Delete Node by Value in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal when deleting a node by value in a linked list?

To find the first node with the given value and remove it from the list, adjusting pointers so the list remains connected.

beginner

In a singly linked list, what special case must you handle when deleting a node by value?

If the node to delete is the head (first node), you must update the head pointer to the next node.

beginner

Why do we keep track of the previous node when deleting a node by value in a singly linked list?

Because to remove the current node, we need to change the previous node's next pointer to skip the current node.

beginner

What happens if the value to delete is not found in the linked list?

No nodes are removed; the list stays the same.

beginner

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

O(n), where n is the number of nodes, because you may need to traverse the entire list to find the node.

What pointer do you update to remove a node in a singly linked list?

A The head pointer always

B The previous node's next pointer

C The current node's next pointer

D The tail pointer

You update the previous node's next pointer to skip the node to be deleted.

If the node to delete is the head node, what should you do?

A Set head to null

B Do nothing

C Set head to previous node

D Set head to head.next

You update the head pointer to the next node to remove the first node.

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

A No change to the list

B The last node is deleted

C The list becomes empty

D An error is thrown

If the value is not found, the list remains unchanged.

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

A O(n)

B O(1)

C O(log n)

D O(n^2)

You may need to traverse the entire list, so it is O(n).

Which of these is NOT needed when deleting a node by value in a singly linked list?

A Pointer to next node

B Pointer to current node

C Pointer to tail node

D Pointer to previous node

The tail pointer is not required for deletion by value.

Explain step-by-step how to delete a node by value in a singly linked list.

Think about how pointers change when removing a node.

What special cases must you consider when deleting a node by value in a linked list?

Consider where the node is located and if the list is empty.

Practice

1. What does the operation Delete Node by Value do in a linked list?

easy

A. Adds a new node with the given value at the end of the list

B. Finds the node with the given value and returns it without removing

C. Removes the first node that contains the given value from the list

D. Reverses the linked list

Solution

1. Step 1: Understand the operation

   Deleting a node by value means searching for the first node that matches the value and removing it from the linked list.

2. Step 2: Compare options

   Only Removes the first node that contains the given value from the list describes removing the first node with the given value. Others describe adding, finding without removing, or reversing, which are different operations.

3. Final Answer:

   Removes the first node that contains the given value from the list -> Option C

4. Quick Check:

   Delete Node by Value = Remove first matching node [OK]

Hint: Deleting by value removes first matching node only [OK]

Common Mistakes:

• Confusing deletion by value with adding a node
• Thinking it removes all nodes with that value
• Assuming it reverses the list

2. Which of the following is the correct C# syntax to delete a node by value from a singly linked list?

easy

A. void DeleteNodeByValue(int value) { /* code */ }

B. public void DeleteNode(int value) { /* code */ }

C. public int DeleteNode(int value) { /* code */ }

D. DeleteNode(int value) => { /* code */ }

Solution

1. Step 1: Check method signature conventions in C#

   In C#, methods usually have an access modifier like 'public' and a return type. Deleting a node usually returns void.

2. Step 2: Analyze options

   public void DeleteNode(int value) { /* code */ } uses 'public void DeleteNode(int value)', which is correct. void DeleteNodeByValue(int value) { /* code */ } misses access modifier. public int DeleteNode(int value) { /* code */ } returns int which is uncommon here. DeleteNode(int value) => { /* code */ } uses invalid lambda syntax for method.

3. Final Answer:

   public void DeleteNode(int value) { /* code */ } -> Option B

4. Quick Check:

   Correct method signature = public void DeleteNode(int value) { /* code */ } [OK]

Hint: Use public void method with parameter for deletion [OK]

Common Mistakes:

• Omitting access modifier
• Using wrong return type
• Incorrect method syntax

3. Given the linked list: 10 -> 20 -> 30 -> 40 -> null, what is the list after deleting the node with value 30 using the DeleteNodeByValue method?

medium

A. 10 -> 20 -> 40 -> null

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

C. 20 -> 30 -> 40 -> null

D. 10 -> 30 -> 40 -> null

Solution

1. Step 1: Identify the node to delete

   The node with value 30 is the third node in the list.

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

   After deleting 30, the node with value 20 points directly to 40, so the list becomes 10 -> 20 -> 40 -> null.

3. Final Answer:

   10 -> 20 -> 40 -> null -> Option A

4. Quick Check:

   Deleted 30, list skips it [OK]

Hint: Remove node and link previous to next node [OK]

Common Mistakes:

• Not updating links after deletion
• Deleting wrong node
• Leaving deleted node in list

4. Consider this code snippet to delete a node by value in a singly linked list:

public void DeleteNode(int value) {
  Node current = head;
  Node previous = null;
  while (current != null && current.data != value) {
    previous = current;
    current = current.next;
  }
  if (current == null) return;
  previous.next = current.next;
}

What error will occur when deleting the head node?

medium

A. Compilation error due to missing return type

B. No error, deletion works fine

C. Infinite loop because current never changes

D. NullReferenceException because previous is null when deleting head

Solution

1. Step 1: Analyze deletion of head node

   If the node to delete is head, previous is still null because loop never runs.

2. Step 2: Check assignment previous.next = current.next

   Since previous is null, trying to access previous.next causes NullReferenceException.

3. Final Answer:

   NullReferenceException because previous is null when deleting head -> Option D

4. Quick Check:

   Deleting head needs special case to avoid null error [OK]

Hint: Handle head deletion separately to avoid null errors [OK]

Common Mistakes:

• Not checking if node to delete is head
• Assuming previous is never null
• Ignoring null pointer exceptions

5. You have a singly linked list: 5 -> 10 -> 15 -> 20 -> 25 -> null. You want to delete the node with value 5 (head), then delete the node with value 20. What will the list look like after these two deletions?

hard

A. 10 -> 15 -> 25 -> null

B. 5 -> 10 -> 15 -> 25 -> null

C. 10 -> 15 -> 20 -> 25 -> null

D. 15 -> 25 -> null

Solution

1. Step 1: Delete head node with value 5

   Removing head means head moves to next node, so list becomes 10 -> 15 -> 20 -> 25 -> null.

2. Step 2: Delete node with value 20

   Find node 20 and remove it by linking 15 to 25, resulting in 10 -> 15 -> 25 -> null.

3. Final Answer:

   10 -> 15 -> 25 -> null -> Option A

4. Quick Check:

   Deleted head and middle node correctly [OK]

Hint: Delete head first, then link previous to next for others [OK]

Common Mistakes:

• Not updating head after deleting first node
• Not linking nodes correctly after deletion
• Deleting wrong nodes