Insert at End 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 moving forward and backward through the list.

beginner

What steps are needed to insert a new node at the end of a doubly linked list?

1. Create a new node with the given data.
2. If the list is empty, make the new node the head.
3. Otherwise, traverse to the last node.
4. Set the last node's next pointer to the new node.
5. Set the new node's previous pointer to the last node.
6. Set the new node's next pointer to nil.

intermediate

Why do we need to update both next and previous pointers when inserting at the end?

Because each node points both forward and backward, we must update the last node's next pointer to the new node and the new node's previous pointer back to the last node. This keeps the list connected in both directions.

intermediate

What happens if you forget to update the new node's previous pointer during insertion at the end?

The new node will not link back to the previous node, breaking the backward traversal of the list. This can cause errors when moving backward through the list.

beginner

Show the Go code snippet to create a new node for a doubly linked list.

type Node struct { data int prev *Node next *Node } func newNode(data int) *Node { return &Node{data: data, prev: nil, next: nil} }

What pointer of the last node should be updated when inserting a new node at the end?

A next

B prev

C data

D head

The last node's next pointer should point to the new node to link it at the end.

If the doubly linked list is empty, what should the new node become after insertion?

A Middle node

B Tail node

C Head node

D Disconnected node

If the list is empty, the new node becomes the head (and tail) of the list.

Which pointer of the new node must be set to the previous last node?

A next

B head

C tail

D prev

The new node's prev pointer must point to the previous last node to maintain backward links.

What is the next pointer of the new last node after insertion?

A Points to head

B Points to nil (null)

C Points to itself

D Points to previous node

The new last node's next pointer should be nil to mark the end of the list.

What is the time complexity to insert at the end of a doubly linked list if no tail pointer is maintained?

A O(n)

B O(1)

C O(log n)

D O(n^2)

Without a tail pointer, you must traverse the list to find the last node, which takes O(n) time.

Explain the process of inserting a new node at the end of a doubly linked list.

Think about how pointers connect nodes forward and backward.

Describe what happens if you forget to update the previous pointer of the new node during insertion at the end.

Consider how the doubly linked list allows moving both ways.

Practice

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

easy

A. To add a new element after the last node

B. To remove the first node

C. To reverse the list

D. To find the middle node

Solution

1. Step 1: Understand the structure of a doubly linked list

   A doubly linked list has nodes connected forward and backward, allowing traversal in both directions.

2. Step 2: Identify the effect of inserting at the end

   Inserting at the end means adding a new node after the current last node, updating pointers accordingly.

3. Final Answer:

   To add a new element after the last node -> Option A

4. Quick Check:

   Insert at end = add after last node [OK]

Hint: Insert at end means add after last node [OK]

Common Mistakes:

• Confusing insertion at end with deletion
• Thinking insertion reverses the list
• Assuming insertion happens at the start

2. Which of the following Go code snippets correctly updates the 'next' pointer when inserting a new node at the end of a doubly linked list?

easy

A. newNode.next = last

B. newNode.prev = last.next

C. last.next = newNode

D. last.prev = newNode

Solution

1. Step 1: Identify the last node's next pointer update

   When inserting at the end, the last node's next pointer should point to the new node.

2. Step 2: Match correct pointer assignment

   Setting last.next = newNode correctly links the last node to the new node.

3. Final Answer:

   last.next = newNode -> Option C

4. Quick Check:

   last.next points to new node [OK]

Hint: Last node's next points to new node [OK]

Common Mistakes:

• Assigning newNode.prev to last.next incorrectly
• Setting newNode.next to last (wrong direction)
• Changing last.prev instead of last.next

3. What will be the printed output of the doubly linked list after inserting nodes with values 10, 20, and 30 at the end in order? Assume the print function shows values from head to tail.

medium

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

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

C. 10 <-> 30 <-> 20 <-> null

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

Solution

1. Step 1: Insert 10 at end when list is empty

   List becomes: 10 -> null

2. Step 2: Insert 20 at end

   List becomes: 10 <-> 20 -> null

3. Step 3: Insert 30 at end

   List becomes: 10 <-> 20 <-> 30 -> null

4. Final Answer:

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

5. Quick Check:

   Insert order preserved from head to tail [OK]

Hint: Insertion at end keeps order as inserted [OK]

Common Mistakes:

• Assuming insertion reverses order
• Printing null at start instead of end
• Mixing node order in output

4. Given the following Go code snippet for inserting at the end of a doubly linked list, what is the error?

func InsertEnd(head *Node, data int) *Node {
    newNode := &Node{data: data}
    if head == nil {
        head = newNode
    } else {
        temp := head
        for temp.next != nil {
            temp = temp.next
        }
        temp.next = newNode
        newNode.prev = temp
    }
    return head
}

medium

A. The newNode's prev pointer is incorrectly assigned

B. The newNode's next pointer is not set to nil

C. The loop condition should be temp.prev != nil

D. The function does not update the head when list is empty

Solution

1. Step 1: Check newNode pointer initialization

   In Go, uninitialized pointers default to nil, but explicitly setting newNode.next = nil is safer.

2. Step 2: Identify missing explicit next pointer assignment

   The code does not set newNode.next = nil, which can cause issues if newNode is reused or inspected.

3. Final Answer:

   The newNode's next pointer is not set to nil -> Option B

4. Quick Check:

   Always set newNode.next = nil at insertion [OK]

Hint: Always set newNode.next = nil when inserting at end [OK]

Common Mistakes:

• Ignoring explicit nil assignment for newNode.next
• Confusing prev and next pointers
• Incorrect loop condition for traversal

5. You have a doubly linked list with nodes 5 <-> 15 <-> 25. You want to insert a new node with value 35 at the end. After insertion, which pointers must be updated to maintain list integrity?

hard

A. Set last node's next to new node, new node's prev to last node, new node's next to nil

B. Set new node's next to last node, last node's prev to new node

C. Set head's prev to new node, new node's next to head

D. Set new node's prev and next to nil only

Solution

1. Step 1: Identify the last node before insertion

   The last node is the one with value 25.

2. Step 2: Update pointers to insert new node at end

   Set last node's next to new node, new node's prev to last node, and new node's next to nil to mark end.

3. Final Answer:

   Set last node's next to new node, new node's prev to last node, new node's next to nil -> Option A

4. Quick Check:

   Last.next = new, new.prev = last, new.next = nil [OK]

Hint: Update last.next, new.prev, and new.next for end insertion [OK]

Common Mistakes:

• Reversing prev and next pointers
• Not setting new node's next to nil
• Changing head pointers incorrectly