Balanced Parentheses Problem Using Stack in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA C

void push(char stack[], int *top, char value) {
    stack[++(*top)] = [1];
}

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

Astack

B*top

Cvalue

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*top' instead of 'value' to push.

Using 'stack' or 'top' incorrectly.

We push the given value onto the stack at the incremented top index.

2fill in blank

medium

Complete the code to check if the stack is empty.

DSA C

int isEmpty(int top) {
    return [1] == -1;
}

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

Astack

Btop

Cvalue

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Checking 'stack' or 'value' instead of 'top'.

Using 'size' which is unrelated here.

The stack is empty when 'top' is -1.

3fill in blank

hard

Fix the error in the pop function to return the top element and update the top index.

DSA C

char pop(char stack[], int *top) {
    if (*top == -1) return '\0';
    return stack[[1]--];
}

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

A(*top)

Btop

Cstack

D*top

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'top' instead of '*top' causes wrong indexing.

Using 'stack' or '(*top)' incorrectly.

We return the element at '*top' and then decrement '*top'.

4fill in blank

hard

Fill both blanks to check if the popped opening bracket matches the current closing bracket.

DSA C

int isMatchingPair(char opening, char closing) {
    return (opening == [1] && closing == [2]) ||
           (opening == '{' && closing == '}') ||
           (opening == '[' && closing == ']');
}

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

A'('

B'['

C')'

D']'

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing opening and closing brackets.

Using double quotes instead of single quotes.

We check if opening is '(' and closing is ')'.

5fill in blank

hard

Fill all three blanks to complete the main loop that checks balanced parentheses.

DSA C

for (int i = 0; i < length; i++) {
    if (expression[i] == [1] || expression[i] == [2] || expression[i] == [3]) {
        push(stack, &top, expression[i]);
    } else {
        if (isEmpty(top)) return 0;
        char popped = pop(stack, &top);
        if (!isMatchingPair(popped, expression[i])) return 0;
    }
}

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

A'('

B'['

C'{'

D')'

Attempts:

3 left

💡 Hint

Common Mistakes

Including closing brackets in the push condition.

Using wrong bracket characters.

We push opening brackets '(', '[', '{' onto the stack.

Practice

1. What is the main purpose of using a stack in the Balanced Parentheses problem?

easy

A. To keep track of opening parentheses and match them with closing ones

B. To count the total number of parentheses in the expression

C. To reverse the order of parentheses in the expression

D. To store only the closing parentheses encountered

Solution

1. Step 1: Understand the role of stack in parentheses matching

   A stack helps to remember the last opening parenthesis to match it with the next closing one.

2. Step 2: Identify why other options are incorrect

   Counting or reversing parentheses does not solve matching pairs; storing only closing parentheses misses the matching logic.

3. Final Answer:

   To keep track of opening parentheses and match them with closing ones -> Option A

4. Quick Check:

   Stack tracks openings for matching = B [OK]

Hint: Stack remembers last opening bracket for matching [OK]

Common Mistakes:

• Thinking stack counts parentheses instead of matching
• Using stack to store only closing parentheses
• Ignoring order of parentheses

2. Which of the following code snippets correctly pushes an opening parenthesis '(' onto a stack named stack in C?

easy

A. push(stack, '(');

B. stack.push('(');

C. stack->push('(');

D. push('(', stack);

Solution

1. Step 1: Recognize typical C stack push syntax

   In C, stack operations are usually functions like push(stack, element).

2. Step 2: Analyze each option

   push(stack, '('); matches the common C function call syntax. Options B and C use dot or arrow notation typical in C++ or OOP, not plain C. push('(', stack); has arguments reversed.

3. Final Answer:

   push(stack, '('); -> Option A

4. Quick Check:

   C stack push syntax = A [OK]

Hint: In C, use push(stack, element) function call [OK]

Common Mistakes:

• Using C++ style stack.push() in C
• Reversing arguments in push function
• Using arrow operator on non-pointer stack

3. What will be the output of the following C code snippet that checks balanced parentheses?

char expr[] = "({[]})";
int result = isBalanced(expr);
printf("%d", result);

Assume isBalanced returns 1 if balanced, 0 otherwise.

medium

A. Runtime error

B. 0

C. Compilation error

D. 1

Solution

1. Step 1: Analyze the expression "({[]})"

   The expression has matching pairs: '(' with ')', '{' with '}', '[' with ']'. All are properly nested.

2. Step 2: Understand isBalanced function behavior

   Since the expression is balanced, isBalanced returns 1, so printf prints 1.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   Balanced expression returns 1 = A [OK]

Hint: Balanced nested pairs return 1 from isBalanced [OK]

Common Mistakes:

• Assuming only () pairs are valid
• Confusing balanced with equal count
• Expecting 0 for nested but balanced

4. Identify the bug in this C function snippet for checking balanced parentheses:

int isBalanced(char* expr) {
  Stack s;
  for (int i = 0; expr[i] != '\0'; i++) {
    if (expr[i] == '(') push(&s, '(');
    else if (expr[i] == ')') {
      if (isEmpty(&s)) return 0;
      pop(&s);
    }
  }
  return isEmpty(&s);
}

medium

A. Return statement should be inside the loop

B. Stack s is not initialized before use

C. pop() is called without checking if stack is empty

D. The function does not check for other types of brackets

Solution

1. Step 1: Check stack initialization

   The stack variable 's' is declared but not initialized, so push/pop operations may fail.

2. Step 2: Verify other parts

   Other options are less critical: checking only () is a limitation but not a bug; pop is called after empty check; return outside loop is correct.

3. Final Answer:

   Stack s is not initialized before use -> Option B

4. Quick Check:

   Uninitialized stack causes errors = D [OK]

Hint: Always initialize stack before push/pop [OK]

Common Mistakes:

• Ignoring stack initialization
• Assuming pop safe without empty check
• Thinking return must be inside loop

5. Given the expression "[{()}](({}))", which of the following modifications to the isBalanced function will correctly handle all types of brackets: (), {}, and []?

hard

A. Ignore brackets inside quotes and only check outer brackets

B. Count total opening and closing brackets separately and compare counts

C. Push opening brackets and check matching closing brackets by popping and comparing types

D. Replace all brackets with parentheses before checking

Solution

1. Step 1: Understand bracket matching logic

   To handle multiple bracket types, push opening brackets and on closing bracket, pop and check if types match.

2. Step 2: Evaluate other options

   Counting brackets ignores order; ignoring brackets inside quotes is unrelated; replacing brackets loses type info.

3. Final Answer:

   Push opening brackets and check matching closing brackets by popping and comparing types -> Option C

4. Quick Check:

   Match types by push/pop for all brackets = C [OK]

Hint: Match each closing bracket with last opening of same type [OK]

Common Mistakes:

• Counting brackets without order check
• Ignoring bracket types
• Replacing brackets loses matching info