Delete by Value in Doubly Linked List in DSA C# - Build from Scratch

Delete by Value in Doubly Linked List

📖 Scenario: You are managing a playlist of songs using a doubly linked list. Each song has a unique title. Sometimes, you want to remove a song from the playlist by its title.

🎯 Goal: Build a doubly linked list of songs and write code to delete a song by its title.

📋 What You'll Learn

Create a doubly linked list with exactly these songs in order: "SongA", "SongB", "SongC", "SongD", "SongE"

Create a string variable called songToDelete with the value "SongC"

Write code to delete the node with the title matching songToDelete from the doubly linked list

Print the titles of the songs in the list from head to tail after deletion, separated by " -> ", ending with " -> null"

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, browsers, and other apps to manage items where you can move forward and backward easily.

💼 Career

Understanding linked list operations like deletion is fundamental for software engineering roles that involve data structure manipulation and memory management.

1

Create the doubly linked list with songs

Create a doubly linked list class with nodes that hold a string Title. Then create a list with these exact songs in order: "SongA", "SongB", "SongC", "SongD", "SongE". Store the head node in a variable called head.

DSA C#

// Define Node class and create doubly linked list with songs
// Your code here

Hint

Start by defining a Node class with Title, Prev, and Next fields. Then create nodes for each song and link them in order.

2

Create the song title to delete

Create a string variable called songToDelete and set it to "SongC".

DSA C#

public class Node
{
    public string Title;
    public Node Prev;
    public Node Next;

    public Node(string title)
    {
        Title = title;
        Prev = null;
        Next = null;
    }
}

Node head = new Node("SongA");
Node nodeB = new Node("SongB");
Node nodeC = new Node("SongC");
Node nodeD = new Node("SongD");
Node nodeE = new Node("SongE");

head.Next = nodeB;
nodeB.Prev = head;
nodeB.Next = nodeC;
nodeC.Prev = nodeB;
nodeC.Next = nodeD;
nodeD.Prev = nodeC;
nodeD.Next = nodeE;
nodeE.Prev = nodeD;

// Create songToDelete variable
// Your code here

Hint

Just create a string variable named songToDelete and assign it the value "SongC".

3

Delete the node with the matching song title

Write code to delete the node from the doubly linked list whose Title matches the string in songToDelete. Update the links of the previous and next nodes accordingly. If the node to delete is the head, update head to the next node.

DSA C#

public class Node
{
    public string Title;
    public Node Prev;
    public Node Next;

    public Node(string title)
    {
        Title = title;
        Prev = null;
        Next = null;
    }
}

Node head = new Node("SongA");
Node nodeB = new Node("SongB");
Node nodeC = new Node("SongC");
Node nodeD = new Node("SongD");
Node nodeE = new Node("SongE");

head.Next = nodeB;
nodeB.Prev = head;
nodeB.Next = nodeC;
nodeC.Prev = nodeB;
nodeC.Next = nodeD;
nodeD.Prev = nodeC;
nodeD.Next = nodeE;
nodeE.Prev = nodeD;

string songToDelete = "SongC";

// Delete node with Title == songToDelete
// Your code here

Hint

Traverse the list starting from head. When you find the node with Title equal to songToDelete, update the Next of the previous node and the Prev of the next node to remove it. If it's the head node, update head.

4

Print the doubly linked list after deletion

Print the titles of the songs in the doubly linked list starting from head. Separate each title with " -> " and end the output with " -> null".

DSA C#

public class Node
{
    public string Title;
    public Node Prev;
    public Node Next;

    public Node(string title)
    {
        Title = title;
        Prev = null;
        Next = null;
    }
}

Node head = new Node("SongA");
Node nodeB = new Node("SongB");
Node nodeC = new Node("SongC");
Node nodeD = new Node("SongD");
Node nodeE = new Node("SongE");

head.Next = nodeB;
nodeB.Prev = head;
nodeB.Next = nodeC;
nodeC.Prev = nodeB;
nodeC.Next = nodeD;
nodeD.Prev = nodeC;
nodeD.Next = nodeE;
nodeE.Prev = nodeD;

string songToDelete = "SongC";

Node current = head;
while (current != null)
{
    if (current.Title == songToDelete)
    {
        if (current.Prev != null)
        {
            current.Prev.Next = current.Next;
        }
        else
        {
            head = current.Next;
        }
        if (current.Next != null)
        {
            current.Next.Prev = current.Prev;
        }
        break;
    }
    current = current.Next;
}

// Print the list from head
// Your code here

Hint

Start from head and move through each node using Next. Print each Title followed by " -> ". After the last node, print "null".

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