Delete from Beginning of Doubly Linked List in DSA Go - Execution Trace

Concept Flow - Delete from Beginning of Doubly Linked List

Start at head node

↓

Check if list is empty?

↓

Move head to head.next

↓

Set new head.prev = null

↓

Old head node removed

↓

Done

Start at the head node, check if list is empty. If not, move head to next node and update pointers to remove old head.

Execution Sample

DSA Go

func deleteFromBeginning(head *Node) *Node {
    if head == nil {
        return nil
    }
    newHead := head.next
    if newHead != nil {
        newHead.prev = nil
    }
    return newHead
}

Deletes the first node of a doubly linked list and returns the new head.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Start with list	10 <-> 20 <-> 30	head points to node 10	┌────────┐ ┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ data:30│ │ prev:∅ │←── │ prev:10 │←── │ prev:20 │ │ next:20│──→ │ next:30 │──→ │ next:∅ │ └────────┘ └────────┘ └────────┘ head
2	Check if list empty	10 <-> 20 <-> 30	head != nil, continue	Same as step 1
3	Move head to head.next	20 <-> 30	head updated to node 20	┌────────┐ ┌────────┐ │ data:20│ │ data:30│ │ prev:10│←── │ prev:20 │←── │ next:30│──→ │ next:∅ │ └────────┘ └────────┘ head
4	Set new head.prev = nil	20 <-> 30	node 20.prev set to ∅	┌────────┐ ┌────────┐ │ data:20│ │ data:30│ │ prev:∅ │←── │ prev:20 │←── │ next:30│──→ │ next:∅ │ └────────┘ └────────┘ head
5	Old head removed	20 <-> 30	Old node 10 disconnected	┌────────┐ ┌────────┐ │ data:20│ │ data:30│ │ prev:∅ │←── │ prev:20 │←── │ next:30│──→ │ next:∅ │ └────────┘ └────────┘ head
6	Return new head	20 <-> 30	head points to node 20	Same as step 5

💡 Old head node removed, new head updated, list updated correctly

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
head	node 10	node 20	node 20	node 20
head.prev	∅	10	∅	∅
head.next	node 20	node 30	node 30	node 30
oldHead	node 10	node 10	node 10	removed

Key Moments - 3 Insights

Why do we set new head.prev to null after moving head?

Because the new head is now the first node, it should not point back to the old head. See step 4 in execution_table where new head.prev is set to ∅.

What happens if the list is empty when we try to delete?

The function checks if head is nil at step 2 and returns nil immediately, so no deletion happens.

Why do we not need to update the old head's next or prev pointers explicitly?

Because after moving head and updating new head.prev, the old head is disconnected by losing references and will be removed by garbage collection. See step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of head after step 3?

A node 10

B node 20

C nil

D node 30

💡 Hint

Check the 'Pointer Changes' column at step 3 in execution_table

At which step does the new head.prev pointer get set to null?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Operation' and 'Pointer Changes' columns in execution_table

If the list was empty initially, what would the function return?

A The original head node

B A new node

C nil

D An error

💡 Hint

See step 2 where the function checks if head is nil

Concept Snapshot

Delete from Beginning of Doubly Linked List:
- Check if list is empty (head == nil)
- Move head to head.next
- Set new head.prev = nil
- Old head node is disconnected and removed
- Return new head
- Handles empty list safely

Full Transcript

To delete the first node in a doubly linked list, start at the head. If the list is empty, return nil immediately. Otherwise, move the head pointer to the next node. Then set the new head's prev pointer to null to disconnect it from the old head. The old head node is now disconnected and will be removed by garbage collection. Finally, return the new head pointer. This process safely updates the list and handles empty lists without errors.

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 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:

   Head points to new first node after deletion [OK]

Hint: Head always points to the first node [OK]

Common Mistakes:

• Thinking head becomes null without checking list emptiness
• Assuming head points to last node after deletion
• Not updating head pointer after deletion

2. Which of the following Go code snippets correctly deletes the first node from a doubly linked list named dll?

easy

A. dll.head.prev = nil; dll.head = dll.head.next

B. dll.head = dll.head.next; if dll.head != nil { dll.head.prev = nil }

C. dll.head.next.prev = nil; dll.head = dll.head.next

D. dll.head = nil; dll.tail = nil

Solution

1. Step 1: Move head to next node

   Set dll.head = dll.head.next to remove first node reference.

2. Step 2: Update new head's prev pointer

   If new head exists, set dll.head.prev = nil to detach backward link.

3. Final Answer:

   dll.head = dll.head.next; if dll.head != nil { dll.head.prev = nil } -> Option B

4. Quick Check:

   Correct pointer updates for deletion [OK]

Hint: Update head then clear new head's prev [OK]

Common Mistakes:

• Setting prev pointer before moving head
• Not checking if new head is nil
• Clearing head and tail without condition

3. Given the following doubly linked list with nodes 10 <-> 20 <-> 30, what will be the list after deleting the first node?

type Node struct {
  data int
  prev *Node
  next *Node
}

head := &Node{data: 10}
second := &Node{data: 20}
third := &Node{data: 30}
head.next = second
second.prev = head
second.next = third
third.prev = second

// Delete from beginning
head = head.next
head.prev = nil

// Print list
curr := head
for curr != nil {
  fmt.Print(curr.data)
  if curr.next != nil {
    fmt.Print(" <-> ")
  }
  curr = curr.next
}

medium

A. 20 <-> 30

B. 10 <-> 20 <-> 30

C. 30

D. nil

Solution

1. Step 1: Initial list setup

   List is 10 <-> 20 <-> 30 with head at 10.

2. Step 2: Delete first node

   Head moves to 20, and 20's prev is set to nil, detaching 10.

3. Final Answer:

   20 <-> 30 -> Option A

4. Quick Check:

   First node removed, list starts at 20 [OK]

Hint: Head moves to next, prev cleared [OK]

Common Mistakes:

• Not clearing prev pointer of new head
• Printing old head node
• Confusing next and prev pointers

4. Identify the error in this Go code snippet that tries to delete the first node from a doubly linked list:

if dll.head != nil {
  dll.head = dll.head.next
  dll.head.prev = nil
}

medium

A. It should assign dll.head.prev to dll.head.next

B. It should set dll.head.next.prev to nil instead

C. No error, code is correct

D. It does not check if dll.head.next is nil before accessing prev

Solution

1. Step 1: Check condition before accessing pointers

   The code moves head to next node without checking if next node exists.

2. Step 2: Accessing prev of nil causes runtime error

   If dll.head.next is nil, then dll.head becomes nil and accessing dll.head.prev causes panic.

3. Final Answer:

   It does not check if dll.head.next is nil before accessing prev -> Option D

4. Quick Check:

   Always check for nil before pointer access [OK]

Hint: Check nil before accessing pointers [OK]

Common Mistakes:

• Assuming next node always exists
• Not handling empty list after deletion
• Confusing prev and next pointers

5. You have a doubly linked list with nodes 5 <-> 15. You want to delete the first node and then insert a new node with value 10 at the beginning. Which sequence of operations correctly updates the list?

hard

A. Delete first node by moving head to next; create new node with prev = head, next = nil; set head.next to new node

B. Delete first node by setting head to nil; create new node with next = nil, prev = nil; set head to new node

C. Delete first node by moving head to next and clearing prev; then create new node with next = head, prev = nil; update head.prev to new node; finally set head to new node

D. Delete first node by moving head to next; create new node with next = nil, prev = nil; set head to new node

Solution

1. Step 1: Delete first node properly

   Move head to next node and set new head's prev to nil to detach old first node.

2. Step 2: Insert new node at beginning

   Create new node with next pointing to current head and prev nil; update current head's prev to new node; update head pointer to new node.

3. Final Answer:

   Delete first node by moving head to next and clearing prev; then create new node with next = head, prev = nil; update head.prev to new node; finally set head to new node -> Option C

4. Quick Check:

   Proper delete then insert at head [OK]

Hint: Delete then insert by updating head and pointers [OK]

Common Mistakes:

• Not updating prev pointer of old head after insertion
• Setting new node's prev incorrectly
• Overwriting head without linking new node