Delete from Beginning of Doubly Linked List in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a pointer to the next node, and a pointer to the previous node. This allows traversal in both directions.

beginner

What happens when you delete the first node from a doubly linked list?

The head pointer moves to the second node, and the new head's previous pointer is set to nil. The original first node is removed from the list.

intermediate

Why do we need to update the previous pointer of the new head to nil after deletion?

Because the new head is now the first node, it should not point back to any node. Setting its previous pointer to nil ensures the list starts correctly.

intermediate

Show the Go code snippet to delete the first node from a doubly linked list.

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

beginner

What should you check before deleting the first node in a doubly linked list?

You should check if the list is empty (head is nil). If it is empty, there is nothing to delete.

What pointer of the new head node must be set to nil after deleting the first node in a doubly linked list?

A Previous pointer

B Next pointer

C Data pointer

D Head pointer

The previous pointer of the new head node must be set to nil because it becomes the first node and should not point back to any node.

What should you do if the doubly linked list is empty and you try to delete from the beginning?

A Do nothing

B Throw an error

C Delete the head anyway

D Create a new node

If the list is empty (head is nil), there is nothing to delete, so the function should simply do nothing.

After deleting the first node, what does the head pointer point to?

A Null

B The last node

C The second node

D The deleted node

After deletion, the head pointer moves to the second node, which becomes the new first node.

In Go, how do you update the head pointer inside a function when deleting the first node?

A Pass head by value and update it

B Return the new head and assign outside

C Use a global variable

D Pass head as a pointer to pointer and update *head

Passing head as a pointer to pointer allows the function to update the original head pointer directly.

Which of these is NOT a part of a doubly linked list node?

A Data

B Random pointer

C Next pointer

D Previous pointer

A standard doubly linked list node has data, next pointer, and previous pointer. A random pointer is not standard.

Explain step-by-step how to delete the first node from a doubly linked list.

Think about what happens to the head and the pointers after deletion.

Write a Go function to delete the first node from a doubly linked list and explain each line.

Focus on pointer updates and edge cases.

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