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 data structure that works like a stack of plates. You add (push) and remove (pop) items only from the top. It follows Last In, First Out (LIFO) order.

beginner

Why use a linked list to implement a stack?

Using a linked list lets the stack grow and shrink easily without fixed size limits. Each item points to the next, so adding or removing from the top is fast and simple.

beginner

What does the Push operation do in a stack implemented with a linked list?

Push adds a new node at the start (top) of the linked list. The new node points to the old top, and then becomes the new top.

beginner

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

Pop removes the top node from the linked list. The top pointer moves to the next node. The removed node's value is returned.

beginner

How do you check if the stack is empty in a linked list implementation?

If the top pointer is null, it means the stack has no nodes and is empty.

What order does a stack follow?

A Last In, First Out (LIFO)

B First In, First Out (FIFO)

C Random Order

D Sorted Order

A stack always removes the most recently added item first, which is Last In, First Out.

In a linked list stack, where is the new node added during Push?

A In the middle of the list

B At the end of the list

C At the start (top) of the list

D Nowhere, it replaces the old top

Push adds the new node at the start, making it the new top.

What does the Pop operation return?

A The size of the stack

B The value of the bottom node

C Nothing

D The value of the top node

Pop removes and returns the value of the top node.

How do you know if the stack is empty?

A Top pointer is null

B Top pointer points to a node

C Stack size is greater than zero

D Stack has one node

If the top pointer is null, the stack has no nodes and is empty.

What is the time complexity of Push and Pop in a linked list stack?

A O(n^2)

B O(1)

C O(log n)

D O(n)

Both Push and Pop operate at the top node, so they run in constant time O(1).

Explain how a stack is implemented using a linked list and how Push and Pop operations work.

Think about adding and removing plates from the top of a stack.

Describe how to check if a linked list stack is empty and why this check is important.

Empty means no plates on the stack.

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 automatically sorts the elements

Solution

1. Step 1: Understand stack size limitation in arrays

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

2. Step 2: Linked list dynamic size

   Linked lists can grow or shrink dynamically by adding or removing nodes, so the stack size is limited only by memory.

3. Final Answer:

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

4. Quick Check:

   Linked list stacks grow dynamically [OK]

Hint: Linked list stacks grow dynamically, arrays do not [OK]

Common Mistakes:

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

2. Which of the following is the correct way to push an element onto a stack implemented using a linked list in C#?

easy

A. Create a new node, set its next to current top, then update top to new node

B. Create a new node, set its previous to current top, then update top to new node

C. Add the new element at the end of the linked list

D. Replace the top node's value with the new element

Solution

1. Step 1: Understand push operation in linked list stack

   Push adds a new node at the beginning (top) of the linked list.

2. Step 2: Correct pointer update

   The new node's next points to the current top, then top is updated to new node.

3. Final Answer:

   Create a new node, set its next to current top, then update top to new node -> Option A

4. Quick Check:

   Push adds node at front [OK]

Hint: Push adds node at front, update top pointer [OK]

Common Mistakes:

• Using previous pointer instead of next
• Adding node at end instead of front
• Overwriting top node value instead of adding new node

3. What will be the output after the following C# code executes?

StackNode top = null;
// Push 10
StackNode node1 = new StackNode(10);
node1.next = top;
top = node1;
// Push 20
StackNode node2 = new StackNode(20);
node2.next = top;
top = node2;
// Pop
int popped = top.data;
top = top.next;
// Print top data
Console.WriteLine(top.data);

medium

A. NullReferenceException

B. 20

C. 10

D. 0

Solution

1. Step 1: Trace push operations

   Push 10: top points to node with data 10.
   Push 20: top points to node with data 20, next points to node 10.

2. Step 2: Trace pop operation

   Pop removes top node (20), top updated to next node (10).

3. Step 3: Print top data

   Top now points to node with data 10, so output is 10.

4. Final Answer:

   10 -> Option C

5. Quick Check:

   After pop, top data = 10 [OK]

Hint: Pop removes top, next node becomes new top [OK]

Common Mistakes:

• Printing popped value instead of new top
• Confusing push order
• Not updating top after pop

4. Identify the error in this pop method for a stack implemented using a linked list:

public int Pop() {
    if (top == null) {
        Console.WriteLine("Stack Underflow");
    }
    int value = top.data;
    top = top.next;
    return value;
}

medium

A. It does not return a value when stack is empty

B. It does not update the top pointer

C. It incorrectly prints "Stack Overflow" instead of "Underflow"

D. It removes the bottom node instead of the top

Solution

1. Step 1: Check behavior when stack is empty

   If top is null, method prints message but continues to access top.data causing error or returns no value.

2. Step 2: Missing return for empty stack case

   Method should return or throw exception when stack is empty to avoid runtime error.

3. Final Answer:

   It does not return a value when stack is empty -> Option A

4. Quick Check:

   Empty stack must return or throw [OK]

Hint: Always return or throw on empty stack pop [OK]

Common Mistakes:

• Ignoring empty stack case
• Confusing overflow with underflow
• Not updating top pointer

5. You want to implement a method ReverseStack that reverses the elements of a stack implemented using a linked list without using extra data structures. Which approach is correct?

hard

A. Use a queue to dequeue and enqueue elements to reverse order

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

C. Create a new linked list and push elements from old stack to new stack

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

Solution

1. Step 1: Understand constraints

   No extra data structures allowed, so using recursion (call stack) is allowed but no explicit new lists or queues.

2. Step 2: Recursion approach

   Recursively pop all elements, then insert them back at bottom to reverse order without extra storage.

3. Step 3: Other options use extra structures

   Swapping data or using new lists or queues violates constraints or is incorrect for stack reversal.

4. Final Answer:

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

5. Quick Check:

   Recursion reverses stack without extra structures [OK]

Hint: Use recursion to reverse stack without extra data structures [OK]

Common Mistakes:

• Using extra linked lists or queues
• Trying to swap node data incorrectly
• Ignoring no extra structure constraint