Delete from Beginning of Doubly Linked List in DSA Java - Time & Space Complexity

Time Complexity: Delete from Beginning of Doubly Linked List

O(1)

Understanding Time Complexity

We want to understand how long it takes to remove the first item from a doubly linked list.

How does the time needed change as the list grows larger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class DoublyLinkedList {
    Node head;

    void deleteFromBeginning() {
        if (head == null) return;
        head = head.next;
        if (head != null) {
            head.prev = null;
        }
    }
}
    

This code removes the first node from a doubly linked list by updating the head pointer and adjusting links.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Updating the head pointer and adjusting one or two links.
• How many times: This happens only once per call, no loops or recursion.

How Execution Grows With Input

Removing the first node always takes the same small number of steps, no matter how big the list is.

Input Size (n)	Approx. Operations
10	3-4 steps
100	3-4 steps
1000	3-4 steps

Pattern observation: The time stays the same no matter how large the list grows.

Final Time Complexity

Time Complexity: O(1)

This means deleting the first node takes a fixed amount of time regardless of list size.

Common Mistake

[X] Wrong: "Deleting from the beginning takes longer as the list grows because we have to move through the list."

[OK] Correct: We only change a few pointers at the start; we do not traverse the list at all.

Interview Connect

Knowing that deleting from the start of a doubly linked list is quick shows you understand how linked lists work and pointer updates.

Self-Check

"What if we had to delete the last node instead? How would the time complexity change?"

Practice

1. What happens to the head pointer when you delete the first node from a doubly linked list?

easy

A. It moves to the second node in the list

B. It becomes null immediately

C. It points to the last node

D. It remains unchanged

Solution

1. Step 1: Understand the head pointer role

   The head pointer always points to the first node in the doubly linked list.

2. Step 2: Effect of deleting the first node

   When the first node is deleted, the head pointer must move to the next node, which becomes the new first node.

3. Final Answer:

   It moves to the second node in the list -> Option A

4. Quick Check:

   Deleting first node updates head to next node [OK]

Hint: Head always points to first node; update it after deletion [OK]

Common Mistakes:

• Not updating head pointer after deletion
• Setting head to null even if list has more nodes
• Confusing head with tail pointer

2. Which of the following is the correct Java statement to update the prev pointer of the new head node after deleting the first node in a doubly linked list?

easy

A. head = null.prev;

B. head.next = null;

C. head.prev = null;

D. head.prev.next = null;

Solution

1. Step 1: Identify the new head node

   After deleting the first node, the new head is the second node in the list.

2. Step 2: Set the previous pointer of new head to null

   Since the new head is the first node now, its prev pointer must be null to indicate no node before it.

3. Final Answer:

   head.prev = null; -> Option C

4. Quick Check:

   New head prev pointer must be null [OK]

Hint: New head's prev pointer is always null [OK]

Common Mistakes:

• Setting head.next to null instead of head.prev
• Using invalid syntax like null.prev
• Trying to access prev.next which may cause error

3. Consider the following Java code snippet for deleting the first node from a doubly linked list:

if (head == null) return;
head = head.next;
if (head != null) head.prev = null;

What will be the state of the list after deleting the first node from the list 1 <-> 2 <-> 3?

medium

A. 1 <-> 3

B. Empty list

C. 3 only

D. 2 <-> 3

Solution

1. Step 1: Initial list and deletion

   The list is 1 <-> 2 <-> 3. Deleting the first node means removing node with value 1.

2. Step 2: Update head and prev pointer

   Head moves to node 2. Then head.prev is set to null, so node 2 has no previous node.

3. Final Answer:

   2 <-> 3 -> Option D

4. Quick Check:

   First node removed, head points to second node [OK]

Hint: After deletion, head points to second node, prev set null [OK]

Common Mistakes:

• Not setting head.prev to null causing broken links
• Assuming node 1 still exists
• Confusing next and prev pointers

4. The following code attempts to delete the first node from a doubly linked list but causes a runtime error:

if (head == null) return;
head = head.next;
head.prev = null;

What is the error and how to fix it?

medium

A. Syntax error due to missing semicolon; add semicolon after head.prev = null

B. NullPointerException when list has one node; fix by checking if head is null before setting head.prev

C. Logical error: head should not be updated; remove head = head.next

D. No error; code works fine

Solution

1. Step 1: Identify cause of NullPointerException

   If the list has only one node, after head = head.next, head becomes null. Then head.prev causes NullPointerException.

2. Step 2: Fix by checking head before accessing prev

   Add a condition to check if head is not null before setting head.prev = null.

3. Final Answer:

   NullPointerException when list has one node; fix by checking if head is null before setting head.prev -> Option B

4. Quick Check:

   Always check for null before accessing object members [OK]

Hint: Check head != null before head.prev = null to avoid errors [OK]

Common Mistakes:

• Not checking for null causing runtime errors
• Assuming list always has multiple nodes
• Ignoring exception details

5. You have a doubly linked list with nodes 10 <-> 20 <-> 30 <-> 40. After deleting the first node twice using the correct deletion method, what will be the printed list?

hard

A. 30 <-> 40

B. 20 <-> 30 <-> 40

C. 40 only

D. Empty list

Solution

1. Step 1: First deletion

   Deleting first node removes 10. Head moves to 20, head.prev set to null. List becomes 20 <-> 30 <-> 40.

2. Step 2: Second deletion

   Deleting first node again removes 20. Head moves to 30, head.prev set to null. List becomes 30 <-> 40.

3. Final Answer:

   30 <-> 40 -> Option A

4. Quick Check:

   Two deletions remove first two nodes [OK]

Hint: Each deletion moves head forward and updates prev to null [OK]

Common Mistakes:

• Deleting only once instead of twice
• Not updating head.prev after second deletion
• Printing old list without changes