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.