Stack operations (push, pop, peek) in Data Structures Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a stack in data structures?

A stack is a collection of elements that follows the Last In, First Out (LIFO) principle, where the last element added is the first one to be removed.

beginner

What does the 'push' operation do in a stack?

The 'push' operation adds a new element to the top of the stack.

beginner

What is the purpose of the 'pop' operation in a stack?

The 'pop' operation removes and returns the top element from the stack.

beginner

Explain the 'peek' operation in a stack.

The 'peek' operation returns the top element of the stack without removing it, allowing you to see what is on top.

beginner

Why is the stack called Last In, First Out (LIFO)?

Because the last element pushed (added) onto the stack is the first one to be popped (removed), like a stack of plates where you take the top plate first.

Which operation adds an element to the top of a stack?

A Insert

B Pop

C Push

D Peek

The 'push' operation adds a new element to the top of the stack.

What does the 'pop' operation do?

A Returns the bottom element

B Removes and returns the top element

C Adds an element to the stack

D Checks if the stack is empty

The 'pop' operation removes and returns the top element from the stack.

What is the result of the 'peek' operation?

A Returns the top element without removing it

B Removes the top element

C Adds an element to the bottom

D Clears the stack

The 'peek' operation returns the top element without removing it.

Which principle does a stack follow?

A Last In, First Out (LIFO)

B Last In, Last Out (LILO)

C First In, First Out (FIFO)

D Random Access

A stack follows the Last In, First Out (LIFO) principle.

If you push elements 1, 2, 3 in order, which element will pop return first?

A 1

B None

C 2

D 3

The last pushed element (3) is the first to be popped in a stack.

Describe the three main operations of a stack and their effects.

Think about how you add, remove, and check the top item in a stack of objects.

Explain why a stack is called Last In, First Out (LIFO) with a real-life example.

Imagine a pile of books or plates and how you take them off.

Practice

1. What does the pop operation do in a stack?

easy

A. Shows the bottom item without removing it

B. Adds a new item to the bottom of the stack

C. Removes and returns the top item from the stack

D. Clears all items from the stack

Solution

1. Step 1: Understand the pop operation

   The pop operation removes the last item added to the stack, which is the top item.

2. Step 2: Confirm the behavior of pop

   It returns this top item and removes it from the stack, following last-in, first-out order.

3. Final Answer:

   Removes and returns the top item from the stack -> Option C

4. Quick Check:

   pop = remove top item [OK]

Hint: Pop always removes the last item added [OK]

Common Mistakes:

• Thinking pop adds an item
• Confusing pop with peek
• Assuming pop removes bottom item

2. Which of the following is the correct way to add an item 5 to a stack named s using the push operation?

easy

A. s.pop(5)

B. s.push(5)

C. s.peek(5)

D. s.add(5)

Solution

1. Step 1: Identify the push syntax

   Push adds an item to the top of the stack, usually called with push(item).

2. Step 2: Match the correct method call

   Only s.push(5) correctly adds 5 to the stack.

3. Final Answer:

   s.push(5) -> Option B

4. Quick Check:

   push = add item [OK]

Hint: Push means add, so look for push() method [OK]

Common Mistakes:

• Using pop instead of push
• Using peek to add items
• Using add which is not standard for stacks

3. Given the following stack operations:

s.push(10)
s.push(20)
s.pop()
s.push(30)
top = s.peek()

What is the value of top after these operations?

medium

A. 30

B. 20

C. None

D. 10

Solution

1. Step 1: Trace the stack changes step-by-step

   After push(10), stack = [10]. After push(20), stack = [10, 20]. After pop(), top 20 is removed, stack = [10]. After push(30), stack = [10, 30].

2. Step 2: Determine the top item with peek

   Peek returns the top item without removing it, so top = 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed after pop is 30 [OK]

Hint: Track stack after each operation carefully [OK]

Common Mistakes:

• Forgetting pop removes top
• Assuming peek removes item
• Mixing order of operations

4. Consider this code snippet for a stack:

stack = []
stack.push(1)
stack.push(2)
item = stack.pop()

What is the error in this code?

medium

A. stack is not initialized correctly

B. pop() does not return the top item

C. stack.pop() removes the bottom item, not top

D. push() is not a method of list in Python

Solution

1. Step 1: Check stack initialization

   The stack is initialized as an empty list, which is correct in Python.

2. Step 2: Verify method usage on list

   Python lists use append() to add items, not push(). Using push() causes an error.

3. Final Answer:

   push() is not a method of list in Python -> Option D

4. Quick Check:

   Python list uses append(), not push() [OK]

Hint: Remember Python list uses append(), not push() [OK]

Common Mistakes:

• Assuming push() works on Python lists
• Confusing pop() behavior
• Thinking stack initialization is wrong

5. You want to reverse a list [1, 2, 3] using a stack. Which sequence of stack operations correctly reverses the list?

hard

A. Push all items in order, then pop all items to get reversed list

B. Pop all items first, then push them back in order

C. Peek all items, then push them again

D. Push items in reverse order, then pop all items

Solution

1. Step 1: Understand stack reversal logic

   Pushing items in original order stores them with last item on top. Popping all items removes them in reverse order.

2. Step 2: Apply push then pop to reverse list

   Push 1, then 2, then 3. Popping returns 3, 2, 1, which is the reversed list.

3. Final Answer:

   Push all items in order, then pop all items to get reversed list -> Option A

4. Quick Check:

   Push then pop reverses order [OK]

Hint: Push in order, pop all to reverse list [OK]

Common Mistakes:

• Pushing in reverse order first
• Using peek to reverse
• Popping before pushing