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, what happens if you try to push an element when the stack is full?

This situation is called stack overflow. The push operation cannot add a new element because there is no space left.

beginner

Show the state of the stack after pushing elements 5, then 10, then 15 (starting from empty).

After pushing 5: 5 -> null
After pushing 10: 10 -> 5 -> null
After pushing 15: 15 -> 10 -> 5 -> null

beginner

What is the time complexity of the push operation on a stack?

The push operation runs in O(1) time because it only adds an element to the top without traversing the stack.

intermediate

Explain the difference between push operation in an array-based stack and a linked-list-based stack.

In an array-based stack, push adds the element at the next free index if space is available. In a linked-list-based stack, push creates a new node and links it as the new top.

What does the push operation do in a stack?

A Adds an element to the top

B Removes the top element

C Checks if the stack is empty

D Returns the bottom element

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

What is stack overflow?

A Trying to pop when stack is empty

B Trying to push when stack is full

C Adding an element to the bottom

D Removing all elements at once

Stack overflow happens when pushing to a full stack.

What is the time complexity of push operation?

A O(n)

B O(log n)

C O(n^2)

D O(1)

Push operation takes constant time O(1).

In a linked-list stack, what does push do?

A Creates a new node and links it as top

B Traverses to the bottom

C Deletes the last node

D Copies all nodes

Push creates a new node and makes it the new top.

If a stack currently has elements 3 -> 2 -> 1 -> null, what will be the stack after push(4)?

A 3 -> 2 -> null

B 3 -> 2 -> 1 -> 4 -> null

C 4 -> 3 -> 2 -> 1 -> null

D 1 -> 2 -> 3 -> 4 -> null

Push adds 4 on top, so 4 becomes the new top.

Describe step-by-step what happens internally when you push an element onto a stack implemented using a linked list.

Think about how the new node connects to the existing stack.

Explain what stack overflow means and how it affects the push operation in an array-based stack.

Consider the fixed size of the array.

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