Stack Concept and LIFO Principle in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this stack operation sequence?

Consider a stack initially empty. We perform these operations in order:

push(5), push(10), pop(), push(7), pop(), push(3), push(8), pop()

What is the content of the stack from top to bottom after these operations?

DSA Python

stack = []
stack.append(5)
stack.append(10)
stack.pop()
stack.append(7)
stack.pop()
stack.append(3)
stack.append(8)
stack.pop()
print(stack)

A [5, 3]

B [5, 7]

C [5, 3, 8]

D [10, 7]

💡 Hint

Remember pop removes the top element of the stack.

The stack evolves as follows: push 5 -> [5], push 10 -> [5,10], pop -> removes 10 -> [5], push 7 -> [5,7], pop -> removes 7 -> [5], push 3 -> [5,3], push 8 -> [5,3,8], pop -> removes 8 -> [5,3].

🧠 Conceptual

intermediate

Which statement best describes the LIFO principle in stacks?

Choose the option that correctly explains the Last In First Out (LIFO) principle used in stacks.

A The first element added is the first one to be removed.

B Elements are removed in random order regardless of insertion.

C Elements are removed in the same order they were added.

D The last element added is the first one to be removed.

💡 Hint

Think about which element is on top of the stack.

LIFO means the last element pushed onto the stack is the first one to be popped off.

🔧 Debug

advanced

What error does this stack code produce?

Consider this Python code using a list as a stack:

stack = [1, 2, 3]
stack.pop(3)

What error will this code raise when run?

DSA Python

stack = [1, 2, 3]
stack.pop(3)

A IndexError: pop index out of range

B TypeError: pop() takes no arguments

C No error, outputs 3

D AttributeError: 'list' object has no attribute 'pop'

💡 Hint

Check the valid indices for pop in a list of length 3.

pop(3) tries to remove element at index 3, but valid indices are 0,1,2. This causes IndexError.

❓ Predict Output

advanced

What is the printed output after these stack operations?

Given this code:

stack = []
for i in range(1, 5):
    if i % 2 == 0:
        stack.append(i)
    else:
        stack.pop() if stack else None
print(stack)

What is the final content of the stack?

DSA Python

stack = []
for i in range(1, 5):
    if i % 2 == 0:
        stack.append(i)
    else:
        stack.pop() if stack else None
print(stack)

A []

B [4]

C [2, 4]

D [1, 3]

💡 Hint

Trace each iteration carefully, noting when pop happens.

i=1 (odd): stack empty, pop skipped -> [] i=2 (even): append 2 -> [2] i=3 (odd): pop top (2) -> [] i=4 (even): append 4 -> [4]

🚀 Application

expert

How many items remain in the stack after this sequence?

A stack starts empty. We perform these operations:

push(1), push(2), push(3), pop(), push(4), pop(), pop(), push(5), push(6), pop()

How many items are left in the stack at the end?

DSA Python

stack = []
stack.append(1)
stack.append(2)
stack.append(3)
stack.pop()
stack.append(4)
stack.pop()
stack.pop()
stack.append(5)
stack.append(6)
stack.pop()
print(len(stack))

A 0

B 3

C 2

D 1

💡 Hint

Count pushes and pops carefully step by step.

Operations stepwise:
push 1 -> [1]
push 2 -> [1,2]
push 3 -> [1,2,3]
pop -> removes 3 -> [1,2]
push 4 -> [1,2,4]
pop -> removes 4 -> [1,2]
pop -> removes 2 -> [1]
push 5 -> [1,5]
push 6 -> [1,5,6]
pop -> removes 6 -> [1,5]
Stack size is 2.