Stacks in Python

A stack is a linear data structure that follows the LIFO (Last In, First Out) principle. The last element inserted is the first one to be removed. Stacks are used in many real-world applications like undo systems, backtracking, expression parsing, and function calls.

Concept of a Stack (LIFO)

• Push: add an element to the top
• Pop: remove the top element
• Peek/Top: view the top element without removing
• isEmpty: check if stack is empty

Implementation of Stack Using List

Python lists naturally support stack operations because append() adds to the end and pop() removes from the end.

Example:

stack = []

stack.append(10)  # push
stack.append(20)
stack.append(30)

print(stack)  # [10, 20, 30]

stack.pop()   # pop
print(stack)  # [10, 20]

Common Stack Operations

Push

stack.append(5)

Pop

top = stack.pop()

Peek

top = stack[-1]

isEmpty

is_empty = len(stack) == 0

Applications of Stacks

Expression Evaluation (Balanced Parentheses)

Example:

def is_balanced(s):
    stack = []
    for ch in s:
        if ch == '(':
            stack.append(ch)
        elif ch == ')':
            if not stack:
                return False
            stack.pop()
    return len(stack) == 0

print(is_balanced("(())"))  # True
print(is_balanced("(()"))   # False

Undo Mechanism

• Push actions to stack
• Pop to undo last action

Backtracking

Used in problems like:

• Maze solving
• N-Queens
• DFS traversal

Summary

Stacks are simple but powerful. Once you master them, many problems (parentheses, next greater element, backtracking) become much easier.