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

Concept Flow - Why Doubly Linked List Over Singly Linked List

Start with Singly Linked List

↓

Can only move forward

↓

Operations like backward traversal or easy deletion

↓

Hard or inefficient

↓

Use Doubly Linked List

↓

Nodes have prev and next pointers

↓

Can move forward and backward easily

↓

Efficient deletion and traversal both ways

↓

More memory used but more flexible

Shows why singly linked lists limit movement to forward only, making some operations hard, and how doubly linked lists add backward links to solve this.

Execution Sample

DSA C#

Node head = new Node(1);
head.next = new Node(2);
head.next.next = new Node(3);
// Singly linked list
// Trying to delete node 2 requires traversal from head

// Doubly linked list nodes have prev and next pointers
// Deletion is easier with direct access to prev

Shows creation of singly linked list and notes difficulty deleting a middle node without backward links; doubly linked list solves this.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create singly linked list with nodes 1 -> 2 -> 3	1 -> 2 -> 3	head points to node 1	1 -> 2 -> 3 -> null
2	Attempt to delete node 2 in singly linked list	1 -> 2 -> 3	Traverse from head to node 1, then node 2	Need to update node 1's next to node 3
3	Delete node 2 by changing node 1's next to node 3	1 -> 3	node 1.next = node 3	1 -> 3 -> null
4	Create doubly linked list with nodes 1 <-> 2 <-> 3	1 <-> 2 <-> 3	head points to node 1; node 2.prev = node 1; node 2.next = node 3	null <- 1 <-> 2 <-> 3 -> null
5	Delete node 2 in doubly linked list	1 <-> 3	node 1.next = node 3; node 3.prev = node 1	null <- 1 <-> 3 -> null
6	Traverse backward from node 3 to node 1	1 <-> 3	Using prev pointers	3 -> 1
7	End	1 <-> 3	No changes	null <- 1 <-> 3 -> null

💡 Traversal and deletion complete; doubly linked list allows backward movement and easier deletion.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
head	null	points to node 1	points to node 1	points to node 1	points to node 1	points to node 1
node1.next	null	points to node 2	points to node 3	points to node 2	points to node 3	points to node 3
node2.prev	null	null	null	points to node 1	null	null
node2.next	null	points to node 3	null	points to node 3	null	null
node3.prev	null	null	null	points to node 2	points to node 1	points to node 1

Key Moments - 3 Insights

Why is deleting a middle node harder in a singly linked list?

Because you cannot move backward, you must start from head and find the previous node to update its next pointer, as shown in execution_table step 2 and 3.

How does a doubly linked list make backward traversal possible?

Each node has a prev pointer pointing to the previous node, allowing movement backward as shown in execution_table step 6.

Does a doubly linked list use more memory than a singly linked list?

Yes, because each node stores an extra pointer (prev), but this trade-off allows more flexible operations, as seen in the pointer changes in steps 4 and 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the visual state after deleting node 2 in the singly linked list?

A 1 -> 2 -> 3 -> null

B 1 -> 3 -> null

C null <- 1 <-> 3 -> null

D 1 <-> 2 <-> 3

💡 Hint

Refer to execution_table row 3 under Visual State column.

At which step does the doubly linked list allow backward traversal?

A Step 6

B Step 2

C Step 3

D Step 5

💡 Hint

Check execution_table step 6 describing backward traversal.

If the doubly linked list did not have prev pointers, which operation would be affected?

A Creating nodes

B Forward traversal

C Backward traversal

D Setting head pointer

💡 Hint

Look at variable_tracker showing prev pointers and execution_table step 6.

Concept Snapshot

Singly linked lists have nodes with only next pointers, allowing forward movement only.
Doubly linked lists add prev pointers to nodes, enabling backward traversal.
Backward links make deletion and reverse traversal easier and more efficient.
Doubly linked lists use more memory but offer more flexibility.
Use doubly linked lists when you need two-way traversal or easy node deletion.

Full Transcript

A singly linked list lets you move only forward through nodes. This makes some tasks, like deleting a middle node, harder because you must find the previous node by starting at the head. A doubly linked list adds a backward pointer (prev) to each node. This lets you move both forward and backward easily. Deleting nodes becomes simpler because you can access the previous node directly. However, doubly linked lists use more memory because of the extra pointer. The trade-off is more flexible and efficient operations. The execution steps show creating both lists, deleting a node, and traversing backward in the doubly linked list.

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