Balanced Parentheses Problem Using Stack in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code checking balanced parentheses?

Consider the following JavaScript code that checks if parentheses in a string are balanced using a stack. What will be printed after running it?

DSA Javascript

function isBalanced(s) {
  const stack = [];
  for (const char of s) {
    if (char === '(') {
      stack.push(char);
    } else if (char === ')') {
      if (stack.length === 0) return false;
      stack.pop();
    }
  }
  return stack.length === 0;
}

console.log(isBalanced('(()())'));

A SyntaxError

B false

C TypeError

D true

💡 Hint

Think about how the stack is used to match opening and closing parentheses.

The code pushes '(' onto the stack and pops when it sees ')'. The string '(()())' is balanced, so the stack is empty at the end, returning true.

❓ Predict Output

intermediate

What is the output when input has extra closing parentheses?

What will the following code print when checking the string '(()))' for balanced parentheses?

DSA Javascript

function isBalanced(s) {
  const stack = [];
  for (const char of s) {
    if (char === '(') {
      stack.push(char);
    } else if (char === ')') {
      if (stack.length === 0) return false;
      stack.pop();
    }
  }
  return stack.length === 0;
}

console.log(isBalanced('(()))'));

A false

B true

C ReferenceError

D RangeError

💡 Hint

Check what happens when a closing parenthesis appears but stack is empty.

The string has one extra closing parenthesis, so the function returns false.

🧠 Conceptual

advanced

Which data structure is best for checking balanced parentheses and why?

Why is a stack the best data structure to check if parentheses in a string are balanced?

A Because it allows last-in, first-out access to match the most recent opening parenthesis with a closing one.

B Because it stores all characters in sorted order for easy lookup.

C Because it uses a queue to process parentheses in the order they appear.

D Because it uses a hash map to count parentheses types.

💡 Hint

Think about how parentheses must be matched in reverse order of appearance.

A stack works as last-in, first-out, so the most recent '(' is matched with the next ')'. This matches the nesting structure of parentheses.

❓ Predict Output

advanced

What is the output of this code with multiple types of brackets?

What will the following code print when checking the string '{[()()]}' for balanced brackets of types (), {}, and []?

DSA Javascript

function isBalanced(s) {
  const stack = [];
  const pairs = {')': '(', ']': '[', '}': '{'};
  for (const char of s) {
    if (['(', '[', '{'].includes(char)) {
      stack.push(char);
    } else if ([')', ']', '}'].includes(char)) {
      if (stack.length === 0 || stack.pop() !== pairs[char]) return false;
    }
  }
  return stack.length === 0;
}

console.log(isBalanced('{[()()]}'));

A SyntaxError

B false

C true

D TypeError

💡 Hint

Check how the code matches each closing bracket with the correct opening bracket.

All brackets are correctly matched and nested, so the function returns true.

🔧 Debug

expert

What error does this code raise when checking '(()'?

What error will this code raise when checking the string '(()' for balanced parentheses?

DSA Javascript

function isBalanced(s) {
  const stack = [];
  for (const char of s) {
    if (char === '(') {
      stack.push(char);
    } else if (char === ')') {
      stack.pop();
    }
  }
  return stack.length === 0;
}

console.log(isBalanced('(()'));

A TypeError

B false

C true

D RangeError

💡 Hint

Check what happens if there are unmatched opening parentheses left in the stack.

The code does not check if stack is empty before popping, but since input has extra '(', no pop error occurs. The function returns false because stack is not empty.

Practice

1. What is the main purpose of using a stack to solve the Balanced Parentheses Problem?

easy

A. To sort the parentheses in order

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

C. To reverse the string of parentheses

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

Solution

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

   A stack helps store opening parentheses as they appear, so we can check if each closing parenthesis matches the last opened one.

2. Step 2: Explain why other options are incorrect

   Counting or reversing parentheses does not help check if they are balanced. Sorting changes order and breaks the original sequence.

3. Final Answer:

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

4. Quick Check:

   Stack tracks openings for matching [OK]

Hint: Stack stores openings to match closings quickly [OK]

Common Mistakes:

• Thinking stack counts parentheses instead of matching
• Assuming reversing string solves balance
• Confusing sorting with balancing

2. Which of the following is the correct way to push an element onto a stack in JavaScript?

easy

A. stack.add(element);

B. stack.push(element);

C. stack.insert(element);

D. stack.append(element);

Solution

1. Step 1: Recall JavaScript array method for adding elements at the end

   JavaScript arrays use push() to add elements to the end, which simulates stack push.

2. Step 2: Identify incorrect methods

   add(), insert(), and append() are not valid JavaScript array methods.

3. Final Answer:

   stack.push(element); -> Option B

4. Quick Check:

   Use push() to add to stack [OK]

Hint: Use push() to add elements to stack in JavaScript [OK]

Common Mistakes:

• Using add() or append() which are not JavaScript array methods
• Confusing push() with pop()
• Trying to use insert() which doesn't exist

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

function isBalanced(s) {
  const stack = [];
  for (const char of s) {
    if (char === '(') stack.push(char);
    else if (char === ')') {
      if (stack.length === 0) return false;
      stack.pop();
    }
  }
  return stack.length === 0;
}

console.log(isBalanced('(()())'));
console.log(isBalanced('(()'));
console.log(isBalanced(')('));

medium

A. true, true, false

B. false, true, false

C. true, false, false

D. false, false, true

Solution

1. Step 1: Trace the function for each input string

   For '(()())': all parentheses match, stack ends empty, returns true.
   For '(()': one '(' remains unmatched, returns false.
   For ')(': first char is ')', stack empty, returns false.

2. Step 2: Confirm outputs match true, false, false

   Outputs are true, false, false respectively.

3. Final Answer:

   true, false, false -> Option C

4. Quick Check:

   Stack empty at end means balanced [OK]

Hint: Stack empty at end means balanced parentheses [OK]

Common Mistakes:

• Assuming unmatched openings are balanced
• Ignoring early unmatched closing parentheses
• Confusing true/false outputs

4. The following code is intended to check balanced parentheses but has a bug. What is the bug?

function checkBalance(str) {
  const stack = [];
  for (let i = 0; i < str.length; i++) {
    if (str[i] === '(') {
      stack.push('(');
    } else {
      if (stack.length === 0) return false;
      stack.pop();
    }
  }
  return true;
}

medium

A. It returns true even if stack is not empty after loop

B. It pushes closing parentheses instead of opening

C. It does not check for closing parentheses

D. It uses wrong loop syntax

Solution

1. Step 1: Analyze the return statement after loop

   The function returns true without checking if stack is empty, so leftover '(' are ignored.

2. Step 2: Explain why this causes incorrect results

   If there are unmatched '(' left in stack, string is unbalanced but function returns true incorrectly.

3. Final Answer:

   It returns true even if stack is not empty after loop -> Option A

4. Quick Check:

   Check stack.length === 0 before return true [OK]

Hint: Always check stack empty before returning true [OK]

Common Mistakes:

• Forgetting to check stack length after loop
• Returning true too early
• Ignoring unmatched opening parentheses

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

hard

A. Add support for matching pairs of '()', '{}', and '[]' with stack push/pop logic

B. Ignore all characters except '(' and ')' during stack operations

C. Use two separate stacks for different types of brackets

D. Reverse the string before checking with the existing algorithm

Solution

1. Step 1: Understand the input string contains multiple bracket types

   The string has '()', '{}', and '[]' brackets, so the algorithm must handle all types.

2. Step 2: Explain how to handle multiple bracket types with one stack

   Push opening brackets onto stack, and on closing bracket, check if it matches the last opening bracket type before popping.

3. Final Answer:

   Add support for matching pairs of '()', '{}', and '[]' with stack push/pop logic -> Option A

4. Quick Check:

   One stack handles all bracket types [OK]

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

Common Mistakes:

• Ignoring bracket types other than parentheses
• Using multiple stacks unnecessarily
• Reversing string instead of matching pairs