Insert at End of Doubly Linked List in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C++

Node* newNode = new Node([1]);

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

ANULL

Bdata

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL instead of data when creating the node.

Using head or tail instead of data.

We create a new node by passing the data value to the Node constructor.

2fill in blank

medium

Complete the code to check if the list is empty.

DSA C++

if (head == [1]) {

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

Atail

BnewNode

Chead

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if head equals newNode or tail.

Checking if head equals head itself.

If head is NULL, the list is empty.

3fill in blank

hard

Fix the error in linking the new node at the end.

DSA C++

tail->next = [1];

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

AnewNode

BNULL

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning tail->next to head or tail instead of newNode.

Assigning tail->next to NULL which breaks the list.

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

4fill in blank

hard

Fill both blanks to update the new node's previous pointer and move the tail.

DSA C++

newNode->prev = [1];
tail = [2];

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

Atail

Bhead

CnewNode

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode->prev to head instead of tail.

Not updating tail to newNode.

The new node's previous points to the old tail, and tail moves to newNode.

5fill in blank

hard

Fill all three blanks to complete the insert at end function.

DSA C++

void insertAtEnd(Node*& head, Node*& tail, int data) {
    Node* newNode = new Node([1]);
    if (head == [2]) {
        head = tail = newNode;
    } else {
        tail->next = [3];
        newNode->prev = tail;
        tail = newNode;
    }
}

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

Adata

BNULL

CnewNode

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names in conditions.

Not linking tail->next to newNode.

Create newNode with data, check if list empty (head == NULL), link tail->next to newNode.

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: Purpose of insertion 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 at end reverses the list
• Assuming insertion happens at the start

2. Which of the following is the correct way to update the tail pointer when inserting a new node at the end of a doubly linked list in C++?

easy

A. tail = NULL;

B. tail = tail->prev;

C. tail = head;

D. tail = newNode;

Solution

1. Step 1: Identify the new tail after insertion

   When a new node is added at the end, it becomes the new tail of the list.

2. Step 2: Update the tail pointer

   Assign the tail pointer to point to the new node to reflect the updated end of the list.

3. Final Answer:

   tail = newNode; -> Option D

4. Quick Check:

   New tail points to newNode [OK]

Hint: Tail always points to the last inserted node [OK]

Common Mistakes:

• Setting tail to previous node instead of new node
• Resetting tail to head incorrectly
• Assigning tail to NULL after insertion

3. What will be the output of the following code snippet after inserting 30 at the end of the doubly linked list?

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

Node* head = new Node{10, NULL, NULL};
Node* tail = head;

Node* newNode = new Node{30, tail, NULL};
tail->next = newNode;
tail = newNode;

// Print list from head
Node* temp = head;
while(temp != NULL) {
  std::cout << temp->data << " ";
  temp = temp->next;
}

medium

A. 10 30

B. 30 10

C. 10

D. 30

Solution

1. Step 1: Initial list and insertion

   Initially, list has one node with data 10. New node with 30 is added at end, linked properly.

2. Step 2: Traversing and printing

   Starting from head (10), print data then move to next node (30), print data, then NULL ends loop.

3. Final Answer:

   10 30 -> Option A

4. Quick Check:

   List after insertion = 10 30 [OK]

Hint: Traverse from head to see all nodes in order [OK]

Common Mistakes:

• Printing nodes in reverse order
• Not updating tail->next pointer
• Forgetting to update tail pointer

4. Identify the error in the following code snippet that inserts a node at the end of a doubly linked list:

void insertEnd(Node*& head, Node*& tail, int val) {
  Node* newNode = new Node{val, NULL, NULL};
  if (head == NULL) {
    head = newNode;
    tail = newNode;
  } else {
    tail->next = newNode;
    newNode->prev = tail;
    head = newNode;  // Error here
  }
}

medium

A. head should be updated to newNode

B. newNode->prev should be NULL

C. tail should be updated to newNode

D. tail->next should be NULL

Solution

1. Step 1: Analyze pointer updates in else block

   tail->next points to newNode and newNode->prev points to tail correctly.

2. Step 2: Identify incorrect pointer assignment

   head is wrongly updated to newNode; it should remain unchanged. Instead, tail should be updated to newNode.

3. Final Answer:

   tail should be updated to newNode -> Option C

4. Quick Check:

   Tail points to last node after insertion [OK]

Hint: Update tail, not head, when inserting at end [OK]

Common Mistakes:

• Updating head instead of tail
• Not linking newNode's prev pointer
• Forgetting to set tail->next

5. Given a doubly linked list with nodes 5 <-> 10 <-> 15, write the sequence of node values after inserting 20 at the end and then 2 at the end. What is the final list?

hard

A. 2 <-> 5 <-> 10 <-> 15 <-> 20

B. 5 <-> 10 <-> 15 <-> 20 <-> 2

C. 5 <-> 10 <-> 15 <-> 2 <-> 20

D. 20 <-> 2 <-> 5 <-> 10 <-> 15

Solution

1. Step 1: Insert 20 at the end

   Starting list: 5 <-> 10 <-> 15. After inserting 20 at end: 5 <-> 10 <-> 15 <-> 20.

2. Step 2: Insert 2 at the end

   Now add 2 at end: 5 <-> 10 <-> 15 <-> 20 <-> 2.

3. Final Answer:

   5 <-> 10 <-> 15 <-> 20 <-> 2 -> Option B

4. Quick Check:

   Insertions add nodes at list end in order [OK]

Hint: Insertions add nodes at the end, order preserved [OK]

Common Mistakes:

• Inserting new nodes at the start instead of end
• Mixing order of inserted nodes
• Confusing next and prev pointers