Delete Node at End in DSA C - Time & Space Complexity

Time Complexity: Delete Node at End

O(n)

Understanding Time Complexity

When we delete a node at the end of a linked list, we want to know how the time taken changes as the list grows.

We ask: How much work does the computer do when the list gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

This code deletes the last node of a singly linked list by traversing to the second last node and removing the last.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that moves through the list nodes.
• How many times: It runs once for each node until the second last node, so roughly n-1 times for n nodes.

How Execution Grows With Input

As the list gets longer, the loop runs more times, almost once per node except the last.

Input Size (n)	Approx. Operations
10	About 9 steps through nodes
100	About 99 steps through nodes
1000	About 999 steps through nodes

Pattern observation: The work grows roughly in a straight line with the number of nodes.

Final Time Complexity

Time Complexity: O(n)

This means the time to delete the last node grows directly with the list size.

Common Mistake

[X] Wrong: "Deleting the last node is always quick because it's at the end."

[OK] Correct: We must find the node before the last, which takes time proportional to the list length.

Interview Connect

Understanding this helps you explain how linked lists work and why some operations take longer as data grows.

Self-Check

"What if we kept a pointer to the last node? How would the time complexity change when deleting the last node?"

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