Balanced Parentheses Problem Using Stack in DSA C# - Build from Scratch

Balanced Parentheses Problem Using Stack

📖 Scenario: Imagine you are building a simple code editor that needs to check 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 to hold characters

Use a string variable with a specific sequence of parentheses

Implement logic to push opening brackets and pop when matching closing brackets appear

Print True if parentheses are balanced, otherwise False

💡 Why This Matters

🌍 Real World

Checking balanced parentheses is important in code editors, compilers, and calculators to ensure expressions are correct.

💼 Career

Understanding stacks and balanced parentheses is a common interview question and fundamental for parsing tasks in software development.

1

Create the parentheses string

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

DSA C#

# Your code here

Hint

Use string parentheses = "(()())"; to create the string.

2

Create an empty stack for characters

Create a stack of characters called stack using Stack<char>.

DSA C#

string parentheses = "(()())";
// Your code here

Hint

Use Stack<char> stack = new Stack<char>(); to create the stack.

3

Check parentheses using the stack

Write a for loop with variable ch to iterate over each character in parentheses. Inside the loop, push ch to stack if it is '('; if ch is ')', pop from stack only if stack is not empty. If stack is empty when trying to pop, set a boolean variable isBalanced to false and break the loop. Initialize isBalanced as true before the loop.

DSA C#

using System.Collections.Generic;

string parentheses = "(()())";
Stack stack = new Stack();

// Your code here

Hint

Use a for loop and check each character. Push '(' to stack, pop for ')'. If pop is not possible, set isBalanced to false and stop.

4

Print if parentheses are balanced

After the loop, print True if isBalanced is true and stack is empty; otherwise print False.

DSA C#

using System.Collections.Generic;

string parentheses = "(()())";
Stack stack = new Stack();

bool isBalanced = true;
for (int i = 0; i < parentheses.Length; i++)
{
    char ch = parentheses[i];
    if (ch == '(')
    {
        stack.Push(ch);
    }
    else if (ch == ')')
    {
        if (stack.Count == 0)
        {
            isBalanced = false;
            break;
        }
        stack.Pop();
    }
}

// Your code here

Hint

Check if isBalanced is true and stack is empty. Print True or False accordingly.

Practice

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

easy

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

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

C. To reverse the order of parentheses in the string

D. To sort the parentheses based on their type

Solution

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

   A stack helps keep track of opening parentheses so that each closing parenthesis can be matched correctly in order.

2. Step 2: Identify why other options are incorrect

   Counting or reversing parentheses does not solve the matching problem, and sorting is unrelated to balance checking.

3. Final Answer:

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

4. Quick Check:

   Stack tracks openings for matching closings [OK]

Hint: Stack stores openings to match closings in order [OK]

Common Mistakes:

• Thinking stack counts parentheses instead of matching
• Confusing stack with queue for this problem
• Assuming stack reverses parentheses order

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

easy

A. stack.Push('(');

B. stack.Add('(');

C. stack.Insert('(');

D. stack.Enqueue('(');

Solution

1. Step 1: Recall stack method to add elements

   In C#, the method to add an element to a stack is Push().

2. Step 2: Identify incorrect methods

   Add() and Insert() are not stack methods; Enqueue() is for queues.

3. Final Answer:

   stack.Push('('); -> Option A

4. Quick Check:

   Use Push() to add to stack [OK]

Hint: Use Push() to add elements to stack in C# [OK]

Common Mistakes:

• Using queue methods like Enqueue() on stack
• Trying to use Add() or Insert() which don't exist for stack
• Confusing stack with list methods

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

string input = "({[]})";
Stack<char> stack = new Stack<char>();
foreach (char c in input) {
  if (c == '(' || c == '{' || c == '[') {
    stack.Push(c);
  } else {
    if (stack.Count == 0) {
      Console.WriteLine("Unbalanced");
      return;
    }
    char top = stack.Pop();
    if ((c == ')' && top != '(') || (c == '}' && top != '{') || (c == ']' && top != '[')) {
      Console.WriteLine("Unbalanced");
      return;
    }
  }
}
if (stack.Count == 0) Console.WriteLine("Balanced"); else Console.WriteLine("Unbalanced");

medium

A. Runtime Error

B. Unbalanced

C. No Output

D. Balanced

Solution

1. Step 1: Trace the input string character by character

   Opening brackets '(', '{', '[' are pushed onto the stack. Closing brackets pop and check matching. The input "({[]})" is perfectly nested.

2. Step 2: Check stack emptiness after processing

   After processing all characters, stack is empty, so output is "Balanced".

3. Final Answer:

   Balanced -> Option D

4. Quick Check:

   All pairs matched, stack empty = Balanced [OK]

Hint: Match each closing with last opening using stack [OK]

Common Mistakes:

• Not checking stack empty before popping
• Mismatching bracket pairs
• Ignoring stack emptiness at end

4. Identify the bug in the following C# code snippet for checking balanced parentheses:

Stack<char> stack = new Stack<char>();
foreach (char c in input) {
  if (c == '(' || c == '{' || c == '[') {
    stack.Push(c);
  } else {
    char top = stack.Pop();
    if ((c == ')' && top != '(') || (c == '}' && top != '{') || (c == ']' && top != '[')) {
      return false;
    }
  }
}
return stack.Count == 0;

medium

A. Not returning true when balanced

B. Incorrect matching of parentheses pairs

C. Missing check if stack is empty before Pop()

D. Using stack.Push() instead of Pop() for closing brackets

Solution

1. Step 1: Check for stack underflow before popping

   The code pops from stack without checking if it's empty, which can cause runtime error if closing bracket appears first.

2. Step 2: Confirm matching logic is correct

   Matching pairs are correctly checked, and return false if mismatch occurs.

3. Final Answer:

   Missing check if stack is empty before Pop() -> Option C

4. Quick Check:

   Always check stack empty before Pop() [OK]

Hint: Check stack empty before Pop() to avoid errors [OK]

Common Mistakes:

• Popping from empty stack causing exceptions
• Ignoring stack.Count == 0 before Pop()
• Confusing Push and Pop methods

5. Given the string "[{()}]({[]})", which of the following modifications to the balanced parentheses algorithm will correctly handle this input and return true?

Options:
A) Ignore all characters except parentheses.
B) Use a stack and match pairs as usual.
C) Use two stacks, one for opening and one for closing parentheses.
D) Use a queue instead of a stack to track parentheses.

hard

A. Use a stack and match pairs as usual.

B. Ignore all characters except parentheses.

C. Use two stacks, one for opening and one for closing parentheses.

D. Use a queue instead of a stack to track parentheses.

Solution

1. Step 1: Understand the input and algorithm

   The input contains only parentheses characters. The standard stack-based matching algorithm works for nested and sequential pairs.

2. Step 2: Evaluate each option's correctness

   Use a stack and match pairs as usual. is correct as it uses the standard stack approach. Ignore all characters except parentheses. is irrelevant since input has only parentheses. Use two stacks, one for opening and one for closing parentheses. complicates unnecessarily. Use a queue instead of a stack to track parentheses. fails because queue does not track last opening bracket.

3. Final Answer:

   Use a stack and match pairs as usual. -> Option A

4. Quick Check:

   Standard stack method handles nested pairs [OK]

Hint: Standard stack method works for all balanced parentheses [OK]

Common Mistakes:

• Trying to use queue which breaks matching order
• Overcomplicating with multiple stacks
• Ignoring characters that are not parentheses