Stack Implementation Using Linked List in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node for the stack.

DSA C

struct Node* newNode(int data) {
    struct Node* node = (struct Node*)malloc(sizeof(struct Node));
    node->data = [1];
    node->next = NULL;
    return node;
}

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

A0

BNULL

Cnode

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning NULL or 0 instead of the input data.

Assigning the node pointer itself.

The new node's data field should be set to the input parameter 'data'.

2fill in blank

medium

Complete the code to check if the stack is empty.

DSA C

int isEmpty(struct Node* top) {
    return [1] == NULL;
}

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

Atop->next

Btop

Ctop->data

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Checking top->next instead of top.

Comparing data field to NULL.

The stack is empty if the top pointer is NULL.

3fill in blank

hard

Fix the error in the push function to correctly add a new node on top of the stack.

DSA C

void push(struct Node** top_ref, int data) {
    struct Node* node = newNode(data);
    node->next = [1];
    *top_ref = node;
}

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

A*top_ref->next

Btop_ref

C*top_ref

Dtop_ref->next

Attempts:

3 left

💡 Hint

Common Mistakes

Using top_ref instead of *top_ref.

Using top_ref->next which is invalid pointer access.

The new node's next should point to the current top node, which is *top_ref.

4fill in blank

hard

Fill both blanks to correctly pop the top element from the stack and return its data.

DSA C

int pop(struct Node** top_ref) {
    if (isEmpty(*top_ref))
        return -1;
    struct Node* temp = [1];
    int popped = temp->data;
    *top_ref = [2];
    free(temp);
    return popped;
}

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

A*top_ref

B(*top_ref)->next

Ctop_ref

D(*top_ref)

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning temp to top_ref instead of *top_ref.

Not updating *top_ref to the next node.

temp should point to the current top node (*top_ref), and top_ref should be updated to the next node (*top_ref)->next.

5fill in blank

hard

Fill in the blank to implement a peek function that returns the top element's data without removing it.

DSA C

int peek(struct Node* top) {
    if (top == NULL)
        return -1;
    return [1];
}

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

Atop->data

Btop->next

Ctop

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Returning top pointer instead of its data.

Returning top->next which is the next node, not the data.

The peek function returns the data of the top node without modifying the stack.

Practice

1. What is the main advantage of implementing a stack using a linked list instead of an array?

easy

A. The stack can grow dynamically without a fixed size limit

B. It uses less memory than an array

C. It allows random access to elements

D. It is easier to sort the stack elements

Solution

1. Step 1: Understand stack size limitation in arrays

   Arrays have fixed size, so stack implemented with arrays has a maximum capacity.

2. Step 2: Linked list dynamic size

   Linked lists allocate memory dynamically, so the stack can grow as needed without fixed size.

3. Final Answer:

   The stack can grow dynamically without a fixed size limit -> Option A

4. Quick Check:

   Dynamic size = Advantage of linked list stack [OK]

Hint: Linked list stacks grow dynamically, arrays don't [OK]

Common Mistakes:

• Thinking linked list uses less memory always
• Confusing stack with queue behavior
• Assuming random access is possible in stack

2. Which of the following is the correct way to declare a node structure for a stack implemented using a linked list in C?

easy

A. struct Node { int data; struct Node next; };

B. struct Node { int data; int next; };

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

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

Solution

1. Step 1: Understand node structure for linked list

   A node must hold data and a pointer to the next node.

2. Step 2: Check pointer types

   Next must be a pointer to struct Node, not an int or struct Node itself.

3. Final Answer:

   struct Node { int data; struct Node* next; }; -> Option C

4. Quick Check:

   Node has data + pointer to next [OK]

Hint: Next pointer must be struct Node* not int or struct Node [OK]

Common Mistakes:

• Using int instead of pointer for next
• Declaring next as struct Node instead of pointer
• Using pointer for data unnecessarily

3. Consider the following C code snippet for pushing elements onto a stack implemented using a linked list:

void push(struct Node** top_ref, int new_data) {
  struct Node* new_node = (struct Node*) malloc(sizeof(struct Node));
  new_node->data = new_data;
  new_node->next = *top_ref;
  *top_ref = new_node;
}

// Initial stack is empty (top_ref = NULL)
push(&top, 10);
push(&top, 20);
push(&top, 30);

What will be the order of elements in the stack from top to bottom after these pushes?

medium

A. 10 -> 20 -> 30 -> NULL

B. 30 -> 20 -> 10 -> NULL

C. 20 -> 30 -> 10 -> NULL

D. NULL -> 10 -> 20 -> 30

Solution

1. Step 1: Trace first push(10)

   Stack top points to node with data 10, next is NULL.

2. Step 2: Trace second push(20) and third push(30)

   Each new node becomes top, so order is 30 (top) -> 20 -> 10 -> NULL.

3. Final Answer:

   30 -> 20 -> 10 -> NULL -> Option B

4. Quick Check:

   Last pushed is top = 30 [OK]

Hint: Last pushed is always on top in linked list stack [OK]

Common Mistakes:

• Assuming order is insertion order (FIFO)
• Confusing next pointer direction
• Forgetting top points to last pushed node

4. The following code snippet is intended to pop an element from a stack implemented using a linked list. Identify the error:

int pop(struct Node** top_ref) {
  struct Node* temp;
  int popped;
  if (*top_ref == NULL) {
    printf("Stack Underflow\n");
    return -1;
  }
  temp = *top_ref;
  popped = temp->data;
  *top_ref = temp->next;
  free(temp);
  return popped;
}

medium

A. Should update temp->next to NULL before freeing

B. Missing check for NULL before accessing temp->data

C. Should not free temp after popping

D. No error, code correctly pops the top element

Solution

1. Step 1: Check for empty stack

   Code correctly checks if *top_ref is NULL and returns underflow error.

2. Step 2: Pop operation correctness

   It saves top node, retrieves data, updates top to next node, frees old top, and returns data.

3. Final Answer:

   No error, code correctly pops the top element -> Option D

4. Quick Check:

   Pop code with free and update top is correct [OK]

Hint: Pop frees top node and updates pointer correctly [OK]

Common Mistakes:

• Forgetting to free popped node causing memory leak
• Not checking for empty stack before pop
• Incorrectly updating top pointer

5. You want to implement a function reverseStack that reverses the elements of a stack implemented using a linked list without using any extra stack or array. Which approach is correct?

hard

A. Use recursion to pop all elements, then push them back in reverse order

B. Traverse the linked list and swap data of first and last nodes repeatedly

C. Create a new linked list and copy elements in reverse order

D. Use a queue to dequeue and enqueue elements to reverse

Solution

1. Step 1: Understand constraints

   No extra stack or array allowed, so iterative copying or extra data structures are not allowed.

2. Step 2: Use recursion to reverse

   Recursion can pop all elements, then push them back in reverse order naturally using call stack.

3. Step 3: Other options use extra data or incorrect method

   Swapping data repeatedly is complex and error-prone; copying creates extra list; queue uses extra structure.

4. Final Answer:

   Use recursion to pop all elements, then push them back in reverse order -> Option A

5. Quick Check:

   Recursion reverses stack without extra data structures [OK]

Hint: Recursion uses call stack to reverse without extra structures [OK]

Common Mistakes:

• Trying to swap node data instead of rearranging nodes
• Using extra arrays or stacks against constraints
• Confusing queue usage for stack reversal