Balanced Parentheses Problem Using Stack in DSA Python - Build from Scratch

Balanced Parentheses Problem Using Stack

📖 Scenario: Imagine you are building a simple code editor that checks if the parentheses in a line of code are balanced. Balanced parentheses mean every opening bracket has a matching closing bracket in the correct order.

🎯 Goal: You will create a program that uses a stack to check if a given string of parentheses is balanced or not.

📋 What You'll Learn

Create a stack using a list

Use a loop to process each character in the input string

Push opening brackets onto the stack

Pop from the stack when a matching closing bracket is found

Check if the stack is empty at the end to decide if parentheses are balanced

💡 Why This Matters

🌍 Real World

Checking balanced parentheses is important in code editors and compilers to ensure code syntax is correct.

💼 Career

Understanding stacks and balanced parentheses is a common interview question and a fundamental concept in programming and software development.

1

Create the input string of parentheses

Create a variable called parentheses and set it to the string '(()())'.

DSA Python

# Create the input string called parentheses
# Your code here

Hint

Use single quotes to create the string exactly as shown.

2

Create an empty stack

Create a variable called stack and set it to an empty list [].

DSA Python

parentheses = '(()())'
# Create an empty list called stack
# Your code here

Hint

A stack can be created using an empty list in Python.

3

Use a loop to check parentheses using the stack

Use a for loop with variable char to iterate over parentheses. Inside the loop, if char is '(', append it to stack. If char is ')', pop from stack if it is not empty. If stack is empty when trying to pop, set a variable balanced to False and break the loop. After the loop, if stack is empty and balanced is not False, set balanced to True, else set it to False.

DSA Python

parentheses = '(()())'
stack = []
# Use a for loop to check each character in parentheses
# Your code here

Hint

Use stack.append() to add and stack.pop() to remove the last item.

Check if stack is empty before popping to avoid errors.

4

Print if the parentheses are balanced

Print "Balanced" if balanced is True, else print "Not Balanced".

DSA Python

parentheses = '(()())'
stack = []
balanced = None
for char in parentheses:
    if char == '(':  
        stack.append(char)
    elif char == ')':
        if stack:
            stack.pop()
        else:
            balanced = False
            break
if balanced is None:
    balanced = len(stack) == 0
# Print "Balanced" if balanced is True, else print "Not Balanced"
# Your code here

Hint

Use an if statement to print the correct message based on the balanced variable.

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