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.

intermediate

Why use a linked list to implement a stack?

Using a linked list allows the stack to grow and shrink dynamically without fixed size limits. It avoids wasted space and resizing overhead compared to arrays.

beginner

What are the main operations of a stack implemented with a linked list?

The main operations are:
1. Push: Add an element to the top.
2. Pop: Remove the top element.
3. Peek/Top: View the top element without removing it.
4. IsEmpty: Check if the stack has no elements.

intermediate

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

Push creates a new node with the data, sets its next pointer to the current top node, then updates the top pointer to this new node. This adds the new element at the top.

intermediate

What happens during the pop operation in a linked list stack?

Pop removes the top node by saving its data, moving the top pointer to the next node, and deleting the old top node. This removes the last added element.

Which principle does a stack follow?

A Priority Based

B First In, First Out (FIFO)

C Random Access

D Last In, First Out (LIFO)

A stack follows the Last In, First Out (LIFO) principle where the last element added is the first to be removed.

In a linked list stack, what does the top pointer represent?

A The last node added to the stack

B The first node in the list

C The bottom node of the stack

D The middle node of the list

The top pointer points to the last node added, which is the top of the stack.

What happens if you pop from an empty stack?

A Returns the bottom element

B Returns an error or null

C Adds a new element

D Does nothing and continues

Popping from an empty stack is invalid and usually returns an error or null to indicate no elements.

Which operation adds an element to the stack?

A Pop

B Peek

C Push

D IsEmpty

Push operation adds a new element to the top of the stack.

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

A O(1)

B O(log n)

C O(n^2)

D O(n)

Both push and pop operations take constant time O(1) because they only change the top pointer and node links.

Explain how to implement a stack using a linked list and describe the push and pop operations.

Think about how nodes link and how the top pointer changes.

What are the advantages of using a linked list to implement a stack compared to using an array?

Consider memory and size flexibility.

Practice

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

easy

A. It uses less memory than an array

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

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 property

   Linked lists can grow or shrink dynamically by adding or removing nodes, so stack size is not fixed.

3. Final Answer:

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

4. Quick Check:

   Linked list stack size = dynamic growth [OK]

Hint: Linked list stacks grow dynamically, arrays have fixed size [OK]

Common Mistakes:

• Thinking linked list uses less memory always
• Assuming stack allows random access
• Confusing sorting with stack operations

2. Which of the following is the correct syntax to create a new node in a linked list stack in C++?

easy

A. Node* newNode = new Node(value);

B. Node newNode = Node(value);

C. Node newNode = new Node(value);

D. Node* newNode = Node(value);

Solution

1. Step 1: Understand dynamic memory allocation in C++

   To create a node dynamically, use 'new' keyword and pointer type.

2. Step 2: Check syntax correctness

   Node* newNode = new Node(value); uses pointer and 'new' correctly: Node* newNode = new Node(value);

3. Final Answer:

   Node* newNode = new Node(value); -> Option A

4. Quick Check:

   Dynamic node creation = new + pointer [OK]

Hint: Use 'new' with pointer to create nodes dynamically [OK]

Common Mistakes:

• Omitting pointer (*) when using new
• Using stack allocation instead of dynamic
• Assigning new object to non-pointer variable

3. What will be the output after executing the following code snippet for a stack implemented using linked list?

Stack s;
s.push(10);
s.push(20);
s.push(30);
s.pop();
s.push(40);
s.display();

Assuming display prints elements from top to bottom.

medium

A. 40 -> 30 -> 20 -> 10 -> null

B. 30 -> 20 -> 10 -> null

C. 20 -> 10 -> null

D. 40 -> 20 -> 10 -> null

Solution

1. Step 1: Trace push and pop operations

   Push 10, stack: 10; push 20, stack: 20->10; push 30, stack: 30->20->10; pop removes 30, stack: 20->10; push 40, stack: 40->20->10.

2. Step 2: Determine display output

   Display prints from top to bottom: 40 -> 20 -> 10 -> null.

3. Final Answer:

   40 -> 20 -> 10 -> null -> Option D

4. Quick Check:

   Stack after ops = 40->20->10 [OK]

Hint: Track push/pop stepwise to find top element [OK]

Common Mistakes:

• Forgetting pop removes top element
• Assuming push adds at bottom
• Mixing order of elements in display

4. Identify the error in the following pop function of a linked list stack:

int pop() {
  if (top == NULL) {
    cout << "Stack Underflow";
    return -1;
  }
  int val = top->data;
  delete top;
  top = top->next;
  return val;
}

medium

A. Returning -1 on empty stack is incorrect

B. Missing check for stack overflow

C. Accessing top->next after deleting top causes undefined behavior

D. Not updating top pointer after pop

Solution

1. Step 1: Analyze order of operations in pop

   Code deletes top node before accessing top->next, which is unsafe.

2. Step 2: Identify correct order

   Must save top->next in a temp pointer before deleting top, then update top.

3. Final Answer:

   Accessing top->next after deleting top causes undefined behavior -> Option C

4. Quick Check:

   Delete before next access = error [OK]

Hint: Save next pointer before deleting current node [OK]

Common Mistakes:

• Deleting node before updating pointer
• Ignoring stack underflow check
• Returning wrong values on empty stack

5. You want to implement a function to reverse a stack implemented using a linked list without using extra data structures. Which approach is correct?

hard

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

B. Traverse the linked list and swap data of nodes pairwise

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

D. Use a queue to reorder elements and then push back to stack

Solution

1. Step 1: Understand constraints

   No extra data structures allowed, so cannot use queue or new list.

2. Step 2: Use recursion to reverse stack

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

3. Step 3: Why other options fail

   Swapping data pairwise does not reverse entire stack; copying needs extra list; queue is extra structure.

4. Final Answer:

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

5. Quick Check:

   Recursion reverses stack without extra structures [OK]

Hint: Recursion can reverse stack without extra memory [OK]

Common Mistakes:

• Using extra data structures despite constraints
• Trying to swap data instead of nodes
• Not understanding recursion stack behavior