Delete from Beginning of Doubly Linked List in DSA Go

Mental Model

To remove the first item, we move the start pointer to the next item and fix links so the list stays connected.

Analogy: Imagine a train where the first carriage is detached and the second carriage becomes the new front, with no loose ends.

null ← 1 ↔ 2 ↔ 3 -> null
↑head

Dry Run Walkthrough

Input: list: 1 ↔ 2 ↔ 3, delete from beginning

Goal: Remove the first node and update the list to start from the second node

Step 1: head moves from node 1 to node 2

null ← 1 ↔ 2 ↔ 3 -> null
     ↑head

Why: We need to point head to the second node to remove the first

Step 2: set previous pointer of new head (node 2) to null

null ← 2 ↔ 3 -> null
↑head

Why: Disconnect the old first node by removing backward link

Step 3: old first node (1) is now detached and ready for garbage collection

null ← 2 ↔ 3 -> null
↑head

Why: The list is updated and the first node is removed

Result:

null ← 2 ↔ 3 -> null
↑head

Annotated Code

DSA Go

package main

import "fmt"

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

type DoublyLinkedList struct {
	head *Node
}

func (dll *DoublyLinkedList) deleteFromBeginning() {
	if dll.head == nil {
		return // list empty, nothing to delete
	}
	// move head to next node
	dll.head = dll.head.next
	// if new head exists, set its prev to nil
	if dll.head != nil {
		dll.head.prev = nil
	}
}

func (dll *DoublyLinkedList) printList() {
	curr := dll.head
	for curr != nil {
		fmt.Printf("%d", curr.data)
		if curr.next != nil {
			fmt.Printf(" ↔ ")
		}
		curr = curr.next
	}
	fmt.Println()
}

func main() {
	dll := DoublyLinkedList{}
	dll.head = &Node{data: 1}
	dll.head.next = &Node{data: 2, prev: dll.head}
	dll.head.next.next = &Node{data: 3, prev: dll.head.next}

	fmt.Print("Original list: ")
	dll.printList()

	dll.deleteFromBeginning()

	fmt.Print("After deleting from beginning: ")
	dll.printList()
}

if dll.head == nil { return }

handle empty list by returning early

dll.head = dll.head.next

advance head pointer to second node to remove first

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

disconnect backward link of new head to detach old first node

Output

Original list: 1 ↔ 2 ↔ 3

After deleting from beginning: 2 ↔ 3

Complexity Analysis

Time: O(1) because we only change a few pointers at the start

Space: O(1) because no extra memory is used besides pointers

vs Alternative: Compared to deleting from the end which requires traversal O(n), deleting from beginning is faster and simpler

Edge Cases

empty list

function returns immediately without error

DSA Go

if dll.head == nil { return }

single element list

head becomes nil, list becomes empty

DSA Go

dll.head = dll.head.next

When to Use This Pattern

When you need to remove the first item quickly from a doubly linked list, use delete from beginning to update head and fix links.

Common Mistakes

Mistake: Not setting the new head's prev pointer to nil after deletion
Fix: Always set new head's prev to nil to avoid backward link to removed node

Mistake: Not handling empty list before deletion
Fix: Check if head is nil before deleting to avoid errors

Summary

Deletes the first node of a doubly linked list by moving head and fixing pointers.

Use when you want to remove the front element efficiently.

The key is to move head forward and disconnect the backward link of the new head.

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