Delete by Value in 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 does 'delete by value' mean in a doubly linked list?

It means finding the first node that contains the given value and removing it from the list, adjusting the pointers of neighboring nodes to keep the list connected.

intermediate

Which pointers need to be updated when deleting a node in a doubly linked list?

You update the previous node's next pointer to the deleted node's next, and the next node's previous pointer to the deleted node's previous.

intermediate

What happens if the node to delete is the head of the doubly linked list?

The head pointer is updated to the next node, and if that node exists, its previous pointer is set to nil.

beginner

Why is it important to handle the case when the node to delete is not found?

Because if the value does not exist, the list should remain unchanged and the program should not crash or behave unexpectedly.

In a doubly linked list, what pointer does each node have besides the data?

A Next and previous pointers

B Only next pointer

C Only previous pointer

D No pointers

Each node has two pointers: one to the next node and one to the previous node.

When deleting a node by value, what is the first step?

A Create a new node

B Find the node with the given value

C Update all pointers immediately

D Delete the head node

You must first find the node that contains the value to delete.

If the node to delete is the last node, what pointer must be updated?

A Next node's previous pointer to null

B No pointers need updating

C Head pointer

D Previous node's next pointer to null

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

What should happen if the value to delete is not found in the list?

A List remains unchanged

B Delete the head node

C Delete the last node

D Crash the program

If the value is not found, the list stays the same and no deletion occurs.

Which pointer is updated when deleting the head node?

A Next node's next pointer

B Tail pointer to previous node

C Head pointer to next node

D Previous node's previous pointer

The head pointer is moved to the next node after deleting the current head.

Explain step-by-step how to delete a node by value in a doubly linked list.

Think about how pointers before and after the node change.

What special cases must you consider when deleting a node by value in a doubly linked list?

Consider positions of the node and list state.

Practice

1. What happens when you delete a node by value in a doubly linked list?

easy

A. The node with the matching value is removed and links are updated

B. All nodes are deleted regardless of value

C. Only the first node is deleted no matter the value

D. The list is reversed instead of deleting

Solution

1. Step 1: Locate the node with the given value

   Traverse the doubly linked list from head to tail to find the node whose data matches the value to delete.

2. Step 2: Update the links to remove the node

   Adjust the previous node's next pointer and the next node's previous pointer to bypass the node to be deleted, effectively removing it from the list.

3. Final Answer:

   The node with the matching value is removed and links are updated -> Option A

4. Quick Check:

   Delete by value means remove matching node and fix links [OK]

Hint: Find node by value, then fix previous and next links [OK]

Common Mistakes:

• Not updating previous node's next pointer
• Not updating next node's previous pointer
• Deleting wrong node when multiple have same value

2. Which of the following Go code snippets correctly deletes a node by value from a doubly linked list?

easy

A. if current.data == value { current.next.next = current.prev.prev }

B. if current.data == value { current.next = current.prev; current.prev = current.next }

C. if current.data == value { current = nil }

D. if current.data == value { current.prev.next = current.next; current.next.prev = current.prev }

Solution

1. Step 1: Check node data against value

   The condition current.data == value correctly identifies the node to delete.

2. Step 2: Update links correctly

   Setting current.prev.next = current.next and current.next.prev = current.prev properly removes the node by linking its neighbors.

3. Final Answer:

   if current.data == value { current.prev.next = current.next; current.next.prev = current.prev } -> Option D

4. Quick Check:

   Correct link updates remove node [OK]

Hint: Update prev.next and next.prev to remove node [OK]

Common Mistakes:

• Swapping prev and next pointers incorrectly
• Setting current to nil without fixing links
• Accessing next or prev without nil checks causing panic

3. Given the doubly linked list: 1 <-> 2 <-> 3 <-> 4 <-> 5, what is the list after deleting the node with value 3?

medium

A. 1 <-> 2 <-> 4 <-> 3 <-> 5

B. 1 <-> 2 <-> 4 <-> 5

C. 2 <-> 3 <-> 4 <-> 5

D. 1 <-> 2 <-> 3 <-> 5

Solution

1. Step 1: Identify node with value 3

   Traverse the list to find the node containing 3, which is between nodes 2 and 4.

2. Step 2: Remove node 3 and update links

   Set node 2's next pointer to node 4 and node 4's previous pointer to node 2, skipping node 3.

3. Final Answer:

   1 <-> 2 <-> 4 <-> 5 -> Option B

4. Quick Check:

   Node 3 removed, neighbors linked [OK]

Hint: Remove node and link neighbors directly [OK]

Common Mistakes:

• Leaving node 3 linked causing broken list
• Deleting wrong node by value
• Not updating both prev and next pointers

4. Consider this Go code snippet to delete a node by value in a doubly linked list:

func deleteByValue(head *Node, value int) *Node {
    current := head
    for current != nil {
        if current.data == value {
            if current.prev != nil {
                current.prev.next = current.next
            } else {
                head = current.next
            }
            if current.next != nil {
                current.next.prev = current.prev
            }
            break
        }
        current = current.next
    }
    return head
}

What is the bug in this code?

medium

A. It does not handle deleting the last node correctly

B. It does not update head when deleting the first node

C. It breaks after deleting only the first matching node

D. It does not check for nil pointers before accessing prev or next

Solution

1. Step 1: Analyze loop and break behavior

   The loop breaks immediately after deleting the first node with matching value, so only one node is deleted even if multiple nodes have the same value.

2. Step 2: Understand implications for multiple matches

   If the list has multiple nodes with the same value, only the first is removed, which may not be the intended behavior if all matches should be deleted.

3. Final Answer:

   It breaks after deleting only the first matching node -> Option C

4. Quick Check:

   Break stops after first delete [OK]

Hint: Check if loop breaks too early when deleting multiple nodes [OK]

Common Mistakes:

• Ignoring multiple nodes with same value
• Not updating head when first node deleted
• Not checking nil before pointer access

5. You have a doubly linked list with nodes: 10 <-> 20 <-> 30 <-> 20 <-> 40. You want to delete all nodes with value 20. Which approach correctly deletes all such nodes?

hard

A. Traverse the list and delete nodes with value 20, continuing without breaking the loop

B. Delete the first node with value 20 and then stop traversal

C. Delete nodes with value 20 only if they are at the head or tail

D. Replace nodes with value 20 with a new node containing 0

Solution

1. Step 1: Traverse entire list without breaking

   To delete all nodes with value 20, you must check every node and not stop after the first deletion.

2. Step 2: Delete matching nodes and update links carefully

   For each node with value 20, update the previous and next pointers to remove it, then continue traversal to find others.

3. Final Answer:

   Traverse the list and delete nodes with value 20, continuing without breaking the loop -> Option A

4. Quick Check:

   Delete all matches by full traversal [OK]

Hint: Do not break loop; delete all matching nodes during traversal [OK]

Common Mistakes:

• Breaking loop after first deletion
• Not updating links properly causing broken list
• Only deleting head or tail nodes