Stack Concept and LIFO Principle in DSA Python - Execution Trace

Concept Flow - Stack Concept and LIFO Principle

Start with empty stack

↓

Push element

→Add element on top

↓

Push more elements?

Yes→Repeat push

No↓

Pop element

→Remove top element

↓

Pop more elements?

Yes→Repeat pop

No↓

End

Stack works by adding elements on top and removing from top, following Last In First Out order.

Execution Sample

DSA Python

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

This code pushes 1, 2, 3 onto the stack and then pops two elements off.

Execution Table

Step	Operation	Nodes in Stack	Pointer Changes	Visual State
1	Start with empty stack	[]	head=None	Top -> None
2	Push 1	[1]	head=1	Top -> 1 -> None
3	Push 2	[1, 2]	head=2	Top -> 2 -> 1 -> None
4	Push 3	[1, 2, 3]	head=3	Top -> 3 -> 2 -> 1 -> None
5	Pop (remove top)	[1, 2]	head=2	Top -> 2 -> 1 -> None
6	Pop (remove top)	[1]	head=1	Top -> 1 -> None
7	End	[1]	head=1	Top -> 1 -> None

💡 No more push or pop operations, stack remains with one element.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
stack	[]	[1]	[1, 2]	[1, 2, 3]	[1, 2]	[1]	[1]
head (top element)	None	1	2	3	2	1	1

Key Moments - 3 Insights

Why does pop remove the last pushed element, not the first?

Because stack follows LIFO (Last In First Out), so the last element pushed (step 4) is the first to be popped (step 5). See execution_table rows 4 and 5.

What happens if we pop when the stack is empty?

Popping an empty stack causes an error or exception because there is no element to remove. This is not shown in the table but is important to handle in code.

Why does the 'head' pointer change after each push or pop?

The 'head' always points to the top element of the stack. When we push, head moves to the new element (rows 2-4). When we pop, head moves down to the next element (rows 5-6).

Visual Quiz - 3 Questions

Look at the execution_table, what is the stack state after step 4?

A [1, 2, 3]

B [1, 2]

C [3, 2, 1]

D [3]

💡 Hint

Check the 'Nodes in Stack' column at step 4 in execution_table.

At which step does the stack first remove an element?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look for the first 'Pop' operation in the 'Operation' column of execution_table.

If we add another push after step 6, what happens to the 'head' pointer?

A It stays at 1

B It moves to the new element pushed

C It becomes None

D It moves to 2

💡 Hint

Refer to variable_tracker and see how 'head' changes after push operations.

Concept Snapshot

Stack is a data structure that stores elements in LIFO order.
Use push to add on top, pop to remove from top.
The top element is always the last pushed.
Empty stack has no elements and pop causes error.
Useful for undo, backtracking, and function calls.

Full Transcript

A stack is like a pile of plates where you add and remove plates only from the top. This is called Last In First Out (LIFO). When you push, you put a new plate on top. When you pop, you take the top plate off. The 'head' pointer always shows the top plate. If you pop when empty, it causes an error. The execution table shows pushing 1, 2, 3 and then popping twice, leaving 1 on the stack.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the first item added is the first to be removed

B. Last In, First Out - the last item added is the first to be removed

C. First In, First Out - the first item added is the first to be removed

D. First In, Last Out - the first item added is the last to be removed

Solution

1. Step 1: Understand LIFO meaning

   LIFO means the last element added to the stack is the first one to be removed.

2. Step 2: Match with options

   Last In, First Out - the last item added is the first to be removed correctly describes this behavior as "Last In, First Out".

3. Final Answer:

   Last In, First Out - the last item added is the first to be removed -> Option B

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking first added is removed first
• Mixing order of insertion and removal

2. Which of the following is the correct way to push an element '5' onto a stack named stack in Python?

easy

A. stack.append(5)

B. stack.add(5)

C. stack.push(5)

D. stack.insert(5)

Solution

1. Step 1: Recall Python list methods for stack

   Python uses list's append() method to add elements to the top of the stack.

2. Step 2: Check options for correct method

   Only stack.append(5) uses append(5), which is correct for pushing onto a stack.

3. Final Answer:

   stack.append(5) -> Option A

4. Quick Check:

   Push in Python stack = append() [OK]

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

Common Mistakes:

• Using push() which is not a Python list method
• Using add() which is for sets
• Using insert() which inserts at specific index

3. What will be the output of the following Python code?

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

medium

A. 30
10
[20]

B. 10
20
[30]

C. 20
30
[10]

D. 30
20
[10]

Solution

1. Step 1: Trace stack after each append

   After append(10): [10], append(20): [10, 20], append(30): [10, 20, 30]

2. Step 2: Trace pop operations

   pop() removes last element: first pop() returns 30, second pop() returns 20, stack left is [10]

3. Final Answer:

   30 20 [10] -> Option D

4. Quick Check:

   Stack pops last in first out [OK]

Hint: Pop removes last pushed element first [OK]

Common Mistakes:

• Thinking pop removes first element
• Confusing order of popped elements
• Not updating stack after pop

4. Identify the error in the following code that implements a stack using a Python list:

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

medium

A. Using push() method which does not exist for Python lists

B. Using pop() method incorrectly

C. Stack should be initialized as a tuple, not list

D. Missing import statement for stack

Solution

1. Step 1: Check methods used on list

   Python lists do not have a push() method; append() is used to add elements.

2. Step 2: Verify pop() usage

   pop() is valid for lists and correctly used here.

3. Final Answer:

   Using push() method which does not exist for Python lists -> Option A

4. Quick Check:

   Python list push() does not exist [OK]

Hint: Use append(), not push(), to add in Python list [OK]

Common Mistakes:

• Assuming push() is valid for Python lists
• Thinking pop() needs arguments
• Confusing list with stack module

5. Given a stack implemented as a Python list, which sequence of operations will reverse the order of elements in the stack?
Assume initial stack is [1, 2, 3] where 3 is the top.
Choose the correct sequence:

hard

A. Append elements to the stack again without popping

B. Use pop() repeatedly and append to the same stack without extra storage

C. Pop all elements into a new stack

D. Sort the stack list in ascending order

Solution

1. Step 1: Understand reversing stack by popping

   Pop elements from original stack and push them into a new stack reverses order.

2. Step 2: Confirm other options

   Appending without popping or sorting does not reverse stack order correctly.

3. Final Answer:

   Pop all elements into a new stack -> Option C

4. Quick Check:

   Two stacks pop/push reverses order [OK]

Hint: Use two stacks to reverse order [OK]

Common Mistakes:

• Trying to reverse by sorting
• Appending without popping
• Using one stack without extra storage