because position 6 is invalid

Solution

1. Step 1: Check if position 6 exists in list

   List has 5 nodes, so position 6 is invalid.

2. Step 2: Behavior on invalid position

   Proper delete function should do nothing if position is invalid, leaving list unchanged.

3. Final Answer:

   The list remains unchanged because position 6 is invalid -> Option D

4. Quick Check:

   Invalid position means no deletion [OK]

Hint: Invalid position means no change to list [OK]

Common Mistakes:

• Deleting last node incorrectly
• Crashing due to no position check
• Deleting head by mistake