Stack Implementation Using Linked List in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this stack push and pop sequence?

Consider a stack implemented using a linked list. The following operations are performed in order:

1. Push 10
2. Push 20
3. Pop
4. Push 30
5. Pop
6. Pop

What is the printed output of the popped elements?

DSA C

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

struct Node* top = NULL;

void push(int x) {
    struct Node* temp = (struct Node*)malloc(sizeof(struct Node));
    temp->data = x;
    temp->next = top;
    top = temp;
}

int pop() {
    if (top == NULL) return -1;
    int val = top->data;
    struct Node* temp = top;
    top = top->next;
    free(temp);
    return val;
}

int main() {
    push(10);
    push(20);
    printf("%d ", pop());
    push(30);
    printf("%d ", pop());
    printf("%d\n", pop());
    return 0;
}

A 20 30 10

B 10 20 30

C 30 20 10

D 20 10 30

💡 Hint

Remember that stack is Last-In-First-Out (LIFO). The last pushed element is popped first.

The push and pop operations follow LIFO. After pushing 10 and 20, popping returns 20. Then pushing 30 and popping returns 30. Finally, popping returns the remaining 10.

🧠 Conceptual

intermediate

What is the time complexity of push and pop operations in a linked list based stack?

For a stack implemented using a linked list, what is the time complexity of the push and pop operations?

A Push is O(1), pop is O(n)

B Both push and pop are O(1)

C Both push and pop are O(n)

D Push is O(n), pop is O(1)

💡 Hint

Think about how many nodes you need to traverse for push or pop.

Both push and pop operate only at the top node, so they take constant time O(1).

🔧 Debug

advanced

What error does this stack pop function cause?

Consider this pop function for a linked list stack:

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

What error will occur when pop is called on an empty stack?

A Segmentation fault (accessing NULL pointer)

B Stack Underflow message printed and returns -1

C Compilation error due to missing return statement

D No error, returns 0

💡 Hint

Look at what happens after the if condition when top is NULL.

If top is NULL, the function prints a message but continues to access top->data which causes a segmentation fault (crash).

❓ Predict Output

advanced

What is the output after these stack operations?

Given a linked list stack, the following code is executed:

push(5);
push(15);
push(25);
pop();
push(35);
pop();
pop();

What is the printed output of the popped elements?

DSA C

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

struct Node* top = NULL;

void push(int x) {
    struct Node* temp = (struct Node*)malloc(sizeof(struct Node));
    temp->data = x;
    temp->next = top;
    top = temp;
}

int pop() {
    if (top == NULL) return -1;
    int val = top->data;
    struct Node* temp = top;
    top = top->next;
    free(temp);
    return val;
}

int main() {
    push(5);
    push(15);
    push(25);
    printf("%d ", pop());
    push(35);
    printf("%d ", pop());
    printf("%d\n", pop());
    return 0;
}

A 15 35 25

B 25 15 35

C 35 25 15

D 25 35 15

💡 Hint

Trace the stack top after each operation carefully.

Push 5,15,25 → stack top is 25. Pop returns 25. Push 35 → top is 35. Pop returns 35. Pop returns 15.

🧠 Conceptual

expert

What is the maximum number of nodes in a linked list stack after these operations?

Starting with an empty linked list stack, the following operations are performed:

push(1);
push(2);
pop();
push(3);
push(4);
pop();
pop();
push(5);
push(6);
pop();

What is the maximum number of nodes present in the stack at any point during these operations?

A 2

B 4

C 3

D 5

💡 Hint

Count the stack size after each push and pop operation.

The stack sizes after each operation are: push(1)=1, push(2)=2, pop=1, push(3)=2, push(4)=3, pop=2, pop=1, push(5)=2, push(6)=3, pop=2. Maximum size is 3.

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