Balanced Parentheses Problem Using Stack in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind using a stack to check for balanced parentheses?

Use a stack to keep track of opening parentheses. For every closing parenthesis, check if it matches the last opening one on the stack. If all match and stack is empty at the end, parentheses are balanced.

beginner

Which characters are typically considered in the balanced parentheses problem?

The characters are usually '(', ')', '{', '}', '[' and ']'. Each opening bracket must be matched with the correct closing bracket.

beginner

What happens if you find a closing parenthesis but the stack is empty?

It means there is no matching opening parenthesis for this closing one, so the parentheses are not balanced.

beginner

Why must the stack be empty at the end of the check for balanced parentheses?

If the stack is not empty, it means there are opening parentheses without matching closing ones, so the parentheses are not balanced.

beginner

How do you check if a closing parenthesis matches the last opening parenthesis on the stack?

Compare the closing parenthesis with the opening one on top of the stack. For example, ')' matches '(', '}' matches '{', and ']' matches '['.

What data structure is best suited for checking balanced parentheses?

A Array

B Queue

C Stack

D Linked List

A stack allows last-in, first-out access which is perfect for matching opening and closing parentheses.

What should you do when you encounter an opening parenthesis in the string?

A Pop from the stack

B Ignore it

C Replace it with a closing parenthesis

D Push it onto the stack

Opening parentheses are pushed onto the stack to be matched later with closing parentheses.

If the stack is empty but you find a closing parenthesis, what does it mean?

A Parentheses are balanced

B There is no matching opening parenthesis

C You should push the closing parenthesis

D Ignore the closing parenthesis

A closing parenthesis without a matching opening one means the parentheses are not balanced.

What does it mean if the stack is not empty after processing the entire string?

A There are unmatched opening parentheses

B All parentheses matched

C There are unmatched closing parentheses

D The string is empty

Unmatched opening parentheses remain in the stack, so the parentheses are not balanced.

Which of these pairs correctly matches opening and closing parentheses?

A '{' and '}'

B '(' and '}'

C '[' and '}'

D '(' and ']'

'{' matches with '}', '[' matches with ']', and '(' matches with ')'.

Explain step-by-step how a stack helps to check if parentheses in a string are balanced.

Think about how last opened parentheses must be closed first.

Describe what conditions cause the parentheses to be considered unbalanced when using a stack.

Consider mismatches and leftover openings.

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