Push Operation on Stack in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after pushing elements on the stack?

Consider the following C++ code that pushes elements onto a stack. What will be the printed stack content after all push operations?

DSA C++

std::stack<int> s;
s.push(10);
s.push(20);
s.push(30);

while(!s.empty()) {
    std::cout << s.top() << " -> ";
    s.pop();
}
std::cout << "null";

A 30 -> 20 -> 10 -> null

B 10 -> 20 -> 30 -> null

C null

D 10 -> null

💡 Hint

Remember that stack is Last-In-First-Out (LIFO). The last pushed element is printed first.

The stack pushes 10, then 20, then 30. The top is 30, so popping prints 30 first, then 20, then 10.

🧠 Conceptual

intermediate

What happens internally when you push an element on a stack?

When you perform a push operation on a stack, which of the following best describes what happens internally?

A The element is added at the bottom of the stack.

B The stack elements are rearranged in ascending order.

C The element replaces the current top element.

D The element is added at the top of the stack.

💡 Hint

Think about where the new element appears relative to existing elements.

Push adds the new element on top of the stack without removing or rearranging existing elements.

🔧 Debug

advanced

Identify the error in this push operation code

What error will this C++ code produce when trying to push elements onto a stack?

DSA C++

std::stack<int> s;
s.push(5);
s.pop();
s.top();
s.push(10);
std::cout << s.top();

A Compilation error: missing include for stack

B Runtime error: accessing top of empty stack

C No error, output is 10

D Logical error: stack size is zero after push

💡 Hint

Check the order of pop and top calls and whether the stack is empty.

After popping the only element, the stack is empty. Calling top() then causes a runtime error.

❓ Predict Output

advanced

What is the stack content after these push and pop operations?

Given the following C++ code, what will be the printed stack content?

DSA C++

std::stack<int> s;
s.push(1);
s.push(2);
s.pop();
s.push(3);

while(!s.empty()) {
    std::cout << s.top() << " -> ";
    s.pop();
}
std::cout << "null";

A 3 -> 1 -> null

B 2 -> 3 -> 1 -> null

C 1 -> 3 -> null

D 3 -> null

💡 Hint

Remember pop removes the top element before the next push.

Push 1, push 2, pop removes 2, push 3. Stack top is 3, then 1.

🚀 Application

expert

How many elements are in the stack after this sequence?

If you start with an empty stack and perform the following operations: push(4), push(7), pop(), push(9), pop(), pop(), push(5), how many elements remain in the stack?

A 2

B 0

C 1

D 3

💡 Hint

Track each operation step-by-step to count remaining elements.

Operations: push 4 (stack:4), push 7 (4,7), pop removes 7 (4), push 9 (4,9), pop removes 9 (4), pop removes 4 (empty), push 5 (5). One element remains.

Practice

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

easy

A. Checks if the stack is empty

B. Removes the top element from the stack

C. Returns the bottom element of the stack

D. Adds an element to the top of the stack

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 existing stack elements.

3. Final Answer:

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

4. Quick Check:

   Push = Add top element [OK]

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

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Assuming push checks stack emptiness

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

easy

A. stk.push(10);

B. stk.add(10);

C. stk.insert(10);

D. stk.append(10);

Solution

1. Step 1: Recall C++ STL stack methods

   The standard method to add an element to a stack is push().

2. Step 2: Match correct syntax

   Using stk.push(10); correctly pushes 10 onto the stack.

3. Final Answer:

   stk.push(10); -> Option A

4. Quick Check:

   push() is the correct method [OK]

Hint: Use push() to add elements in C++ stack [OK]

Common Mistakes:

• Using add(), insert(), or append() which are not stack methods
• Forgetting parentheses after push
• Confusing vector methods with stack

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

std::stack<int> stk;
stk.push(5);
stk.push(10);
stk.push(15);
int topElement = stk.top();

What is the value of topElement?

medium

A. 15

B. 10

C. 0

D. 5

Solution

1. Step 1: Trace push operations

   Elements pushed in order: 5, then 10, then 15. Stack top is last pushed element.

2. Step 2: Identify top element

   The top element after pushes is 15, as it was pushed last.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   Last pushed = top element = 15 [OK]

Hint: Top element is last pushed value [OK]

Common Mistakes:

• Choosing first pushed element as top
• Confusing top() with pop()
• Assuming stack is empty initially

4. Identify the error in the following code snippet that tries to push an element onto a stack:

std::stack<int> stk;
stk.push 20;

medium

A. Using wrong data type for stack

B. Stack not declared properly

C. Missing parentheses in push call

D. No error in the code

Solution

1. Step 1: Check push syntax

   The push method requires parentheses around the argument, like push(20).

2. Step 2: Identify syntax error

   Code uses stk.push 20; missing parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses in push call -> Option C

4. Quick Check:

   push() needs parentheses [OK]

Hint: Always use parentheses with push() [OK]

Common Mistakes:

• Omitting parentheses after push
• Confusing push with pop syntax
• Assuming stack declaration is wrong

5. Given an empty stack, what will be the state of the stack after executing the following code?

std::stack<int> stk;
stk.push(1);
stk.push(2);
stk.push(3);
stk.pop();
stk.push(4);

Choose the correct sequence of elements from top to bottom.

hard

A. 3 -> 2 -> 1 -> null

B. 4 -> 2 -> 1 -> null

C. 1 -> 2 -> 4 -> null

D. 2 -> 4 -> 1 -> null

Solution

1. Step 1: Trace push and pop operations

   Push 1, stack: 1
   Push 2, stack: 2 -> 1
   Push 3, stack: 3 -> 2 -> 1
   Pop removes top (3), stack: 2 -> 1
   Push 4, stack: 4 -> 2 -> 1

2. Step 2: Write final stack state

   Top to bottom: 4 -> 2 -> 1 -> null

3. Final Answer:

   4 -> 2 -> 1 -> null -> Option B

4. Quick Check:

   Push/pop order = 4,2,1 [OK]

Hint: Pop removes top; next push adds new top [OK]

Common Mistakes:

• Ignoring pop effect on stack
• Assuming push adds at bottom
• Mixing order of elements after pop