Delete Node by Value in DSA C - Time & Space Complexity

Time Complexity: Delete Node by Value

O(n)

Understanding Time Complexity

We want to know how the time needed to delete a node by value changes as the list grows.

How does the number of steps grow when the list gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

void deleteNodeByValue(Node** head, int value) {
    Node* temp = *head;
    Node* prev = NULL;
    while (temp != NULL && temp->data != value) {
        prev = temp;
        temp = temp->next;
    }
    if (temp == NULL) return; // value not found
    if (prev == NULL) *head = temp->next; // delete head
    else prev->next = temp->next;
    free(temp);
}
    

This code searches for a node with a given value and removes it from the linked list.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that moves through the list nodes one by one.
• How many times: Up to n times, where n is the number of nodes in the list.

How Execution Grows With Input

As the list gets longer, the search takes more steps because it may check more nodes.

Input Size (n)	Approx. Operations
10	Up to 10 checks
100	Up to 100 checks
1000	Up to 1000 checks

Pattern observation: The number of steps grows roughly in direct proportion to the list size.

Final Time Complexity

Time Complexity: O(n)

This means the time to delete a node grows linearly with the number of nodes in the list.

Common Mistake

[X] Wrong: "Deleting a node by value always takes constant time because we just remove it."

[OK] Correct: We must first find the node by checking nodes one by one, which can take time proportional to the list size.

Interview Connect

Understanding how searching affects deletion time helps you explain linked list operations clearly and confidently.

Self-Check

"What if the list was doubly linked? How would the time complexity of deleting a node by value change?"

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