Stack Implementation Using Linked List in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack and how does it work?

A stack is a collection where elements are added and removed from only one end called the top. It works on Last In, First Out (LIFO) principle, meaning the last element added is the first to be removed.

beginner

What is the role of a linked list in stack implementation?

A linked list stores stack elements dynamically. Each node holds data and a pointer to the next node. The top of the stack points to the first node, allowing easy push and pop operations without fixed size limits.

intermediate

Explain the push operation in a stack implemented with a linked list.

Push adds a new node at the start of the linked list. The new node's next pointer points to the current top node. Then, the top pointer updates to this new node, making it the new top of the stack.

intermediate

How does the pop operation work in a linked list stack?

Pop removes the top node from the linked list. The top pointer moves to the next node after the current top. The removed node's memory is freed, and its data is returned.

beginner

What happens if you try to pop from an empty stack implemented using a linked list?

If the stack is empty (top is NULL), pop operation cannot remove any element. This is called stack underflow, and the program should handle it by showing an error or returning a special value.

What does LIFO stand for in stack operations?

A Last In, First Out

B Last In, Final Out

C List In, First Out

D Linked In, First Out

LIFO means the last element added to the stack is the first one to be removed.

In a linked list stack, where is the new element added during push?

A At the end of the list

B At a random position

C In the middle of the list

D At the start of the list

Push adds the new node at the start of the linked list, updating the top pointer.

What pointer does the top of the stack hold in a linked list implementation?

A Pointer to the last node

B Pointer to the middle node

C Pointer to the first node

D Pointer to NULL always

The top pointer points to the first node in the linked list representing the stack's top.

What is stack underflow?

A Trying to push on a full stack

B Trying to pop from an empty stack

C Trying to peek at the top element

D Trying to push duplicate elements

Stack underflow happens when pop is called on an empty stack.

Which operation frees memory in a linked list stack?

A Pop

B Push

C Peek

D IsEmpty

Pop removes the top node and frees its memory.

Describe how push and pop operations work in a stack implemented using a linked list.

Think about how the linked list nodes connect and how the top pointer changes.

Explain what happens when you try to pop from an empty stack implemented with a linked list and how to handle it.

Consider what the top pointer value means when the stack is empty.

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