Delete by Value in Doubly Linked List in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows moving forward and backward through the list.

beginner

What does 'delete by value' mean in a doubly linked list?

It means finding the first node that contains a specific value and removing it from the list, adjusting the links so the list stays connected.

intermediate

Which pointers need to be updated when deleting a node in a doubly linked list?

You update the previous node's next pointer to skip the deleted node and point to the deleted node's next node. Also, update the next node's previous pointer to point to the deleted node's previous node.

intermediate

What happens if the node to delete is the head of the doubly linked list?

The head pointer must be updated to the next node, and if that node exists, its previous pointer is set to null.

beginner

What should you do if the value to delete is not found in the doubly linked list?

No changes are made to the list because the value does not exist. The list remains the same.

In a doubly linked list, what does each node contain?

A Data, next pointer, previous pointer

B Data and next pointer only

C Data and previous pointer only

D Data only

Each node in a doubly linked list has data, a pointer to the next node, and a pointer to the previous node.

When deleting a node by value, what is the first step?

A Find the node with the given value

B Update the head pointer

C Set all pointers to null

D Create a new node

You must first find the node that contains the value to delete before removing it.

If the node to delete is the last node, what pointer must be updated?

A Head pointer

B Next node's previous pointer to null

C Previous node's next pointer to null

D No pointers need updating

The previous node's next pointer should be set to null to mark the end of the list.

What happens if you delete the only node in a doubly linked list?

A Next pointer points to null

B Head becomes null, list is empty

C List remains unchanged

D Head points to itself

Deleting the only node makes the list empty, so the head pointer becomes null.

Which pointer is NOT used in a doubly linked list node?

A Next pointer

B Data pointer

C Previous pointer

D Random pointer

A standard doubly linked list node does not have a random pointer.

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

Think about how the links before and after the node change.

Describe how pointers change when deleting the head node in a doubly linked list.

Focus on the first node and its neighbors.

Practice

1. What happens when you delete a node by value in a doubly linked list if the value is not found?

easy

A. An error is thrown and the program stops.

B. The first node is deleted regardless of the value.

C. All nodes are deleted from the list.

D. The list remains unchanged because no node matches the value.

Solution

1. Step 1: Search for the node with the given value

   Traverse the list from head to tail to find a node matching the value.

2. Step 2: Check if node is found

   If no node matches the value, no deletion occurs and the list stays the same.

3. Final Answer:

   The list remains unchanged because no node matches the value. -> Option D

4. Quick Check:

   Delete by value with no match = no change [OK]

Hint: If value not found, list stays same [OK]

Common Mistakes:

• Assuming deletion happens even if value not found
• Trying to delete head without checking value
• Expecting an error instead of no action

2. Which of the following is the correct way to update the previous pointer when deleting a middle node in a doubly linked list in C#?

easy

A. nodeToDelete.Next.Prev = nodeToDelete.Prev;

B. nodeToDelete.Prev.Next = nodeToDelete.Next.Prev;

C. nodeToDelete.Next = nodeToDelete.Prev.Next;

D. nodeToDelete.Prev = nodeToDelete.Next.Prev;

Solution

1. Step 1: Understand pointers in doubly linked list

   When deleting a node, the next node's previous pointer must point to the deleted node's previous node.

2. Step 2: Correct pointer update

   Setting nodeToDelete.Next.Prev = nodeToDelete.Prev correctly links the next node back to the previous node.

3. Final Answer:

   nodeToDelete.Next.Prev = nodeToDelete.Prev; -> Option A

4. Quick Check:

   Update next node's Prev to deleted node's Prev [OK]

Hint: Next node's Prev points to deleted node's Prev [OK]

Common Mistakes:

• Mixing up Next and Prev pointers
• Assigning Prev to Next's Prev incorrectly
• Not updating both Next and Prev pointers

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

medium

A. 10 <-> 20 <-> 30 <-> 40 <-> null

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

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

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

Solution

1. Step 1: Locate node with value 30

   Traverse the list and find the node containing 30 between 20 and 40.

2. Step 2: Remove node 30 and update pointers

   Set 20's Next to 40 and 40's Prev to 20, skipping 30.

3. Final Answer:

   10 <-> 20 <-> 40 <-> null -> Option B

4. Quick Check:

   Deleted 30, linked 20 and 40 directly [OK]

Hint: Remove node, link previous and next nodes [OK]

Common Mistakes:

• Not updating both Prev and Next pointers
• Leaving deleted node in the list
• Deleting wrong node by mistake

4. Consider this code snippet for deleting a node by value in a doubly linked list:

if (current == null) return;
if (current.Prev != null) current.Prev.Next = current.Next;
if (current.Next != null) current.Next.Prev = current.Prev;
if (current == head) head = current.Next;

What is the bug in this code?

medium

A. It may cause a null reference if current is null before checks.

B. It does not update the head when deleting the first node.

C. It does not handle deleting the last node correctly.

D. It does not remove the node from memory.

Solution

1. Step 1: Verify null safety

   The initial check if (current == null) return; and conditional pointer accesses prevent null references.

2. Step 2: Check head and pointer updates

   Head is updated correctly when deleting the first node, and pointers are linked properly for middle nodes.

3. Step 3: Identify the issue with last node

   The code sets current.Prev.Next = null but does not update a potential tail pointer to current.Prev, leaving the tail pointing to the deleted node.

4. Final Answer:

   It does not handle deleting the last node correctly. -> Option C

5. Quick Check:

   Missing tail update for last node deletion [OK]

Hint: Update tail pointer when deleting last node [OK]

Common Mistakes:

• Ignoring null checks before pointer access
• Not updating head when deleting first node
• Assuming garbage collection removes node immediately

5. You have a doubly linked list with nodes: 5 <-> 10 <-> 15 <-> 20 <-> 25 <-> null. You want to delete all nodes with values greater than 15. Which approach correctly deletes these nodes by value?

hard

A. Traverse from head, delete nodes with value > 15, updating pointers carefully.

B. Delete nodes by value without updating pointers, then reset head.

C. Only delete the first node with value > 15 and stop.

D. Traverse from tail, delete nodes with value < 15.

Solution

1. Step 1: Traverse list from head to tail

   Visit each node and check if its value is greater than 15.

2. Step 2: Delete matching nodes and update pointers

   For each node with value > 15, remove it by linking its previous node to its next node, updating both Next and Prev pointers.

3. Step 3: Continue traversal carefully

   After deletion, continue traversal without skipping nodes to delete all matching nodes.

4. Final Answer:

   Traverse from head, delete nodes with value > 15, updating pointers carefully. -> Option A

5. Quick Check:

   Delete all matching nodes with pointer updates [OK]

Hint: Traverse and delete nodes carefully, update pointers [OK]

Common Mistakes:

• Deleting nodes without updating pointers
• Stopping after first deletion
• Deleting wrong nodes by incorrect condition