Delete Node by Value in DSA C - Execution Trace

Concept Flow - Delete Node by Value

Start at head

↓

Check if head is NULL

↓

Stop

↓

Delete head

↓

Update head

↓

Stop

↓

Delete node

↓

Stop

Start from the head node, check if it matches the value to delete. If yes, delete head. Otherwise, traverse to find the node and delete it if found.

Execution Sample

DSA C

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

void deleteNode(struct Node** head, int val) {
  // delete node with value val
}

Deletes the first node with the given value from a singly linked list.

Execution Table

Step	Current Node Value	Condition	Action	List State
1	10	head != NULL	Check head value	10 -> 20 -> 30 -> 40 -> NULL
2	10	10 == 20?	No, move to next	10 -> 20 -> 30 -> 40 -> NULL
3	20	20 == 20?	Yes, delete this node	10 -> 30 -> 40 -> NULL
4	30	Continue traversal?	No, node deleted	10 -> 30 -> 40 -> NULL
5	NULL	End of list	Stop	10 -> 30 -> 40 -> NULL

💡 Node with value 20 found and deleted, traversal stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
head	10 -> 20 -> 30 -> 40 -> NULL	10 -> 20 -> 30 -> 40 -> NULL	10 -> 20 -> 30 -> 40 -> NULL	10 -> 30 -> 40 -> NULL	10 -> 30 -> 40 -> NULL
current	10	10	20	20	30
previous	NULL	NULL	10	10	10

Key Moments - 3 Insights

Why do we check if the head node contains the value before traversing?

Because if the head node has the value, we must update the head pointer to the next node. This is shown in execution_table steps 1-2 where the head value is checked.

What happens if the value is not found in the list?

The traversal reaches the end (NULL) without deleting any node, as shown in execution_table step 5 where the list remains unchanged and the function stops.

Why do we keep track of the previous node during traversal?

To update the previous node's next pointer to skip the deleted node. This is shown in variable_tracker where 'previous' points to the node before 'current' to adjust links.

Visual Quiz - 3 Questions

Look at the execution_table, what is the list state after deleting the node with value 20?

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

B 10 -> 30 -> 40 -> NULL

C 20 -> 30 -> 40 -> NULL

D 10 -> 40 -> NULL

💡 Hint

Check the 'List State' column at step 3 in execution_table.

At which step does the traversal stop after deleting the node?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column where it says 'node deleted' and traversal stops.

If the value to delete was 10 (head node), what would happen to the head pointer?

A It stays the same

B It moves to the next node

C It becomes NULL immediately

D It points to the deleted node

💡 Hint

Refer to key_moments about deleting the head node and updating the head pointer.

Concept Snapshot

Delete Node by Value in Linked List:
- Start at head node
- If head matches value, update head to next
- Else traverse list with previous and current pointers
- When node found, link previous->next to current->next
- Free deleted node
- Stop when node deleted or end reached

Full Transcript

To delete a node by value in a singly linked list, start checking from the head node. If the head node contains the value, update the head pointer to the next node and delete the head. Otherwise, traverse the list keeping track of the previous and current nodes. When the node with the target value is found, update the previous node's next pointer to skip the current node, then delete the current node. If the value is not found, the list remains unchanged. This process ensures the linked list remains connected and valid after deletion.

Practice

1. What is the main purpose of the Delete Node by Value operation in a linked list?

easy

A. To remove the first node that contains the given value from the list

B. To add a new node with the given value at the end of the list

C. To find the position of a node with the given value

D. To reverse the linked list

Solution

1. Step 1: Understand the operation's goal

   The operation aims to remove a node that matches a specific value from the linked list.

2. Step 2: Identify the correct description

   Removing the first node with the given value matches To remove the first node that contains the given value from the list, while others describe different operations.

3. Final Answer:

   To remove the first node that contains the given value from the list -> Option A

4. Quick Check:

   Delete Node by Value = Remove matching node [OK]

Hint: Deleting by value means removing the first matching node [OK]

Common Mistakes:

• Confusing deletion by value with insertion
• Thinking it reverses the list
• Assuming it finds position only

2. Which of the following is the correct way to start deleting a node by value in a singly linked list in C?

easy

A. Reverse the list before deletion

B. Directly free the head pointer without checks

C. Create a new node with the value to delete

D. Set a pointer to the head and check if head's value matches

Solution

1. Step 1: Understand deletion start

   We must check if the head node itself contains the value to delete before moving forward.

2. Step 2: Identify correct approach

   Setting a pointer to head and checking its value is the correct first step; other options are incorrect or irrelevant.

3. Final Answer:

   Set a pointer to the head and check if head's value matches -> Option D

4. Quick Check:

   Start at head to delete by value [OK]

Hint: Always check head node first when deleting by value [OK]

Common Mistakes:

• Freeing head without checking value
• Creating new nodes unnecessarily
• Reversing list before deletion

3. Consider the following linked list: 10 -> 20 -> 30 -> 40 -> NULL. After deleting the node with value 30, what will be the new list?

medium

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

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

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

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

Solution

1. Step 1: Locate node with value 30

   Traverse the list: 10, 20, then 30 is found as the third node.

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

   Link node with value 20 directly to node with value 40, skipping 30.

3. Final Answer:

   10 -> 20 -> 40 -> NULL -> Option B

4. Quick Check:

   Deleted 30, list skips it [OK]

Hint: Link previous node to next to remove target node [OK]

Common Mistakes:

• Not updating previous node's next pointer
• Deleting wrong node
• Leaving deleted node in list

4. Given this code snippet to delete a node by value, what is the main error?

void deleteNode(Node** head, int val) {
  Node* temp = *head;
  Node* prev = NULL;
  while (temp != NULL && temp->data != val) {
    prev = temp;
    temp = temp->next;
  }
  free(temp);
  if (prev != NULL) {
    prev->next = temp->next;
  } else {
    *head = temp->next;
  }
}

medium

A. prev pointer is never updated

B. The loop condition is incorrect and never runs

C. free(temp) is called before updating pointers, causing invalid access

D. The function does not handle empty list

Solution

1. Step 1: Analyze order of free and pointer update

   The code frees temp before updating prev->next or head, so temp->next access is invalid.

2. Step 2: Identify correct fix

   Pointers must be updated before freeing the node to avoid accessing freed memory.

3. Final Answer:

   free(temp) is called before updating pointers, causing invalid access -> Option C

4. Quick Check:

   Free after pointer update to avoid errors [OK]

Hint: Update pointers before freeing node to avoid invalid access [OK]

Common Mistakes:

• Freeing node before fixing links
• Ignoring NULL list case
• Not updating prev pointer

5. You have a singly linked list: 5 -> 10 -> 15 -> 20 -> 25 -> NULL. Write a function to delete all nodes with value greater than 15. What will be the final list after deletion?

hard

A. 5 -> 10 -> 15 -> NULL

B. 5 -> 10 -> 20 -> 25 -> NULL

C. 15 -> 20 -> 25 -> NULL

D. 5 -> 10 -> 15 -> 20 -> 25 -> NULL

Solution

1. Step 1: Identify nodes with value > 15

   Nodes with values 20 and 25 are greater than 15 and must be deleted.

2. Step 2: Delete nodes and link remaining nodes

   After removing 20 and 25, the list becomes 5 -> 10 -> 15 -> NULL.

3. Final Answer:

   5 -> 10 -> 15 -> NULL -> Option A

4. Quick Check:

   Removed nodes > 15, list ends at 15 [OK]

Hint: Delete nodes by checking value > 15 and relink list [OK]

Common Mistakes:

• Deleting wrong nodes
• Not updating links after deletion
• Assuming only one node deleted