Check if Stack is Empty or Full in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after pushing elements on the stack?

Consider a stack with a maximum size of 3. We push elements 10, 20, and 30 in order. What will be the printed stack after these operations?

DSA Python

class Stack:
    def __init__(self, max_size):
        self.stack = []
        self.max_size = max_size
    def push(self, item):
        if len(self.stack) < self.max_size:
            self.stack.append(item)
    def __str__(self):
        return ' -> '.join(map(str, self.stack)) + ' -> null'

s = Stack(3)
s.push(10)
s.push(20)
s.push(30)
print(s)

A 10 -> 20 -> null

B 30 -> 20 -> 10 -> null

C 10 -> 20 -> 30 -> null

D Stack is empty

💡 Hint

Remember, push adds elements to the top of the stack, but printing shows from bottom to top in this code.

The stack stores elements in order pushed. The __str__ method prints from bottom to top, so the output is 10 -> 20 -> 30 -> null.

❓ Predict Output

intermediate

What does the isEmpty method return after popping all elements?

Given a stack with max size 2, we push 5 and 15, then pop twice. What does isEmpty() return?

DSA Python

class Stack:
    def __init__(self, max_size):
        self.stack = []
        self.max_size = max_size
    def push(self, item):
        if len(self.stack) < self.max_size:
            self.stack.append(item)
    def pop(self):
        if self.stack:
            return self.stack.pop()
        return None
    def isEmpty(self):
        return len(self.stack) == 0

s = Stack(2)
s.push(5)
s.push(15)
s.pop()
s.pop()
print(s.isEmpty())

A True

B False

C None

D Error

💡 Hint

Check if the stack has any elements left after popping twice.

After pushing 5 and 15, popping twice removes both elements. The stack is empty, so isEmpty() returns True.

❓ Predict Output

advanced

What error occurs when pushing beyond stack capacity?

What happens when we push an element into a full stack of max size 1?

DSA Python

class Stack:
    def __init__(self, max_size):
        self.stack = []
        self.max_size = max_size
    def push(self, item):
        if len(self.stack) < self.max_size:
            self.stack.append(item)
        else:
            raise OverflowError('Stack is full')

s = Stack(1)
s.push(100)
s.push(200)

A IndexError: list index out of range

B No error, stack contains [100, 200]

C TypeError: unsupported operand type(s)

D OverflowError: Stack is full

💡 Hint

Check the condition before appending and what happens if the stack is full.

The push method raises OverflowError when trying to add beyond max_size.

❓ Predict Output

advanced

What is the output of isFull after pushing elements?

Given a stack with max size 2, after pushing one element, what does isFull() return?

DSA Python

class Stack:
    def __init__(self, max_size):
        self.stack = []
        self.max_size = max_size
    def push(self, item):
        if len(self.stack) < self.max_size:
            self.stack.append(item)
    def isFull(self):
        return len(self.stack) == self.max_size

s = Stack(2)
s.push(42)
print(s.isFull())

A False

B True

C Error

D None

💡 Hint

Compare current stack size with max_size after one push.

After pushing one element, stack size is 1, max_size is 2, so isFull() returns False.

🧠 Conceptual

expert

Which condition correctly checks if a stack is empty?

You want to write a method isEmpty() for a stack implemented as a list named 'stack'. Which condition correctly returns True if the stack is empty and False otherwise?

A return self.stack[0] == None

B return len(self.stack) == 0

C return self.stack == None

D return len(self.stack) > 0

💡 Hint

Think about how to check if a list has no elements.

An empty list has length zero, so checking len(self.stack) == 0 correctly detects emptiness.