Stack in Python

A stack follows LIFO:
Last In, First Out.

Real examples:

• undo/redo
• browser back button
• function call stack

Implement stack with list (simple)

stack = []
stack.append("A")  # push
stack.append("B")
print(stack)

top = stack.pop()  # pop
print(top)
print(stack)

Stack example: bracket matching

Goal: check if parentheses are balanced.

def is_balanced(s: str) -> bool:
    stack = []
    pairs = {")": "(", "]": "[", "}": "{"}

    for ch in s:
        if ch in "([{":
            stack.append(ch)
        elif ch in ")]}":
            if not stack or stack[-1] != pairs[ch]:
                return False
            stack.pop()

    return len(stack) == 0

print(is_balanced("(a[b]{c})"))  # True
print(is_balanced("(a[b]{c}"))   # False

Graph: stack LIFO

flowchart TD
  A[Push A] --> B[Stack: A]
  B --> C[Push B]
  C --> D[Stack: A,B]
  D --> E[Pop -> B]

Remember

• stack = append + pop
• stack[-1] is the top
• used in parsing and “undo” logic