Delete from 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 happens when you delete the last node from a doubly linked list?

The last node is removed, the second last node's next pointer is set to null, and the list's tail pointer is updated to the second last node.

intermediate

Why do we need to update the previous node's next pointer when deleting from the end?

Because the new last node should not point to the deleted node. Its next pointer must be null to mark the end of the list.

intermediate

What is the time complexity of deleting the last node in a doubly linked list if we have a tail pointer?

O(1) because we can directly access the last node using the tail pointer without traversing the list.

advanced

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

func (dll *DoublyLinkedList) DeleteFromEnd() { if dll.tail == nil { return // list empty } if dll.tail.prev == nil { dll.head = nil dll.tail = nil return } dll.tail = dll.tail.prev dll.tail.next = nil }

What pointer must be updated when deleting the last node in a doubly linked list?

A The head pointer

B The previous node's next pointer

C The next node's previous pointer

D The tail's next pointer

The previous node's next pointer must be set to null to mark the new end of the list.

If the doubly linked list has only one node, what happens when you delete from the end?

A Nothing changes

B The head points to itself

C The list becomes empty

D The tail points to itself

Deleting the only node empties the list, so head and tail become null.

What is the time complexity of deleting the last node if you do NOT have a tail pointer?

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.

Which pointer in the last node is always null in a doubly linked list?

A tail

B prev

C head

D next

The last node's next pointer is always null to indicate the end of the list.

In Go, what happens if you try to delete from an empty doubly linked list?

A The function returns immediately without error

B The program crashes

C The list becomes non-empty

D The head pointer is set to a new node

The code checks if the tail is nil and returns immediately, so no error occurs.

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

Think about what pointers need to change to remove the last node safely.

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

Focus on pointer updates and edge cases.

Practice

1. What happens to the tail pointer when you delete the last node from a doubly linked list?

easy

A. The tail pointer becomes null immediately.

B. The tail pointer moves to the next node.

C. The tail pointer does not change.

D. The tail pointer moves to the previous node.

Solution

1. Step 1: Understand the tail pointer role

   The tail pointer points to the last node in the doubly linked list.

2. Step 2: Effect of deleting the last node

   When the last node is deleted, the tail pointer must move to the previous node to remain valid.

3. Final Answer:

   The tail pointer moves to the previous node. -> Option D

4. Quick Check:

   Tail moves backward after deletion [OK]

Hint: Tail always points to last node; update to previous on delete [OK]

Common Mistakes:

• Not updating tail pointer after deletion
• Setting tail to next node which is null
• Leaving tail pointer unchanged

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

easy

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

B. dll.tail = dll.tail.prev; dll.tail.next = nil

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

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

Solution

1. Step 1: Identify correct pointer updates for tail deletion

   Deleting last node means tail moves to previous node and new tail's next is nil.

2. Step 2: Check code correctness

   dll.tail = dll.tail.prev; dll.tail.next = nil sets tail to previous node and sets tail.next to nil, which is correct.

3. Final Answer:

   dll.tail = dll.tail.prev; dll.tail.next = nil -> Option B

4. Quick Check:

   Tail moves back, next cleared [OK]

Hint: Tail moves to prev; set tail.next to nil [OK]

Common Mistakes:

• Moving tail forward instead of backward
• Not setting tail.next to nil
• Changing head pointer instead of tail

3. What is the output of the following Go code that deletes the last node from a doubly linked list?

package main
import "fmt"

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

type DoublyLinkedList struct {
    head *Node
    tail *Node
;
}

func (dll *DoublyLinkedList) DeleteFromEnd() {
    if dll.tail == nil {
        return
    }
    if dll.tail.prev == nil {
        dll.head = nil
        dll.tail = nil
        return
    }
    dll.tail = dll.tail.prev
    dll.tail.next = nil
}

func (dll *DoublyLinkedList) Print() {
    current := dll.head
    for current != nil {
        fmt.Print(current.data, " -> ")
        current = current.next
    }
    fmt.Println("null")
}

func main() {
    n1 := &Node{data: 1}
    n2 := &Node{data: 2}
    n3 := &Node{data: 3}
    n1.next = n2
    n2.prev = n1
    n2.next = n3
    n3.prev = n2
    dll := DoublyLinkedList{head: n1, tail: n3}
    dll.DeleteFromEnd()
    dll.Print()
}

medium

A. 1 -> 2 -> null

B. 1 -> 2 -> 3 -> null

C. 2 -> 3 -> null

D. null

Solution

1. Step 1: Initial list and deletion

   List is 1 -> 2 -> 3 -> null. DeleteFromEnd removes last node (3).

2. Step 2: After deletion

   Tail moves to node with data 2, tail.next set to nil. List becomes 1 -> 2 -> null.

3. Final Answer:

   1 -> 2 -> null -> Option A

4. Quick Check:

   Last node removed, tail updated [OK]

Hint: Deleting end removes last node, tail moves back [OK]

Common Mistakes:

• Not updating tail.next to nil causing dangling pointer
• Printing old list without deletion
• Confusing head and tail pointers

4. The following Go code attempts to delete the last node from a doubly linked list but causes a runtime panic. What is the error?

func (dll *DoublyLinkedList) DeleteFromEnd() {
    if dll.tail == nil {
        return
    }
    dll.tail = dll.tail.prev
    dll.tail.next = nil
}

medium

A. It should delete from the head, not the tail.

B. It incorrectly sets tail.next to nil before moving tail.

C. It does not handle the case when the list has only one node.

D. It should set head to nil instead of tail.

Solution

1. Step 1: Analyze edge case for single node list

   If list has one node, tail.prev is nil, so after tail = tail.prev, tail is nil.

2. Step 2: Cause of panic

   Next line tries to access tail.next but tail is nil, causing runtime panic.

3. Final Answer:

   It does not handle the case when the list has only one node. -> Option C

4. Quick Check:

   Missing single-node check causes nil pointer panic [OK]

Hint: Check for single-node list before updating tail [OK]

Common Mistakes:

• Not checking if tail.prev is nil before access
• Assuming list always has multiple nodes
• Ignoring head pointer update when list becomes empty

5. You have a doubly linked list with nodes: 10 <-> 20 <-> 30 <-> 40. After deleting the last node twice using the correct delete from end method, what will be the printed list?

hard

A. 10 -> 20 -> null

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

C. 20 -> 30 -> null

D. 10 -> 30 -> null

Solution

1. Step 1: Initial list and first deletion

   List is 10 <-> 20 <-> 30 <-> 40. First deletion removes 40, list becomes 10 <-> 20 <-> 30.

2. Step 2: Second deletion

   Second deletion removes 30, list becomes 10 <-> 20.

3. Step 3: Print list

   Printing from head: 10 -> 20 -> null.

4. Final Answer:

   10 -> 20 -> null -> Option A

5. Quick Check:

   Two deletions remove last two nodes [OK]

Hint: Each delete removes last node, tail moves back [OK]

Common Mistakes:

• Deleting only once instead of twice
• Confusing node values after deletion
• Not updating tail pointer correctly