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

Recall & Review

beginner

What is the main goal when deleting a node at the end of a singly linked list?

To remove the last node and update the second last node's next pointer to NULL, effectively shortening the list by one node.

beginner

In a singly linked list, why do we need to traverse the list to delete the last node?

Because singly linked lists do not have backward pointers, we must traverse from the head to find the second last node to update its next pointer.

beginner

What happens if you try to delete the last node in a singly linked list that has only one node?

The head pointer becomes NULL, indicating the list is now empty after deleting the only node.

intermediate

Show the key steps in C to delete the last node of a singly linked list.

1. Check if the list is empty (head == NULL). 2. If only one node, free it and set head to NULL. 3. Otherwise, traverse to second last node. 4. Free last node. 5. Set second last node's next to NULL.

beginner

Why is it important to free the memory of the deleted node in C?

To avoid memory leaks by releasing the allocated memory back to the system after the node is no longer used.

What pointer do you update when deleting the last node in a singly linked list?

A The previous pointer of the last node

B The head pointer

C The next pointer of the last node

D The next pointer of the second last node

You update the next pointer of the second last node to NULL to remove the last node.

If a singly linked list has only one node, what should happen after deleting that node?

A Head pointer should be set to NULL

B Head pointer should point to itself

C Next pointer of the node should be set to NULL

D Nothing changes

After deleting the only node, the list becomes empty, so head should be NULL.

Why can't you directly access the second last node in a singly linked list?

A Because singly linked lists only have next pointers, no backward pointers

B Because the last node is always the head

C Because the list is circular

D Because the second last node is always NULL

Singly linked lists only have next pointers, so you must traverse from head to find the second last node.

What is the time complexity of deleting the last node in a singly linked list?

A O(1)

B O(n)

C O(log n)

D O(n^2)

You must traverse the list to find the second last node, which takes O(n) time.

What happens if you forget to free the deleted node's memory in C?

A The node is automatically freed

B Program crashes immediately

C Memory leak occurs

D The node remains in the list

Not freeing memory causes memory leaks, wasting system resources.

Explain step-by-step how to delete the last node in a singly linked list in C.

Think about what pointers you need to update and when to free memory.

Describe why deleting the last node in a singly linked list takes O(n) time.

Consider how the list is connected and what information is missing.

Practice

1. What is the main purpose of the Delete Node at End operation in a singly linked list?

easy

A. To reverse the linked list

B. To add a new node at the end of the linked list

C. To delete the first node of the linked list

D. To remove the last node from the linked list

Solution

1. Step 1: Understand the operation's goal

   The operation aims to remove the last node in the linked list, which is the node that points to null.

2. Step 2: Identify the correct description

   Removing the last node means deleting the node at the end, not adding or reversing nodes.

3. Final Answer:

   To remove the last node from the linked list -> Option D

4. Quick Check:

   Delete Node at End = Remove last node [OK]

Hint: Deleting at end means removing the last node [OK]

Common Mistakes:

• Confusing delete at end with delete at beginning
• Thinking it adds a node instead of deleting
• Mixing up with reversing the list

2. Which of the following C code snippets correctly deletes the last node of a singly linked list named head?

easy

A. if (head == NULL || head->next == NULL) return; Node* temp = head; while (temp->next->next != NULL) temp = temp->next; free(temp->next); temp->next = NULL;

B. Node* temp = head; while (temp->next != NULL) temp = temp->next; free(temp);

C. free(head); head = NULL;

D. Node* temp = head; while (temp->next != NULL) temp = temp->next; temp->next = NULL;

Solution

1. Step 1: Check for empty or single-node list and traverse to second last node

   if (head == NULL || head->next == NULL) return; Node* temp = head; while (temp->next->next != NULL) temp = temp->next; free(temp->next); temp->next = NULL; checks if list is empty or single node, then moves to the node before last using temp->next->next != NULL.

2. Step 2: Free last node and update pointer

   It frees the last node and sets temp->next to NULL, correctly deleting the last node.

3. Final Answer:

   if (head == NULL || head->next == NULL) return; Node* temp = head; while (temp->next->next != NULL) temp = temp->next; free(temp->next); temp->next = NULL; -> Option A

4. Quick Check:

   Traverse to second last, free last, set next NULL = if (head == NULL || head->next == NULL) return; Node* temp = head; while (temp->next->next != NULL) temp = temp->next; free(temp->next); temp->next = NULL; [OK]

Hint: Stop at second last node before deleting last [OK]

Common Mistakes:

• Freeing node without updating previous node's next pointer
• Not checking for empty list
• Freeing head without traversal

3. What will be the output linked list after deleting the last node from the list: 10 -> 20 -> 30 -> 40 -> NULL using the standard delete at end operation?

medium

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

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

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

D. 10 -> 20 -> NULL

Solution

1. Step 1: Identify the last node

   The last node is 40, which points to NULL.

2. Step 2: Remove the last node and update the second last node's next pointer

   After deletion, 30 becomes the last node and its next pointer is set to NULL.

3. Final Answer:

   10 -> 20 -> 30 -> NULL -> Option B

4. Quick Check:

   Last node 40 removed = 10 -> 20 -> 30 -> NULL [OK]

Hint: Last node removed, second last points to NULL [OK]

Common Mistakes:

• Not updating second last node's next pointer
• Deleting wrong node (like first node)
• Output list unchanged

4. Consider the following code snippet to delete the last node of a singly linked list. What is the error?

void deleteEnd(Node** head) {
  if (*head == NULL) return;
  Node* temp = *head;
  while (temp->next != NULL) {
    temp = temp->next;
  }
  free(temp);
  temp = NULL;
}

medium

A. It does not update the second last node's next pointer to NULL

B. It frees the head node instead of the last node

C. It causes a segmentation fault by accessing NULL pointer

D. It deletes the first node instead of the last

Solution

1. Step 1: Analyze traversal logic

   The loop moves temp to the last node (where temp->next is NULL).

2. Step 2: Check pointer update after freeing

   It frees the last node but does not update the second last node's next pointer to NULL, leaving a dangling pointer.

3. Final Answer:

   It does not update the second last node's next pointer to NULL -> Option A

4. Quick Check:

   Missing next pointer update causes dangling pointer [OK]

Hint: Always update second last node's next to NULL after free [OK]

Common Mistakes:

• Freeing node without updating previous node's next
• Not handling empty list
• Incorrect loop condition causing crash

5. You have a singly linked list with nodes: 5 -> 15 -> 25 -> 35 -> 45 -> NULL. Write the steps to delete the last node and then insert a new node with value 50 at the end. What will be the final list?

hard

A. 5 -> 15 -> 25 -> 35 -> NULL

B. 5 -> 15 -> 25 -> 35 -> 45 -> 50 -> NULL

C. 5 -> 15 -> 25 -> 35 -> 50 -> NULL

D. 50 -> 5 -> 15 -> 25 -> 35 -> NULL

Solution

1. Step 1: Delete the last node (45)

   Traverse to second last node (35), free last node (45), set 35's next to NULL.

2. Step 2: Insert new node with value 50 at end

   Traverse to last node (35), create new node with 50, set 35's next to new node, new node's next to NULL.

3. Final Answer:

   5 -> 15 -> 25 -> 35 -> 50 -> NULL -> Option C

4. Quick Check:

   Delete last, then add 50 at end = 5 -> 15 -> 25 -> 35 -> 50 -> NULL [OK]

Hint: Delete last node, then add new node at end [OK]

Common Mistakes:

• Not updating pointers correctly after deletion
• Appending new node without deleting last
• Inserting new node at wrong position