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

Recall & Review

beginner

What is the push operation 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, where is the new element added during a push operation?

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

intermediate

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

If the stack is full, pushing an element causes a stack overflow error, meaning no more elements can be added.

beginner

Show the state of the stack after pushing elements 1, then 2, then 3 onto an empty stack.

After pushing 1: 1 -> null
After pushing 2: 2 -> 1 -> null
After pushing 3: 3 -> 2 -> 1 -> null

intermediate

Why is the push operation considered a constant time operation (O(1))?

Because it only adds an element at the top without needing to move other elements, so the time taken does not depend on the stack size.

What does the push operation do in a stack?

A Checks if the stack is empty

B Removes the top element

C Adds an element to the top

D Returns the bottom element

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

If a stack currently has elements 5 -> 4 -> null, what will be the stack after push(6)?

A 5 -> 4 -> 6 -> null

B 5 -> 6 -> 4 -> null

C 4 -> 5 -> 6 -> null

D 6 -> 5 -> 4 -> null

Push adds the new element 6 at the top, so it becomes 6 -> 5 -> 4 -> null.

What error occurs when pushing onto a full stack?

A Stack Overflow

B Stack Underflow

C Null Pointer Exception

D Index Out of Bounds

Stack Overflow happens when trying to push onto a full stack.

Which data structure principle does push operation follow?

A Last In First Out (LIFO)

B Priority Queue

C Random Access

D First In First Out (FIFO)

Stack follows Last In First Out (LIFO), so push adds to the top.

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

A O(log n)

B O(1)

C O(n)

D O(n^2)

Push operation takes constant time O(1) because it only adds one element at the top.

Explain the push operation on a stack and what happens to the stack after pushing an element.

Think about how a stack grows when you add something.

Describe what happens when you try to push an element onto a full stack and why it is important to check for this condition.

Consider what happens if the stack has no room left.

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, so new elements are added on top.

2. Step 2: Define push operation

   Push means adding a new element on the top of the stack.

3. Final Answer:

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

4. Quick Check:

   Push = Add top element [OK]

Hint: Push always adds element on top of stack [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Mixing stack with queue operations

2. Which of the following is the correct syntax to push an element 5 into a stack named stack implemented as a Python list?

easy

A. stack.append(5)

B. stack.push(5)

C. stack.add(5)

D. stack.insert(0, 5)

Solution

1. Step 1: Recall Python list methods for stack

   Python lists use append() to add elements at the end, which acts as top of stack.

2. Step 2: Check each option

   push() and add() are not list methods; insert(0, 5) adds at bottom, not top.

3. Final Answer:

   stack.append(5) -> Option A

4. Quick Check:

   Use append() to push in Python list stack [OK]

Hint: Use append() to push in Python list stack [OK]

Common Mistakes:

• Using push() which is not a Python list method
• Using insert(0, value) which adds at bottom
• Confusing add() with append()

3. What will be the content of the stack after executing the following code?

stack = []
stack.append(10)
stack.append(20)
stack.append(30)

medium

A. [20, 10, 30]

B. [30, 20, 10]

C. [10, 20, 30]

D. [10, 30, 20]

Solution

1. Step 1: Trace each append operation

   Start with empty list. Append 10 -> [10], append 20 -> [10, 20], append 30 -> [10, 20, 30].

2. Step 2: Understand stack order

   Elements are added at the end; top is last element (30).

3. Final Answer:

   [10, 20, 30] -> Option C

4. Quick Check:

   Stack after pushes = [10, 20, 30] [OK]

Hint: Append adds at end; last element is top [OK]

Common Mistakes:

• Reversing the order of elements
• Confusing stack top with bottom
• Assuming append adds at start

4. Identify the error in the following code that tries to push an element 15 into a stack implemented as a list:

stack = [5, 10]
stack.push(15)

medium

A. Missing import statement for stack

B. Using push() method which does not exist for list

C. Stack should be initialized as a tuple

D. Using append() instead of push()

Solution

1. Step 1: Check list methods for adding elements

   Python lists do not have a push() method; they use append().

2. Step 2: Analyze the code error

   Calling stack.push(15) will cause an AttributeError because push is undefined.

3. Final Answer:

   Using push() method which does not exist for list -> Option B

4. Quick Check:

   List has append(), not push() [OK]

Hint: Use append() for list push, not push() [OK]

Common Mistakes:

• Assuming push() exists for Python lists
• Thinking stack must be tuple
• Ignoring AttributeError on push()

5. Given a stack implemented as a list, how would you push multiple elements [1, 2, 3] onto the stack so that 3 is on top after all pushes?

hard

A. Use a loop to append elements in order: 1, then 2, then 3

B. Use a loop to append elements in reverse order: 3, then 2, then 1

C. Use extend() method with reversed([1, 2, 3])

D. Insert elements at index 0 in order: 1, then 2, then 3

Solution

1. Step 1: Understand stack push order

   To have 3 on top, push 1 first, then 2, then 3 so 3 is last pushed.

2. Step 2: Analyze each option

   Loop append in order 1,2,3 puts 3 on top. Reverse order puts 1 on top. extend(reversed([1,2,3])) adds 3,2,1 so 1 on top. Insert at index 0 in order ends with [3,2,1], 1 on top.

3. Final Answer:

   Use a loop to append elements in order: 1, then 2, then 3 -> Option A

4. Quick Check:

   Push in order to get last element on top [OK]

Hint: Push elements in order to have last on top [OK]

Common Mistakes:

• Pushing in reverse order causing wrong top
• Using insert(0, val) which adds at bottom
• Using extend(reversed([1, 2, 3])) which puts wrong order on top