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 the position is 1, delete the head node.

beginner

How do you delete a node at position 1 in a singly linked list?

Make the head pointer point to the second node, then delete the original head node to free memory.

intermediate

Why do we need to traverse the list up to the node before the target position when deleting a node?

Because we need to update the 'next' pointer of the previous node to skip the node being deleted.

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 delete the node after unlinking it from the list?

To free the memory allocated for that node and avoid memory leaks.

What should you do if you want to delete the node at position 1 in a singly linked list?

A Do nothing

B Traverse to the last node and delete it

C Update the last node's next pointer to null

D Update head to point to the second node and delete the original head

Deleting the first node means moving the head pointer to the second node and deleting the original head to free memory.

When deleting a node at position n (n > 1), what is the key step?

A Traverse to the node before position n, update its next pointer to skip the node at n, then delete node at n

B Update the head pointer to null

C Delete the head node

D Traverse to the node at position n and delete it directly

You must reach the node before the target to update its next pointer to skip the node being deleted.

What happens if the position to delete is zero or negative?

A No deletion occurs because position is invalid

B Delete the last node

C Delete the head node

D Delete all nodes

Positions less than 1 are invalid for deletion in a singly linked list.

Why is it important to delete the node after unlinking it from the list in C++?

A To keep the node in memory

B To update the head pointer

C To avoid memory leaks

D To traverse the list faster

Deleting the node frees the allocated memory and prevents memory leaks.

If the linked list is empty, what happens when you try to delete a node at any position?

A A new node is created

B The list remains empty; no deletion occurs

C The program crashes

D The head pointer is updated

If the list is empty, there is nothing to delete, so the list stays empty.

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

Think about how pointers change when removing a node.

Describe how to handle edge cases when deleting a node at a specific position in a linked list.

Consider what happens if the position is out of range or list is empty.

Practice

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

easy

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

B. The last node is removed

C. The list remains unchanged

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: Delete 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 A

4. Quick Check:

   Deleting position 1 removes the head [OK]

Hint: Deleting position 1 means removing 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. head = head->prev;

B. if (pos == 1) { head = head->next; }

C. delete head->next->prev;

D. head->next = NULL;

Solution

1. Step 1: Check deletion at position 1

   If position is 1, we update head to head->next to remove the first node.

2. Step 2: Verify syntax correctness

   if (pos == 1) { head = head->next; } correctly updates head pointer; others are invalid or incorrect for singly linked list.

3. Final Answer:

   if (pos == 1) { head = head->next; } -> Option B

4. Quick Check:

   Update head for position 1 deletion [OK]

Hint: For position 1, assign head = head->next [OK]

Common Mistakes:

• Using prev pointer in singly linked list
• Setting next pointer to NULL incorrectly
• Deleting wrong node pointer

3. What is the output linked list after deleting the node at position 3 from the list: 1 -> 2 -> 3 -> 4 -> 5 -> NULL?

medium

A. 2 -> 3 -> 4 -> 5 -> NULL

B. 1 -> 2 -> 3 -> 5 -> NULL

C. 1 -> 3 -> 4 -> 5 -> NULL

D. 1 -> 2 -> 4 -> 5 -> NULL

Solution

1. Step 1: Identify node at position 3

   Node at position 3 is the node with value 3.

2. Step 2: Remove node 3 and link previous to next

   Link node 2's next to node 4, skipping node 3.

3. Final Answer:

   1 -> 2 -> 4 -> 5 -> NULL -> Option D

4. Quick Check:

   Node 3 removed, list links 2 to 4 [OK]

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

Common Mistakes:

• Removing wrong node
• Not updating previous node's next pointer
• Deleting head instead of position 3

4. Given the code snippet below, what is the error when deleting a node at position 4?

void deleteNode(Node*& head, int pos) {
  Node* temp = head;
  for (int i = 1; i < pos - 1; i++) {
    temp = temp->next;
  }
  Node* toDelete = temp->next;
  temp->next = toDelete->next;
  delete toDelete;
}

medium

A. No check if position is valid or if temp is NULL

B. Incorrect loop range causing off-by-one error

C. Deleting head node incorrectly

D. Using delete on a pointer not allocated with new

Solution

1. Step 1: Analyze loop and pointer usage

   The loop moves temp to node before position to delete, but no checks if temp or temp->next is NULL.

2. Step 2: Identify missing validation

   Without checking if position is valid or temp is NULL, code may crash on invalid positions.

3. Final Answer:

   No check if position is valid or if temp is NULL -> Option A

4. Quick Check:

   Missing boundary checks cause runtime errors [OK]

Hint: Always check pointers before accessing next [OK]

Common Mistakes:

• Ignoring invalid position inputs
• Assuming list length is sufficient
• Not checking for NULL pointers

5. You have a singly linked list: 10 -> 20 -> 30 -> 40 -> 50 -> NULL. Write the resulting list after deleting the node at position 5 and then deleting the node at position 1.

hard

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

B. 10 -> 20 -> 30 -> 40 -> NULL

C. 20 -> 30 -> 40 -> NULL

D. 10 -> 20 -> 30 -> NULL

Solution

1. Step 1: Delete node at position 5

   Node at position 5 is 50, removing it results in list: 10 -> 20 -> 30 -> 40 -> NULL.

2. Step 2: Delete node at position 1

   Deleting head node 10 updates head to 20, final list: 20 -> 30 -> 40 -> NULL.

3. Final Answer:

   20 -> 30 -> 40 -> NULL -> Option C

4. Quick Check:

   Delete last then head node sequentially [OK]

Hint: Delete last node first, then head node for correct order [OK]

Common Mistakes:

• Deleting head first then last causing wrong list
• Not updating head after deleting position 1
• Forgetting to remove last node