Stack Implementation Using Array in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack data structure?

A stack is a collection where elements are added and removed from only one end called the top. It follows Last In, First Out (LIFO) order.

beginner

What does the 'push' operation do in a stack implemented using an array?

The 'push' operation adds a new element to the top of the stack if there is space available in the array.

intermediate

What happens when you try to push an element onto a full stack implemented with an array?

It causes a stack overflow because the array has no more space to hold new elements.

beginner

Explain the 'pop' operation in a stack implemented using an array.

The 'pop' operation removes and returns the element at the top of the stack, reducing the stack size by one.

beginner

How do you check if a stack implemented with an array is empty?

Check if the top index is -1. If yes, the stack is empty because no elements have been added.

In a stack implemented using an array, what does the 'top' variable represent?

A Index of the last added element

B Size of the array

C First element in the array

D Number of empty spaces

The 'top' variable holds the index of the last added element in the stack.

What is the initial value of 'top' in an empty stack implemented using an array?

A 1

B 0

C -1

D Array length

The 'top' is set to -1 to indicate the stack is empty and no elements are present.

What happens if you call 'pop' on an empty stack implemented using an array?

A Returns the first element

B Stack underflow error

C Adds a new element

D Increases the top index

Calling 'pop' on an empty stack causes a stack underflow error because there are no elements to remove.

Which operation adds an element to the stack?

A pop

B peek

C isEmpty

D push

'push' adds an element to the top of the stack.

How do you check if the stack is full in an array implementation?

A top == array length - 1

B array length == 0

C top == 0

D top == -1

The stack is full when 'top' equals the last index of the array (array length - 1).

Describe how to implement a stack using an array including key operations.

Think about how you add and remove items from a stack and how the array index changes.

Explain what happens internally when you push and pop elements in a stack implemented with an array.

Focus on how the 'top' variable changes and how elements are accessed.

Practice

1. What is the main purpose of using an array in a stack implementation?

easy

A. To allow random access to elements anywhere in the stack

B. To store elements in a fixed-size, ordered collection

C. To automatically resize the stack when full

D. To sort elements as they are added

Solution

1. Step 1: Understand stack storage

   A stack stores elements in order, last-in first-out, so it needs a structure to hold elements sequentially.

2. Step 2: Role of array in stack

   An array provides a fixed-size, ordered collection to hold stack elements efficiently.

3. Final Answer:

   To store elements in a fixed-size, ordered collection -> Option B

4. Quick Check:

   Stack uses array for fixed order storage [OK]

Hint: Stack uses array for fixed order storage, not random access [OK]

Common Mistakes:

• Thinking array resizes automatically
• Confusing stack with queue behavior
• Assuming array sorts elements

2. Which of the following is the correct way to declare an integer stack array of size 5 in C#?

easy

A. int[] stack = new int[5];

B. int stack = new int[5];

C. int[] stack = int[5];

D. stack int[] = new int(5);

Solution

1. Step 1: Recall C# array declaration syntax

   In C#, arrays are declared with type[], variable name, and new type[size].

2. Step 2: Match correct syntax

   int[] stack = new int[5]; matches this pattern: int[] stack = new int[5];

3. Final Answer:

   int[] stack = new int[5]; -> Option A

4. Quick Check:

   C# array declaration = int[] name = new int[size]; [OK]

Hint: Use type[] name = new type[size]; for arrays in C# [OK]

Common Mistakes:

• Missing [] after type
• Using parentheses instead of brackets
• Incorrect variable declaration order

3. What will be the output after the following operations on a stack implemented using an array of size 3?

Push(10); Push(20); Pop(); Push(30); Pop(); Pop();

Assume the stack is initially empty and prints the popped values.

medium

A. 20 30 10

B. 20 30 20

C. 20 30 30

D. 10 20 30

Solution

1. Step 1: Trace each operation step-by-step

   Push(10) stack=[10]
   Push(20) stack=[10,20]
   Pop() removes 20, output=20, stack=[10]
   Push(30) stack=[10,30]
   Pop() removes 30, output=30, stack=[10]
   Pop() removes 10, output=10, stack=[]

2. Step 2: Collect popped outputs in order

   The popped values printed are 20, then 30, then 10.

3. Final Answer:

   20 30 10 -> Option A

4. Quick Check:

   Stack LIFO pop order = 20 30 10 [OK]

Hint: Pop prints last pushed element, follow LIFO order [OK]

Common Mistakes:

• Confusing push and pop order
• Assuming FIFO instead of LIFO
• Missing last pop output

4. Identify the error in this stack push method code snippet in C#:

public void Push(int item) {
  if (top == stack.Length) {
    Console.WriteLine("Stack Overflow");
  }
  stack[top] = item;
  top++;
}

medium

A. top should be compared with stack.Length - 1

B. top++ should be before assigning stack[top]

C. No return after overflow check causes overwrite

D. stack array should be resized when full

Solution

1. Step 1: Analyze overflow check logic

   The code checks if top == stack.Length and prints overflow, but continues to assign and increment top anyway.

2. Step 2: Identify missing control flow after overflow

   Without a return or exit after overflow message, the code overwrites stack[top] causing error.

3. Final Answer:

   No return after overflow check causes overwrite -> Option C

4. Quick Check:

   Overflow check must stop push to avoid overwrite [OK]

Hint: Stop push after overflow check to prevent overwrite [OK]

Common Mistakes:

• Assuming top comparison off by one
• Moving top++ before assignment causes wrong index
• Ignoring need to stop after overflow

5. You want to implement a stack using an array that doubles its size automatically when full. Which approach correctly achieves this in C#?

hard

A. Increase the size of the existing array using stack.Resize() method

B. Overwrite oldest elements when stack is full to make space

C. Use a List<int> instead of array to avoid resizing manually

D. Create a new array double the size, copy elements, then assign it to the stack variable

Solution

1. Step 1: Understand array resizing in C#

   Arrays have fixed size; to resize, create a new larger array and copy elements over.

2. Step 2: Evaluate options for resizing stack array

   Create a new array double the size, copy elements, then assign it to the stack variable correctly describes creating a new array double the size, copying elements, and reassigning stack variable.

3. Final Answer:

   Create a new array double the size, copy elements, then assign it to the stack variable -> Option D

4. Quick Check:

   Resize array by new array + copy + assign [OK]

Hint: Arrays fixed size; resize by new array and copy [OK]

Common Mistakes:

• Assuming arrays can resize in place
• Using non-existent Resize method on arrays
• Overwriting data instead of resizing