Delete by Value in Doubly Linked List in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after deleting a value from doubly linked list

What is the printed state of the doubly linked list after deleting the value 3?

DSA Java

class Node {
    int data;
    Node prev, next;
    Node(int d) { data = d; prev = next = null; }
}

class DoublyLinkedList {
    Node head;

    void append(int data) {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode;
            return;
        }
        Node last = head;
        while (last.next != null) last = last.next;
        last.next = newNode;
        newNode.prev = last;
    }

    void deleteByValue(int value) {
        Node current = head;
        while (current != null) {
            if (current.data == value) {
                if (current.prev != null) current.prev.next = current.next;
                else head = current.next;
                if (current.next != null) current.next.prev = current.prev;
                break;
            }
            current = current.next;
        }
    }

    void printList() {
        Node temp = head;
        while (temp != null) {
            System.out.print(temp.data + " -> ");
            temp = temp.next;
        }
        System.out.print("null\n");
    }
}

public class Main {
    public static void main(String[] args) {
        DoublyLinkedList dll = new DoublyLinkedList();
        dll.append(1);
        dll.append(2);
        dll.append(3);
        dll.append(4);
        dll.append(5);
        dll.deleteByValue(3);
        dll.printList();
    }
}

A 2 -> 3 -> 4 -> 5 -> null

B 1 -> 2 -> 3 -> 4 -> 5 -> null

C 1 -> 2 -> 4 -> null

D 1 -> 2 -> 4 -> 5 -> null

💡 Hint

Focus on how the deleteByValue method updates the links when the node with value 3 is found.

The deleteByValue method searches for the node with data 3 and removes it by adjusting the previous node's next pointer and the next node's prev pointer. The list after deletion is 1 -> 2 -> 4 -> 5 -> null.

🧠 Conceptual

intermediate

Understanding node deletion in doubly linked list

When deleting a node by value in a doubly linked list, which pointers must be updated to maintain list integrity?

A No pointers need to be updated

B Only the next node's prev pointer

C Both the previous node's next pointer and the next node's prev pointer

D Only the previous node's next pointer

💡 Hint

Think about how nodes are connected in both directions.

In a doubly linked list, each node points to both previous and next nodes. When deleting a node, you must update the previous node's next pointer and the next node's prev pointer to bypass the deleted node.

🔧 Debug

advanced

Identify the error in deleteByValue method

What error will occur when running this deleteByValue method if the value to delete is not found in the list?

DSA Java

void deleteByValue(int value) {
    Node current = head;
    while (current != null) {
        if (current.data == value) {
            if (current.prev != null) current.prev.next = current.next;
            else head = current.next;
            if (current.next != null) current.next.prev = current.prev;
            break;
        }
        current = current.next;
    }
    System.out.println(current.data);
}

A No error, prints the value

B NullPointerException

C ArrayIndexOutOfBoundsException

D Compilation error

💡 Hint

Consider what happens if the value is not found and current becomes null.

If the value is not found, current becomes null after the loop. Then accessing current.data causes a NullPointerException.

❓ Predict Output

advanced

Output after deleting head node by value

What is the printed state of the doubly linked list after deleting the value 1 (head node)?

DSA Java

class Node {
    int data;
    Node prev, next;
    Node(int d) { data = d; prev = next = null; }
}

class DoublyLinkedList {
    Node head;

    void append(int data) {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode;
            return;
        }
        Node last = head;
        while (last.next != null) last = last.next;
        last.next = newNode;
        newNode.prev = last;
    }

    void deleteByValue(int value) {
        Node current = head;
        while (current != null) {
            if (current.data == value) {
                if (current.prev != null) current.prev.next = current.next;
                else head = current.next;
                if (current.next != null) current.next.prev = current.prev;
                break;
            }
            current = current.next;
        }
    }

    void printList() {
        Node temp = head;
        while (temp != null) {
            System.out.print(temp.data + " -> ");
            temp = temp.next;
        }
        System.out.print("null\n");
    }
}

public class Main {
    public static void main(String[] args) {
        DoublyLinkedList dll = new DoublyLinkedList();
        dll.append(1);
        dll.append(2);
        dll.append(3);
        dll.deleteByValue(1);
        dll.printList();
    }
}

A 2 -> 3 -> null

B 1 -> 2 -> 3 -> null

C 3 -> null

D null

💡 Hint

Deleting the head node updates the head pointer to the next node.

Since the head node with value 1 is deleted, the head pointer moves to the node with value 2. The list becomes 2 -> 3 -> null.

🧠 Conceptual

expert

Why is doubly linked list deletion by value more efficient than singly linked list?

Which reason best explains why deleting a node by value in a doubly linked list can be more efficient than in a singly linked list?

A Because doubly linked list nodes have pointers to both previous and next nodes, allowing direct access to update links without traversal to find the previous node.

B Because doubly linked lists use less memory, making deletion faster.

C Because singly linked lists do not allow deletion by value.

D Because doubly linked lists store values in sorted order by default.

💡 Hint

Think about how the previous node is found in singly vs doubly linked lists.

In singly linked lists, to delete a node by value, you must traverse to find the previous node to update its next pointer. In doubly linked lists, each node has a prev pointer, so you can update links directly without extra traversal.

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

2. Step 2: Update 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.

3. Final Answer:

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

4. Quick Check:

   Deleting by value removes matching node [OK]

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

Common Mistakes:

• Not updating both previous and next pointers
• Deleting all nodes instead of one
• Ignoring the case when node is head or tail

2. Which of the following is the correct Java syntax to delete a node by value in a doubly linked list?

easy

A. int deleteByValue(int value) { /* code */ }

B. void deleteByValue(int value) { /* code */ }

C. deleteByValue(int value) { /* code */ }

D. void deleteByValue() { /* code */ }

Solution

1. Step 1: Check method return type and parameters

   The method should accept an integer value to delete and return nothing, so void return type is correct.

2. Step 2: Verify method signature syntax

   Java requires return type before method name and parameter types inside parentheses.

3. Final Answer:

   void deleteByValue(int value) { /* code */ } -> Option B

4. Quick Check:

   Method with void return and int parameter is correct [OK]

Hint: Method must specify return type and parameter types in Java [OK]

Common Mistakes:

• Omitting return type
• Missing parameter type
• Using wrong return type for delete method

3. Given the doubly linked list: 10 <-> 20 <-> 30 <-> 40 <-> null, what is the list after deleting the node with value 30?

medium

A. 10 <-> 20 <-> 40 <-> null

B. 10 <-> 20 <-> 30 <-> 40 <-> null

C. 20 <-> 30 <-> 40 <-> null

D. 10 <-> 40 <-> null

Solution

1. Step 1: Identify node with value 30

   Traverse the list and find the node containing 30 between 20 and 40.

2. Step 2: Remove node 30 and update links

   Set 20's next pointer to 40 and 40's previous pointer to 20, removing 30 from the chain.

3. Final Answer:

   10 <-> 20 <-> 40 <-> null -> Option A

4. Quick Check:

   Node 30 removed, links updated correctly [OK]

Hint: Remove node and link previous to next nodes directly [OK]

Common Mistakes:

• Not updating previous pointer of next node
• Leaving deleted node linked
• Deleting wrong node

4. What is the error in this code snippet for deleting a node by value in a doubly linked list?

void deleteByValue(int value) {
  Node current = head;
  while (current != null && current.data != value) {
    current = current.next;
  }
  if (current == null) return;
  current.prev.next = current.next;
  current.next.prev = current.prev;
}

medium

A. It reverses the list instead of deleting

B. It does not check if current is null before accessing pointers

C. It deletes all nodes with the value instead of one

D. It does not handle deletion when current is head

Solution

1. Step 1: Analyze deletion at head node

   If current is head, current.prev is null, so current.prev.next causes NullPointerException.

2. Step 2: Identify missing condition for head deletion

   Code must check if current.prev is null and update head pointer accordingly.

3. Final Answer:

   It does not handle deletion when current is head -> Option D

4. Quick Check:

   Head deletion needs special handling [OK]

Hint: Check if node is head before updating previous pointer [OK]

Common Mistakes:

• Not handling head or tail nodes separately
• Accessing null pointers without checks
• Assuming node always has previous and next

5. You have a doubly linked list: 5 <-> 10 <-> 15 <-> 20 <-> 25 <-> null. You want to delete the node with value 5, then delete the node with value 25. What is the final list?

hard

A. null

B. 5 <-> 10 <-> 15 <-> 20 <-> null

C. 10 <-> 15 <-> 20 <-> null

D. 10 <-> 15 <-> 20 <-> 25 <-> null

Solution

1. Step 1: Delete node with value 5 (head)

   Since 5 is head, update head to 10 and set 10's prev to null.

2. Step 2: Delete node with value 25 (tail)

   Since 25 is tail, update 20's next to null, removing 25.

3. Final Answer:

   10 <-> 15 <-> 20 <-> null -> Option C

4. Quick Check:

   Head and tail deletions handled correctly [OK]

Hint: Update head or tail pointers when deleting first or last node [OK]

Common Mistakes:

• Not updating head when deleting first node
• Not updating tail when deleting last node
• Breaking links causing list corruption