Why Balanced Parentheses Problem Using Stack in DSA Python?

The Big Idea

What if a simple stack could save you hours of debugging mismatched brackets?

The Scenario

Imagine you are checking if a math expression typed by hand has matching parentheses. You try to count opening and closing brackets manually by scanning the expression from left to right.

The Problem

Counting manually is slow and easy to mess up, especially when parentheses are nested or mixed with different types like {}, [], (). You might miss a closing bracket or get confused about the order.

The Solution

Using a stack, you can push each opening bracket and pop it when you find a matching closing bracket. This keeps track of the order perfectly and quickly tells you if the parentheses are balanced.

Before vs After

✗ Before

expression = "(a+b)*(c-d))"
count = 0
for char in expression:
    if char == '(': count += 1
    elif char == ')': count -= 1
if count == 0:
    print("Balanced")
else:
    print("Not Balanced")

✓ After

expression = "(a+b)*(c-d))"
closing_to_open = {')': '(', ']': '[', '}': '{'}
stack = []
for char in expression:
    if char in '([{':
        stack.append(char)
    elif char in ')]}':
        if not stack or stack.pop() != closing_to_open[char]:
            print("Not Balanced")
            break
else:
    print("Balanced" if not stack else "Not Balanced")

What It Enables

This method lets you quickly and reliably check complex expressions for correct matching and nesting of parentheses.

Real Life Example

Compilers use this technique to verify that code blocks are properly opened and closed with braces, preventing syntax errors before running the program.

Key Takeaways

Manual counting fails with nested or mixed parentheses.

Stack remembers the order of opening brackets for matching.

Stack-based checking is fast, reliable, and essential in parsing code.

Practice

1. What does the Balanced Parentheses problem using a stack check for in a string?

easy

A. Whether every opening bracket has a matching closing bracket in correct order

B. If the string contains only alphabets

C. If the string length is even

D. Whether the string contains any numbers

Solution

1. Step 1: Understand the problem goal

   The Balanced Parentheses problem checks if all opening brackets like '(' have a matching closing bracket ')' in the correct order.

2. Step 2: Role of stack in checking

   A stack helps track opening brackets and ensures each one is closed properly in sequence.

3. Final Answer:

   Whether every opening bracket has a matching closing bracket in correct order -> Option A

4. Quick Check:

   Balanced parentheses = matching pairs in order [OK]

Hint: Stack tracks openings; match with closings in order [OK]

Common Mistakes:

• Thinking it checks string length
• Confusing with character types
• Ignoring order of brackets

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

easy

A. stack.push(x)

B. stack.append(x)

C. stack.add(x)

D. stack.insert(x)

Solution

1. Step 1: Recall Python list methods for stack

   Python uses list to implement stack; to add an element on top, use append().

2. Step 2: Check each option

   add() and push() are not list methods; insert() adds at specific index, not top.

3. Final Answer:

   stack.append(x) -> Option B

4. Quick Check:

   Use append() to push in Python stack [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() without index

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

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

print(is_balanced('(()())'))
print(is_balanced('(()'))

medium

A. True\nFalse

B. False\nTrue

C. True\nTrue

D. False\nFalse

Solution

1. Step 1: Trace first input '(()())'

   Push '(' three times, pop ')' three times, stack empty at end, returns True.

2. Step 2: Trace second input '(()'

   Push '(' twice, one ')' pops one '(', stack not empty at end, returns False.

3. Final Answer:

   True\nFalse -> Option A

4. Quick Check:

   Balanced string returns True, unbalanced returns False [OK]

Hint: Empty stack at end means balanced parentheses [OK]

Common Mistakes:

• Not checking stack empty at end
• Returning true too early
• Ignoring unmatched openings

4. Find the error in this code snippet for checking balanced parentheses:

def check_balance(s):
    stack = []
    for ch in s:
        if ch == '(': stack.push(ch)
        elif ch == ')':
            if stack == []: return False
            stack.pop()
    return len(stack) == 0

medium

A. Using '==' to check empty stack is wrong

B. Missing return statement inside the loop

C. stack.push(ch) is invalid; use stack.append(ch) instead

D. pop() should be called before checking stack empty

Solution

1. Step 1: Identify stack method error

   Python list has no push() method; append() should be used to add elements.

2. Step 2: Verify other parts

   Return statements and empty checks are correct; pop() after check is correct.

3. Final Answer:

   stack.push(ch) is invalid; use stack.append(ch) instead -> Option C

4. Quick Check:

   Use append() to add in Python list stack [OK]

Hint: Use append() not push() for Python list stack [OK]

Common Mistakes:

• Using push() instead of append()
• Checking stack emptiness incorrectly
• Calling pop() without checking stack

5. Given the string s = '{[()()]}', which modification is needed in the balanced parentheses stack algorithm to correctly check this string?

hard

A. Only check for parentheses '()' and ignore other brackets

B. Count total number of brackets without matching pairs

C. Use a queue instead of a stack to check balance

D. Add support for multiple types of brackets and match pairs accordingly

Solution

1. Step 1: Identify bracket types in string

   The string contains curly braces {}, square brackets [], and parentheses ().

2. Step 2: Modify algorithm for multiple bracket types

   The stack algorithm must track each opening bracket type and match it with the correct closing bracket type.

3. Final Answer:

   Add support for multiple types of brackets and match pairs accordingly -> Option D

4. Quick Check:

   Match bracket types correctly for balanced check [OK]

Hint: Match each opening bracket with its correct closing type [OK]

Common Mistakes:

• Checking only parentheses and ignoring others
• Using queue instead of stack
• Counting brackets without matching