Balanced Parentheses Problem Using Stack in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an opening bracket onto the stack.

DSA Python

if char in '({[':
    stack.[1](char)

Drag options to blanks, or click blank then click option'

Ainsert

Bpop

Cremove

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of append() to add elements.

Using remove() which deletes a specific element, not adding.

Using insert() without specifying index properly.

We use append() to add an element to the top of the stack (list).

2fill in blank

medium

Complete the code to check if the stack is empty before popping.

DSA Python

if stack and stack[-1] == pairs[char]:
    stack.[1]()

Drag options to blanks, or click blank then click option'

Aappend

Bpop

Cclear

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using append() which adds instead of removing.

Using clear() which empties the whole list.

Using remove() which removes by value, not by position.

pop() removes the last element from the stack, which is needed here.

3fill in blank

hard

Fix the error in the condition to check if the character is a closing bracket.

DSA Python

if char in [1]:
    if not stack or stack[-1] != pairs[char]:
        return False

Drag options to blanks, or click blank then click option'

A"([{"

B"abc"

C"}])"

D"123"

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for opening brackets instead of closing brackets.

Using unrelated characters like letters or numbers.

Using incorrect bracket types.

We check if char is a closing bracket, so it must be in '})]'.

4fill in blank

hard

Fill both blanks to create a dictionary mapping closing to opening brackets and check stack emptiness.

DSA Python

pairs = [1]
return not [2](stack)

Drag options to blanks, or click blank then click option'

A{"}": "{", ")": "(", "]": "["}

B{"}": "(", ")": "{", "]": "["}

C{"}": "[", ")": "{", "]": "("}

D{"}": "[", ")": "(", "]": "{"}

Elen

Fbool

Gany

Hall

Attempts:

3 left

💡 Hint

Common Mistakes

Incorrect bracket mappings in the dictionary.

Using len(stack) instead of bool(stack) for emptiness check.

Using any() or all() which do not check emptiness.

The pairs dictionary maps closing to opening brackets correctly, and bool(stack) checks if stack is empty.

5fill in blank

hard

Fill all three blanks to complete the balanced parentheses function using stack.

DSA Python

def is_balanced(s):
    stack = []
    pairs = [1]
    for char in s:
        if char in [2]:
            stack.[3](char)
        elif char in pairs:
            if not stack or stack[-1] != pairs[char]:
                return False
            stack.pop()
    return not stack

Drag options to blanks, or click blank then click option'

A"({["

B"}])"

Cappend

D{"}": "{", ")": "(", "]": "["}

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing opening and closing brackets in pairs dictionary.

Using pop instead of append to add elements.

Checking wrong characters for opening brackets.

pairs dictionary maps closing to opening brackets, opening brackets are '({[', and append adds to stack.

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