Push Operation on Stack in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the push operation do in a stack?

The push operation adds a new element to the top of the stack, increasing its size by one.

beginner

In a stack, which end is used for the push operation?

The push operation always adds the element at the top end of the stack.

intermediate

What happens if you try to push an element onto a full stack (stack overflow)?

If the stack has a fixed size and is full, pushing another element causes a stack overflow error, meaning no more elements can be added.

beginner

Show the state of the stack after pushing elements 5, then 10, then 15.

Stack from top to bottom: 15 -> 10 -> 5 -> null

beginner

Why is the push operation considered LIFO in stacks?

Because the last element pushed onto the stack is the first one to be removed, following the Last In, First Out (LIFO) principle.

What does the push operation do in a stack?

A Removes an element from the bottom

B Adds an element to the bottom

C Removes an element from the top

D Adds an element to the top

Push adds an element to the top of the stack.

If a stack is empty, what happens when you push an element?

A The element is added as the top element

B The stack throws an error

C The element is added at the bottom

D Nothing happens

Pushing on an empty stack adds the element as the top (and only) element.

What is the order of elements after pushing 1, then 2, then 3 onto a stack?

A 2 -> 3 -> 1 -> null

B 1 -> 2 -> 3 -> null

C 3 -> 2 -> 1 -> null

D 1 -> 3 -> 2 -> null

The last pushed element (3) is on top, so the order is 3 -> 2 -> 1 -> null.

What error occurs if you push onto a full stack?

A Null pointer exception

B Stack overflow

C Stack underflow

D Index out of range

Pushing onto a full stack causes a stack overflow error.

Which principle does the push operation follow in a stack?

A LIFO (Last In, First Out)

B Priority Queue

C Random Access

D FIFO (First In, First Out)

Stacks follow LIFO, so push adds to the top and pop removes from the top.

Explain the push operation on a stack and how it affects the stack's state.

Think about where the new element goes and what happens to the stack size.

Describe what happens when you push multiple elements onto an empty stack, step by step.

Visualize adding one element at a time on top.

Practice

1. What does the Push operation do in a stack?

easy

A. Adds an element to the top of the stack

B. Removes the top element from the stack

C. Returns the bottom element of the stack

D. Checks if the stack is empty

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, where elements are added or removed from the top.

2. Step 2: Define Push operation

   The Push operation adds a new element on top of the stack, increasing its size by one.

3. Final Answer:

   Adds an element to the top of the stack -> Option A

4. Quick Check:

   Push = Add top element [OK]

Hint: Push means add on top of stack [OK]

Common Mistakes:

• Confusing Push with Pop operation
• Thinking Push removes elements
• Assuming Push accesses bottom element

2. Which of the following is the correct syntax to push an integer value 10 onto a stack named stack in C#?

easy

A. stack.Insert(10);

B. stack.Add(10);

C. stack.Push(10);

D. stack.Push = 10;

Solution

1. Step 1: Recall Stack method for adding elements

   In C#, the Stack class uses the method Push() to add elements.

2. Step 2: Check syntax correctness

   The correct syntax is calling Push with the value inside parentheses: stack.Push(10);

3. Final Answer:

   stack.Push(10); -> Option C

4. Quick Check:

   Push method call syntax = stack.Push(value) [OK]

Hint: Use stack.Push(value) to add elements [OK]

Common Mistakes:

• Using Add or Insert which are not stack methods
• Assigning value directly to Push
• Missing parentheses in method call

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

Stack<int> stack = new Stack<int>();
stack.Push(5);
stack.Push(10);
stack.Push(15);
Console.WriteLine(string.Join(", ", stack));

medium

A. 5, 10, 15

B. 15, 10, 5

C. 10, 15, 5

D. 15, 5, 10

Solution

1. Step 1: Understand push order and stack iteration

   Elements pushed are 5, then 10, then 15. Stack stores last pushed on top.

2. Step 2: Determine iteration order when printing

   Stack enumerates from top to bottom, so output order is 15, 10, 5.

3. Final Answer:

   15, 10, 5 -> Option B

4. Quick Check:

   Stack top to bottom = 15, 10, 5 [OK]

Hint: Stack prints top element first [OK]

Common Mistakes:

• Assuming stack prints in push order
• Confusing stack with queue order
• Forgetting stack is LIFO

4. Identify the error in the following C# code that tries to push elements onto a stack:

Stack<string> stack = new Stack<string>();
stack.Push "hello";
stack.Push("world");

medium

A. Push method does not exist

B. Stack cannot hold strings

C. Missing semicolon after second Push

D. Missing parentheses in first Push call

Solution

1. Step 1: Check syntax of Push calls

   The first Push call is written as stack.Push "hello"; missing parentheses.

2. Step 2: Confirm correct method usage

   Push requires parentheses around argument: stack.Push("hello");

3. Final Answer:

   Missing parentheses in first Push call -> Option D

4. Quick Check:

   Method calls need parentheses [OK]

Hint: Always use parentheses with method calls [OK]

Common Mistakes:

• Ignoring parentheses syntax
• Thinking stack can't hold strings
• Assuming missing semicolon causes error here

5. Given a stack initially empty, what will be the content of the stack after executing the following code?

Stack<int> stack = new Stack<int>();
for (int i = 1; i <= 3; i++)
{
    stack.Push(i * 2);
}
stack.Push(10);
stack.Push(stack.Pop() + 5);

hard

A. 15, 6, 4, 2

B. 10, 8, 4, 2

C. 15, 10, 6, 4, 2

D. 10, 15, 6, 4, 2

Solution

1. Step 1: Trace the for loop pushing values

   Loop pushes 2, 4, 6 in order. Stack top is 6, bottom is 2.

2. Step 2: Push 10, then pop top and push pop+5

   Push 10: stack top to bottom 10, 6, 4, 2. Pop removes 10, 10 + 5 = 15, push 15: top to bottom 15, 6, 4, 2.

3. Final Answer:

   15, 6, 4, 2 -> Option A

4. Quick Check:

   Stack after operations = 15, 6, 4, 2 [OK]

Hint: Track push/pop stepwise to find final stack [OK]

Common Mistakes:

• Confusing pop value used for new push
• Forgetting pop removes top element
• Assuming push adds without removing