Insert at Beginning of Doubly Linked List in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the given data.

DSA Go

newNode := &Node{data: [1]

Drag options to blanks, or click blank then click option'

Adata

Bvalue

Citem

DnodeData

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name not defined in the function like 'value' or 'item'.

Forgetting to assign the data to the new node.

The new node's data field should be assigned the variable 'data' passed to the function.

2fill in blank

medium

Complete the code to link the new node's next pointer to the current head.

DSA Go

newNode.next = [1]

Drag options to blanks, or click blank then click option'

Anil

Btail

CnewNode

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Linking to tail instead of head.

Linking next to nil which breaks the list.

The new node's next pointer should point to the current head node.

3fill in blank

hard

Fix the error in setting the previous pointer of the old head node.

DSA Go

if head != nil {
    head.[1] = newNode
}

Drag options to blanks, or click blank then click option'

Aprev

Bnext

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting head.next instead of head.prev.

Assigning head.data which is incorrect.

The previous pointer of the old head should point back to the new node.

4fill in blank

hard

Fill both blanks to update the head pointer and return it.

DSA Go

head = [1]
return [2]

Drag options to blanks, or click blank then click option'

AnewNode

Bhead

ColdHead

Dnil

Attempts:

3 left

💡 Hint

Common Mistakes

Returning oldHead instead of updated head.

Assigning head to nil which breaks the list.

The head pointer should be updated to the new node and returned.

5fill in blank

hard

Fill all three blanks to complete the InsertAtBeginning function for a doubly linked list.

DSA Go

func InsertAtBeginning(head *Node, data int) *Node {
    newNode := &Node{data: [1]
    newNode.next = [2]
    if head != nil {
        head.prev = newNode
    }
    head = [3]
    return head
}

Drag options to blanks, or click blank then click option'

Adata

Bhead

CnewNode

Dnil

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names for data or pointers.

Not updating head to newNode before returning.

The function creates a new node with data, links it to the current head, updates the previous pointer, updates head to newNode, and returns head.

Practice

1. What is the main purpose of inserting a node at the beginning of a doubly linked list?

easy

A. To add a new node before the current head node

B. To add a new node after the tail node

C. To delete the first node of the list

D. To reverse the entire list

Solution

1. Step 1: Understand the head node position

   The head node is the first node in the doubly linked list.

2. Step 2: Insert new node before head

   Inserting at the beginning means placing the new node before the current head and updating pointers.

3. Final Answer:

   To add a new node before the current head node -> Option A

4. Quick Check:

   Insert at beginning = add before head [OK]

Hint: Beginning means before the head node [OK]

Common Mistakes:

• Confusing beginning with end insertion
• Thinking insertion deletes nodes
• Assuming insertion reverses list

2. Which of the following Go code snippets correctly updates the previous pointer of the old head node when inserting a new node at the beginning?

easy

A. newNode.prev = oldHead

B. oldHead.prev = newNode

C. newNode.next = oldHead.prev

D. oldHead.next = newNode

Solution

1. Step 1: Identify pointer to update

   When inserting at beginning, old head's previous pointer must point to new node.

2. Step 2: Correct assignment

   Setting oldHead.prev = newNode correctly links old head back to new node.

3. Final Answer:

   oldHead.prev = newNode -> Option B

4. Quick Check:

   Old head prev points to new node [OK]

Hint: Old head's prev points to new node [OK]

Common Mistakes:

• Assigning newNode.prev incorrectly
• Mixing next and prev pointers
• Updating oldHead.next instead of prev

3. Consider this Go code inserting nodes at the beginning of a doubly linked list:

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

func insertAtBeginning(head **Node, data int) {
  newNode := &Node{data: data}
  newNode.next = *head
  if *head != nil {
    (*head).prev = newNode
  }
  *head = newNode
}

func main() {
  var head *Node
  insertAtBeginning(&head, 10)
  insertAtBeginning(&head, 20)
  insertAtBeginning(&head, 30)
  // Print list from head
  curr := head
  for curr != nil {
    fmt.Print(curr.data, " -> ")
    curr = curr.next
  }
  fmt.Print("null")
}

What is the printed output?

medium

A. 20 -> 30 -> 10 -> null

B. 10 -> 20 -> 30 -> null

C. null

D. 30 -> 20 -> 10 -> null

Solution

1. Step 1: Trace insertions order

   Nodes inserted in order: 10, then 20 at beginning, then 30 at beginning.

2. Step 2: List after insertions

   After all insertions, list is 30 (head) -> 20 -> 10 -> null.

3. Final Answer:

   30 -> 20 -> 10 -> null -> Option D

4. Quick Check:

   Last inserted node is new head [OK]

Hint: Last inserted node becomes new head [OK]

Common Mistakes:

• Assuming insertion adds at end
• Confusing order of nodes
• Ignoring pointer updates

4. The following Go function attempts to insert a node at the beginning of a doubly linked list but causes a runtime panic:

func insertAtBeginning(head *Node, data int) {
  newNode := &Node{data: data}
  newNode.next = head
  head.prev = newNode
  head = newNode
}

What is the main error causing the panic?

medium

A. head is nil and head.prev causes nil pointer dereference

B. newNode.next is not assigned correctly

C. head = newNode does not update original head pointer

D. data field is not initialized

Solution

1. Step 1: Check nil pointer usage

   If head is nil, accessing head.prev causes runtime panic (nil pointer dereference).

2. Step 2: Identify cause of panic

   Function does not check if head is nil before head.prev = newNode, causing panic.

3. Final Answer:

   head is nil and head.prev causes nil pointer dereference -> Option A

4. Quick Check:

   Accessing prev on nil head causes panic [OK]

Hint: Check for nil before accessing head.prev [OK]

Common Mistakes:

• Ignoring nil check before pointer access
• Thinking assignment head = newNode updates caller
• Assuming data field causes panic

5. You have a doubly linked list with nodes: 5 <-> 10 <-> 15. You want to insert a new node with value 3 at the beginning. After insertion, what will be the correct sequence of nodes when traversed from head to tail?

hard

A. 15 -> 10 -> 5 -> 3 -> null

B. 5 -> 10 -> 15 -> 3 -> null

C. 3 -> 5 -> 10 -> 15 -> null

D. 3 -> 15 -> 10 -> 5 -> null

Solution

1. Step 1: Understand insertion at beginning

   New node with value 3 is added before current head (5).

2. Step 2: Update pointers and traverse

   After insertion, list is 3 (head) -> 5 -> 10 -> 15 -> null.

3. Final Answer:

   3 -> 5 -> 10 -> 15 -> null -> Option C

4. Quick Check:

   New node becomes new head [OK]

Hint: New node becomes new head, old head shifts right [OK]

Common Mistakes:

• Inserting at end instead of beginning
• Reversing node order incorrectly
• Not updating head pointer