Pop Using Linked List Node in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C

struct Node* pop(struct Node** top) {
    if (*top == NULL) {
        return NULL;
    }
    struct Node* temp = *top;
    *top = [1];
    return temp;
}

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

A(*top)->next

B(*top)->prev

Ctop->next

Dtop->prev

Attempts:

3 left

💡 Hint

Common Mistakes

Using (*top)->prev instead of (*top)->next.

Using top->next which is incorrect because top is a double pointer.

Not updating the top pointer at all.

To pop the top node, we move the top pointer to the next node in the list, which is (*top)->next.

2fill in blank

medium

Complete the code to free the popped node after removing it from the stack.

DSA C

void pop_and_free(struct Node** top) {
    if (*top == NULL) {
        return;
    }
    struct Node* temp = *top;
    *top = (*top)->next;
    [1];
}

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

Afree(temp)

Bfree(*top)

Cfree(temp->next)

Dfree(top)

Attempts:

3 left

💡 Hint

Common Mistakes

Freeing *top instead of temp.

Freeing temp->next which is still part of the stack.

Freeing the double pointer top which is incorrect.

After updating the top pointer, we free the memory of the popped node stored in temp.

3fill in blank

hard

Fix the error in the pop function to correctly handle empty stack and return the popped node.

DSA C

struct Node* pop(struct Node** top) {
    if ([1]) {
        return NULL;
    }
    struct Node* temp = *top;
    *top = (*top)->next;
    return temp;
}

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

Atop == NULL

B*top == NULL

C*top != NULL

Dtop != NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Checking top == NULL which checks the pointer to pointer, not the node.

Using != NULL instead of == NULL.

Not checking for empty stack at all.

We check if the stack is empty by testing if *top is NULL before popping.

4fill in blank

hard

Fill both blanks to update the top pointer and detach the popped node from the list.

DSA C

struct Node* pop(struct Node** top) {
    if (*top == NULL) {
        return NULL;
    }
    struct Node* temp = *top;
    *top = [1];
    [2] = NULL;
    return temp;
}

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

A(*top)->next

B(*top)->prev

Ctemp->next

Dtemp->prev

Attempts:

3 left

💡 Hint

Common Mistakes

Setting temp->next to NULL instead of temp->prev.

Updating top to temp->next instead of (*top)->next.

Not detaching the popped node properly.

We move top to the next node and set the popped node's prev pointer to NULL to detach it.

5fill in blank

hard

Fill all three blanks to pop the top node, update the top pointer, and free the popped node safely.

DSA C

void pop_and_free(struct Node** top) {
    if (*top == NULL) {
        return;
    }
    struct Node* temp = *top;
    *top = [1];
    if (*top != NULL) {
        [2] = NULL;
    }
    [3];
}

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

A(*top)->next

B(*top)->prev

Cfree(temp)

Dtemp->next

Attempts:

3 left

💡 Hint

Common Mistakes

Using (*top)->next instead of temp->next to update top.

Not checking if *top is NULL before setting prev.

Freeing *top instead of temp.

We update top to temp->next, set the new top's prev to NULL, and free the popped node temp.

Practice

1. What does the pop operation do in a linked list implemented as a stack?

easy

A. Deletes the entire linked list

B. Adds a new node at the end of the linked list

C. Removes and returns the first node (top) of the linked list

D. Returns the last node without removing it

Solution

1. Step 1: Understand pop operation in stack

   Pop removes the top element from the stack, which is the first node in a linked list stack.

2. Step 2: Apply to linked list

   In a linked list used as a stack, the first node is removed and returned during pop.

3. Final Answer:

   Removes and returns the first node (top) of the linked list -> Option C

4. Quick Check:

   Pop = remove first node [OK]

Hint: Pop removes the first node in linked list stack [OK]

Common Mistakes:

• Thinking pop removes last node
• Confusing pop with push
• Assuming pop deletes entire list

2. Which of the following is the correct syntax to pop a node from a linked list stack in C?

easy

A. node* pop(node** head) { node* temp = *head; *head = (*head)->next; return temp; }

B. node pop(node head) { head = head->next; return head; }

C. void pop(node* head) { head = head->next; }

D. node* pop(node* head) { head = head->next; return head; }

Solution

1. Step 1: Check parameter passing for head pointer

   To modify the head pointer, we pass a pointer to pointer (node** head).

2. Step 2: Verify pop logic

   Store current head in temp, move head to next node, return temp node.

3. Final Answer:

   node* pop(node** head) { node* temp = *head; *head = (*head)->next; return temp; } -> Option A

4. Quick Check:

   Pointer to pointer needed to update head [OK]

Hint: Use node** to update head pointer in pop [OK]

Common Mistakes:

• Passing head by value instead of pointer to pointer
• Returning wrong node or void
• Not updating head pointer correctly

3. What will be the output of the following code snippet?

typedef struct node { int data; struct node* next; } node;

node* pop(node** head) {
  if (*head == NULL) return NULL;
  node* temp = *head;
  *head = (*head)->next;
  return temp;
}

int main() {
  node n3 = {3, NULL};
  node n2 = {2, &n3};
  node n1 = {1, &n2};
  node* head = &n1;
  node* popped = pop(&head);
  printf("%d -> %d -> %d -> NULL", head->data, head->next->data, head->next->next->data);
  return 0;
}

medium

A. 2 -> 3 -> 0 -> NULL

B. 2 -> 3 -> NULL -> NULL

C. 2 -> 3 -> NULL

D. 2 -> 3 -> (segmentation fault)

Solution

1. Step 1: Trace pop operation

   Pop removes node with data 1, head now points to node with data 2.

2. Step 2: Trace printf arguments

   head->data = 2, head->next->data = 3, head->next->next = NULL, accessing ->data causes segmentation fault after printing "2 -> 3 -> " .

3. Final Answer:

   2 -> 3 -> (segmentation fault) -> Option D

4. Quick Check:

   Accessing NULL->data causes crash [OK]

Hint: Accessing next of last node causes crash [OK]

Common Mistakes:

• Assuming NULL prints as 'NULL' string
• Ignoring segmentation fault on NULL dereference
• Misreading linked list after pop

4. Identify the bug in this pop function and select the correct fix:

node* pop(node* head) {
  node* temp = head;
  head = head->next;
  return temp;
}

medium

A. Pass head as node** and update *head inside function

B. Return head instead of temp

C. Set temp->next = NULL before return

D. No bug, function is correct

Solution

1. Step 1: Analyze head pointer update

   Function updates local copy of head, original head outside remains unchanged.

2. Step 2: Fix pointer update

   Pass pointer to pointer (node**) to modify original head pointer.

3. Final Answer:

   Pass head as node** and update *head inside function -> Option A

4. Quick Check:

   Use node** to update head pointer [OK]

Hint: Use node** to update head pointer in pop [OK]

Common Mistakes:

• Updating local copy of head only
• Not passing pointer to pointer
• Returning wrong node

5. Given a linked list stack with nodes containing integers, write the logic to pop all nodes and print their data in order. Which approach is correct?

hard

A. Traverse list without popping and print data

B. Use a loop: while head != NULL, pop node, print data, free node

C. Pop once, print data, then stop

D. Recursively call pop until head is NULL, then print all data after recursion

Solution

1. Step 1: Understand popping all nodes

   Pop removes top node each time, so loop until list is empty.

2. Step 2: Print and free nodes in loop

   Print data of popped node immediately, then free memory to avoid leaks.

3. Final Answer:

   Use a loop: while head != NULL, pop node, print data, free node -> Option B

4. Quick Check:

   Loop pop-print-free until empty [OK]

Hint: Loop pop until head NULL, print and free each node [OK]

Common Mistakes:

• Recursing without printing in correct order
• Not freeing nodes causing memory leaks
• Stopping after one pop