Stack Implementation Using Array in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack in data structures?

A stack is a collection where elements are added and removed in a last-in, first-out (LIFO) order. Think of it like a stack of plates where you add or remove the top plate only.

beginner

What are the two main operations of a stack?

The two main operations are push (to add an element on top) and pop (to remove the top element).

intermediate

How is a stack implemented using an array?

A stack using an array keeps track of the top index. Push adds an element at the next top position, and pop removes the element at the current top, then moves the top down.

intermediate

What happens if you try to push an element when the stack is full in an array implementation?

This is called a stack overflow. Since the array has a fixed size, you cannot add more elements beyond its capacity.

beginner

What is the time complexity of push and pop operations in a stack implemented using an array?

Both push and pop operations take constant time, O(1), because they only add or remove the top element without shifting other elements.

Which operation removes the top element from a stack?

A push

B enqueue

C peek

D pop

The pop operation removes the top element from the stack.

What does 'LIFO' stand for in the context of a stack?

A Last In, First Out

B Last In, Final Out

C List In, First Out

D Last Input, First Output

LIFO means Last In, First Out, describing how the last added element is the first to be removed.

What is the initial value of the 'top' index in an empty stack implemented with an array?

A 0

B -1

C 1

D null

The top index starts at -1 to indicate the stack is empty.

What happens when you try to pop from an empty stack?

A Adds a new element

B Stack overflow error

C Stack underflow error

D Nothing happens

Popping from an empty stack causes a stack underflow error.

Which of these is NOT a stack operation?

A insertAtBottom

B pop

C peek

D push

insertAtBottom is not a standard stack operation.

Explain how push and pop operations work in a stack implemented using an array.

Think about how the top index changes with each operation.

Describe what stack overflow and stack underflow mean in the context of array-based stacks.

Consider what happens when you exceed or go below stack limits.

Practice

1. What is the main purpose of using an array to implement a stack?

easy

A. To allow random access to elements

B. To store elements in a first-in, first-out (FIFO) order

C. To sort elements automatically

D. To store elements in a last-in, first-out (LIFO) order

Solution

1. Step 1: Understand stack behavior

   A stack stores elements so that the last element added is the first one removed, called LIFO.

2. Step 2: Role of array in stack

   An array helps store elements in order and access the last added element easily for push/pop operations.

3. Final Answer:

   To store elements in a last-in, first-out (LIFO) order -> Option D

4. Quick Check:

   Stack = LIFO [OK]

Hint: Stack means last-in, first-out order [OK]

Common Mistakes:

• Confusing stack with queue (FIFO)
• Thinking array sorts elements automatically
• Assuming random access is main stack feature

2. Which of the following is the correct way to initialize an empty stack using an array in Python?

easy

A. stack = []

B. stack = {}

C. stack = ()

D. stack = null

Solution

1. Step 1: Identify Python data types for stack

   An array-like structure for stack is a list, initialized with square brackets [].

2. Step 2: Check each option

   stack = [] uses [], which is a list. stack = {} is a dictionary, () is a tuple, null (no data).

3. Final Answer:

   stack = [] -> Option A

4. Quick Check:

   Empty list = empty stack [OK]

Hint: Use [] to create empty list for stack [OK]

Common Mistakes:

• Using {} which creates a dictionary
• Using () which creates a tuple
• Assigning null instead of a list

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

stack = []
stack.append(10)
stack.append(20)
stack.pop()
print(stack)

medium

A. [10, 20]

B. [20]

C. [10]

D. []

Solution

1. Step 1: Trace push operations

   stack.append(10) adds 10, stack becomes [10]. Then stack.append(20) adds 20, stack becomes [10, 20].

2. Step 2: Trace pop operation

   stack.pop() removes last element 20, stack becomes [10].

3. Final Answer:

   [10] -> Option C

4. Quick Check:

   Pop removes last item, stack left with [10] [OK]

Hint: Pop removes last pushed element [OK]

Common Mistakes:

• Thinking pop removes first element
• Confusing append with insert
• Expecting stack to be empty after pop

4. Identify the error in the following stack implementation code:

stack = []
stack.push(5)
stack.append(10)
print(stack)

medium

A. 'push' method does not exist for list

B. Missing initialization of stack

C. Cannot append to list

D. print statement syntax error

Solution

1. Step 1: Check list methods

   Python lists use append() to add elements, not push().

2. Step 2: Identify error

   stack.push(5) causes AttributeError because 'push' is not a list method.

3. Final Answer:

   'push' method does not exist for list -> Option A

4. Quick Check:

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

Hint: Use append() to add elements to list [OK]

Common Mistakes:

• Using push() instead of append()
• Assuming push() is valid for Python lists
• Ignoring error messages

5. Given a stack implemented using an array, how can you check if the stack is empty before popping an element to avoid errors?

hard

A. Check if stack is null

B. Check if stack's length is zero using len(stack) == 0

C. Try popping and catch exceptions if stack is empty

D. Use stack[0] == null to check emptiness

Solution

1. Step 1: Understand stack emptiness check

   To avoid errors, check if stack has no elements by verifying its length is zero.

2. Step 2: Evaluate options

   Check if stack's length is zero using len(stack) == 0 correctly checks length with len(stack) == 0. Try popping and catch exceptions if stack is empty is valid but less efficient. Check if stack is null checks null which is incorrect. Use stack[0] == null to check emptiness checks first element which may cause error if empty.

3. Final Answer:

   Check if stack's length is zero using len(stack) == 0 -> Option B

4. Quick Check:

   Empty stack means length zero [OK]

Hint: Use len(stack) == 0 to check empty stack [OK]

Common Mistakes:

• Checking stack is null instead of empty
• Accessing stack[0] without checking length
• Relying only on try-except without condition