Why Doubly Linked List Over Singly Linked List in DSA C# - See It Work

Why Doubly Linked List Over Singly Linked List

📖 Scenario: Imagine you are managing a playlist of songs where you want to move forward and backward easily between songs.

🎯 Goal: You will build a simple doubly linked list to see how it allows moving both forward and backward, unlike a singly linked list.

📋 What You'll Learn

Create a doubly linked list with three nodes containing values 10, 20, and 30

Add a variable to keep track of the current node starting at the head

Write code to move the current node forward and then backward

Print the values of the current node after moving forward and backward

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players, web browsers (back and forward buttons), and undo-redo systems where moving both ways is needed.

💼 Career

Understanding doubly linked lists helps in software development roles that involve data structure optimization and efficient navigation through data.

1

Create a doubly linked list with three nodes

Create a class called Node with int data, Node next, and Node prev. Then create three nodes with values 10, 20, and 30. Link them so that node1.next = node2, node2.prev = node1, node2.next = node3, and node3.prev = node2. Finally, create a variable head pointing to node1.

DSA C#

// Your code here to create Node class and link three nodes

Hint

Define a Node class with data, next, and prev fields. Then create and link three nodes.

2

Add a variable to track the current node

Create a variable called current and set it to head to start at the first node.

DSA C#

public class Node
{
    public int data;
    public Node next;
    public Node prev;

    public Node(int data)
    {
        this.data = data;
        this.next = null;
        this.prev = null;
    }
}

Node node1 = new Node(10);
Node node2 = new Node(20);
Node node3 = new Node(30);

node1.next = node2;
node2.prev = node1;
node2.next = node3;
node3.prev = node2;

Node head = node1;

// Create a variable current and set it to head
// Your code here

Hint

Just create a variable current and assign it the value of head.

3

Move current node forward and backward

Move the current node forward by setting current = current.next. Then move it backward by setting current = current.prev.

DSA C#

current = current.next;
current = current.prev;
// Print current.data

Hint

Use current = current.next to move forward and current = current.prev to move backward.

4

Print the current node's data after moves

Print the value of current.data after moving forward and backward using Console.WriteLine(current.data).

DSA C#

Hint

Use Console.WriteLine(current.data) to print the current node's value after each move.

Practice

1. Why might you choose a doubly linked list over a singly linked list in a program?

easy

A. Because it allows traversal in both directions easily

B. Because it uses less memory than singly linked list

C. Because it is simpler to implement

D. Because it only stores one pointer per node

Solution

1. Step 1: Understand traversal capabilities

   A doubly linked list stores two pointers per node: one to the next node and one to the previous node, allowing movement forward and backward.

2. Step 2: Compare with singly linked list

   A singly linked list stores only one pointer per node, allowing traversal only in one direction (forward).

3. Final Answer:

   Because it allows traversal in both directions easily -> Option A

4. Quick Check:

   Traversal both ways = A [OK]

Hint: Doubly means two directions for easy backward moves [OK]

Common Mistakes:

• Thinking doubly linked list uses less memory
• Assuming singly linked list supports backward traversal
• Confusing simplicity with functionality

2. Which of the following is the correct way to declare a node class for a doubly linked list in C#?

easy

A. public class Node { public int data; public Node next; }

B. public class Node { public int data; public Node next; public Node prev; }

C. public class Node { public int data; public Node prev; }

D. public class Node { public int data; public Node previous; }

Solution

1. Step 1: Identify required pointers for doubly linked list node

   A doubly linked list node must have a pointer to the next node and a pointer to the previous node.

2. Step 2: Check each option for correct pointers

   public class Node { public int data; public Node next; public Node prev; } declares both next and prev pointers, matching the doubly linked list requirements.

3. Final Answer:

   public class Node { public int data; public Node next; public Node prev; } -> Option B

4. Quick Check:

   Two pointers (next, prev) = C [OK]

Hint: Doubly linked list node has next and prev pointers [OK]

Common Mistakes:

• Missing the prev pointer for doubly linked list
• Using incorrect pointer names like previous instead of prev
• Confusing singly linked list node with doubly linked list node

3. Consider the following C# code snippet for a doubly linked list:

Node head = new Node(1);
Node second = new Node(2);
Node third = new Node(3);

head.next = second;
second.prev = head;
second.next = third;
third.prev = second;

// Now move backward from third node
Node current = third;
while (current != null) {
    Console.Write(current.data + " -> ");
    current = current.prev;
}
Console.WriteLine("null");

What will be the printed output?

medium

A. 3 -> 2 -> 1 -> null

B. 1 -> 2 -> 3 -> null

C. null

D. 3 -> null

Solution

1. Step 1: Understand the list connections

   The nodes are connected as 1 <-> 2 <-> 3, with prev and next pointers set correctly.

2. Step 2: Trace backward traversal from third node

   Starting at node with data 3, moving prev goes to 2, then 1, then null, printing each data value.

3. Final Answer:

   3 -> 2 -> 1 -> null -> Option A

4. Quick Check:

   Backward traversal prints 3,2,1 [OK]

Hint: Backward traversal prints nodes in reverse order [OK]

Common Mistakes:

• Confusing forward and backward traversal output
• Forgetting to print 'null' at the end
• Assuming singly linked list traversal

4. The following code attempts to insert a new node after a given node in a doubly linked list. What is the bug?

void InsertAfter(Node prevNode, int newData) {
    if (prevNode == null) return;
    Node newNode = new Node(newData);
    newNode.next = prevNode.next;
    prevNode.next = newNode;
    newNode.prev = prevNode;
}

medium

A. It incorrectly assigns newNode.prev

B. It does not create the new node correctly

C. It does not check if prevNode is null

D. It does not update the prev pointer of the next node

Solution

1. Step 1: Analyze pointer updates after insertion

   The new node's next pointer is set to prevNode's next, and prevNode's next is updated to newNode, but the next node's prev pointer is not updated.

2. Step 2: Identify missing update for next node's prev pointer

   Without updating newNode.next.prev to point back to newNode, the list's backward links break.

3. Final Answer:

   It does not update the prev pointer of the next node -> Option D

4. Quick Check:

   Missing next node's prev update = A [OK]

Hint: Always update both next and prev pointers when inserting [OK]

Common Mistakes:

• Forgetting to update next node's prev pointer
• Assuming newNode.prev is enough
• Ignoring null checks for next node

5. You want to implement a feature that frequently deletes nodes from the middle of a linked list and also needs to traverse backward efficiently. Which linked list type is best and why?

hard

A. Singly linked list, because it uses less memory

B. Array list, because it supports random access

C. Doubly linked list, because it allows backward traversal and easy node deletion

D. Circular singly linked list, because it loops infinitely

Solution

1. Step 1: Consider deletion and traversal requirements

   Deleting nodes from the middle requires access to previous nodes to update pointers, and backward traversal needs two-way links.

2. Step 2: Match requirements with linked list types

   Doubly linked lists store both next and prev pointers, enabling efficient backward traversal and easy deletion without full traversal.

3. Final Answer:

   Doubly linked list, because it allows backward traversal and easy node deletion -> Option C

4. Quick Check:

   Backward traversal + easy deletion = D [OK]

Hint: Use doubly linked list for easy backward moves and deletions [OK]

Common Mistakes:

• Choosing singly linked list despite backward traversal need
• Confusing circular list benefits
• Ignoring memory vs functionality trade-offs