Pop Using Linked List Node in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove the top node from the linked list stack.

DSA C++

Node* pop(Node*& head) {
    if (head == nullptr) return nullptr;
    Node* temp = head;
    head = head->[1];
    return temp;
}

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

Alink

Bprev

Cchild

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' causes errors because 'prev' is not defined.

Using 'child' or 'link' which are not valid pointers in this node structure.

The 'next' pointer points to the next node in the linked list. To pop the top node, we move head to head->next.

2fill in blank

medium

Complete the code to check if the linked list stack is empty before popping.

DSA C++

bool isEmpty(Node* head) {
    return head == [1];
}

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

A0

Bnullptr

Chead

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or NULL instead of nullptr in modern C++.

Comparing head to itself which is always true.

The head pointer is nullptr when the linked list is empty.

3fill in blank

hard

Fix the error in the pop function to properly delete the popped node.

DSA C++

Node* pop(Node*& head) {
    if (head == nullptr) return nullptr;
    Node* temp = head;
    head = head->next;
    [1] temp;
    return nullptr;
}

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

Aremove

Bfree

Cdelete

Ddispose

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'free' instead of 'delete' in C++.

Not deleting the node causing memory leaks.

In C++, 'delete' is used to free memory allocated with 'new'.

4fill in blank

hard

Fill both blanks to pop the top node and return its value.

DSA C++

int popValue(Node*& head) {
    if (head == nullptr) return -1;
    Node* temp = head;
    int val = temp->[1];
    head = head->[2];
    delete temp;
    return val;
}

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

Adata

Bvalue

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' instead of 'data' for the node's stored value.

Using 'prev' instead of 'next' to move head forward.

The node's stored value is in 'data', and the next node is pointed to by 'next'.

5fill in blank

hard

Fill all three blanks to pop the top node, update head, and safely return the popped node's data.

DSA C++

int popSafe(Node*& head) {
    if (head == nullptr) return -1;
    Node* temp = head;
    int result = temp->[1];
    head = head->[2];
    [3] temp;
    return result;
}

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

Adata

Bnext

Cdelete

Dfree

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'free' instead of 'delete' to free memory.

Returning the node pointer instead of its data value.

We access the node's 'data', move head to 'next', and delete the popped node with 'delete'.

Practice

1. What does the pop() operation do in a linked list when implemented using nodes?

easy

A. Removes the first node and returns its value

B. Adds a new node at the end

C. Deletes all nodes in the list

D. Finds a node with a specific value

Solution

1. Step 1: Understand pop operation in linked list context

   Pop means removing the top or first element from the list, similar to popping from a stack.

2. Step 2: Identify what happens to the first node

   The first node is removed and its value is returned, and the head pointer moves to the next node.

3. Final Answer:

   Removes the first node and returns its value -> Option A

4. Quick Check:

   Pop = Remove first node [OK]

Hint: Pop removes the first node like taking the top card [OK]

Common Mistakes:

• Thinking pop adds a node
• Confusing pop with delete all
• Assuming pop searches for a value

2. Which of the following is the correct syntax to remove the first node in a singly linked list in C++?

easy

A. delete head->next;

B. head->next = head;

C. head = head->next;

D. head = NULL;

Solution

1. Step 1: Understand how to move head pointer

   To remove the first node, head must point to the second node, which is head->next.

2. Step 2: Check each option

   head = head->next; correctly assigns head to head->next. head->next = head; reverses pointers incorrectly. delete head->next; deletes the second node, not the first. head = NULL; sets head to NULL, deleting entire list.

3. Final Answer:

   head = head->next; -> Option C

4. Quick Check:

   Move head to next node = head = head->next; [OK]

Hint: Pop means head points to next node [OK]

Common Mistakes:

• Deleting wrong node
• Setting head to NULL incorrectly
• Reversing pointer assignment

3. What will be the output after executing the following code?

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

Node* head = new Node{10, nullptr};
head->next = new Node{20, nullptr};

int popValue = head->data;
head = head->next;

std::cout << popValue << " -> " << (head ? std::to_string(head->data) : "null") << std::endl;

medium

A. 20 -> 10

B. 10 -> 20

C. 10 -> null

D. 20 -> null

Solution

1. Step 1: Trace initial list nodes

   head points to node with data 10, which points to node with data 20.

2. Step 2: Execute pop operation

   popValue stores 10 (head's data), then head moves to next node (20).

3. Step 3: Print values

   Output prints popValue (10) and head's data (20), separated by ' -> '.

4. Final Answer:

   10 -> 20 -> Option B

5. Quick Check:

   Pop value = 10, new head = 20 [OK]

Hint: Pop returns old head data, head moves to next node [OK]

Common Mistakes:

• Printing new head data as pop value
• Confusing order of output
• Not updating head pointer

4. Identify the error in the following pop function for a singly linked list:

int pop(Node*& head) {
  int val = head->data;
  delete head;
  head = head->next;
  return val;
}

medium

A. Accessing head->next after deleting head

B. Not checking if head is NULL before pop

C. Returning value before deleting node

D. Function should return void

Solution

1. Step 1: Analyze order of operations

   The code deletes head node first, then tries to access head->next, which is invalid after deletion.

2. Step 2: Identify correct order

   Must save head->next before deleting head to avoid accessing freed memory.

3. Final Answer:

   Accessing head->next after deleting head -> Option A

4. Quick Check:

   Delete before accessing next = error [OK]

Hint: Save next pointer before deleting node [OK]

Common Mistakes:

• Deleting node before saving next
• Ignoring NULL head check
• Wrong return type assumption

5. Given a linked list with nodes containing values [5, 10, 15], write the correct sequence of operations to pop all nodes and print their values in order using C++ pointers.
Which code snippet correctly implements this?

hard

A. while(head) { int val = head->data; head = head->next; std::cout << val << " "; }

B. while(head) { std::cout << head->data << " "; delete head; }

C. while(head) { delete head; head = head->next; }

D. while(head) { int val = head->data; Node* temp = head; head = head->next; delete temp; std::cout << val << " "; }

Solution

1. Step 1: Understand safe pop and print

   We must print the current node's data, save the current node pointer, move head to next, then delete the saved node.

2. Step 2: Check each option

   while(head) { int val = head->data; head = head->next; std::cout << val << " "; } does not delete nodes, causing memory leak. while(head) { int val = head->data; Node* temp = head; head = head->next; delete temp; std::cout << val << " "; } correctly saves node in temp, moves head, deletes temp, and prints value. while(head) { delete head; head = head->next; } deletes head before moving, causing invalid access. while(head) { std::cout << head->data << " "; delete head; } deletes head but does not move head pointer, causing infinite loop or crash.

3. Final Answer:

   while(head) { int val = head->data; Node* temp = head; head = head->next; delete temp; std::cout << val << " "; } -> Option D

4. Quick Check:

   Save node, move head, delete node, print value [OK]

Hint: Save node before delete, move head after [OK]

Common Mistakes:

• Deleting node before moving head
• Not deleting nodes causing leaks
• Not moving head pointer