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DSA Pythonprogramming

Stack Concept and LIFO Principle in DSA Python

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Mental Model
A stack is a collection where the last item added is the first one removed. This is called Last In, First Out (LIFO).
Analogy: Think of a stack of plates: you put new plates on top and take plates from the top only. The last plate you put is the first you take.
Top
 ↑
[3]
[2]
[1]
Bottom
Dry Run Walkthrough
Input: Start with empty stack, push 1, push 2, push 3, then pop one item
Goal: Show how items are added and removed following LIFO
Step 1: Push 1 onto stack
Top
 ↑
[1]
Bottom
Why: We add the first item at the top of the stack
Step 2: Push 2 onto stack
Top
 ↑
[2]
[1]
Bottom
Why: New item goes on top, above the previous one
Step 3: Push 3 onto stack
Top
 ↑
[3]
[2]
[1]
Bottom
Why: Keep adding items on top, last pushed is on top
Step 4: Pop item from stack
Top
 ↑
[2]
[1]
Bottom
Why: Remove the top item, which is the last pushed (3)
Result: 
Top
 ↑
[2]
[1]
Bottom
Popped item: 3
Annotated Code
DSA Python
class Stack:
    def __init__(self):
        self.items = []

    def push(self, item):
        # Add item to the top of the stack
        self.items.append(item)

    def pop(self):
        # Remove and return the top item if stack is not empty
        if not self.is_empty():
            return self.items.pop()
        return None

    def is_empty(self):
        # Check if stack has no items
        return len(self.items) == 0

    def __str__(self):
        # Show stack from top to bottom
        return 'Top\n ↑\n' + '\n'.join(f'[{item}]' for item in reversed(self.items)) + '\nBottom'

stack = Stack()
stack.push(1)
stack.push(2)
stack.push(3)
print(stack)
popped = stack.pop()
print(f'Popped item: {popped}')
print(stack)
self.items.append(item)
Add new item on top of stack
if not self.is_empty():
Check stack is not empty before popping
return self.items.pop()
Remove and return top item following LIFO
OutputSuccess
Top ↑ [3] [2] [1] Bottom Popped item: 3 Top ↑ [2] [1] Bottom
Complexity Analysis
Time: O(1) for push and pop because adding/removing from the end of list is constant time
Space: O(n) because stack stores all pushed items
vs Alternative: Compared to a queue where removal is from front (O(n) in list), stack operations are faster and simpler
Edge Cases
Pop from empty stack
Returns None safely without error
DSA Python
if not self.is_empty():
When to Use This Pattern
When you see a problem needing last added item accessed first, use stack and LIFO principle to manage order efficiently.
Common Mistakes
Mistake: Removing items from the start of the list instead of the end
Fix: Use list.pop() without index to remove from the end, preserving LIFO
Summary
Stack stores items so the last added is the first removed (LIFO).
Use stack when you need to reverse order or track recent items first.
Remember: push adds on top, pop removes from top, like a stack of plates.