Balanced Parentheses Problem Using Stack in DSA C# - 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 pushed onto the stack in the balanced parentheses problem?

Only opening parentheses characters like '(', '{', '[' are pushed onto the stack.

beginner

What should happen when a closing parenthesis is encountered during the check?

Pop the top element from the stack and check if it is the matching opening parenthesis. If not, the parentheses are not balanced.

beginner

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

If the stack is not empty, it means there are unmatched opening parentheses left, so the parentheses are not balanced.

intermediate

What is the time complexity of checking balanced parentheses using a stack?

The time complexity is O(n), where n is the length of the string, because each character is processed once.

What data structure is best suited for checking balanced parentheses?

A Queue

B Stack

C Array

D Linked List

A stack allows last-in, first-out access, which matches the need to check the most recent opening parenthesis first.

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

A Push it onto the stack

B Pop from the stack

C Ignore it

D Replace it with a closing parenthesis

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

If the stack is empty when a closing parenthesis is found, what does it mean?

A There is no matching opening parenthesis

B Parentheses are balanced

C The string is empty

D The closing parenthesis is extra but balanced

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

Which of these strings is balanced?

A ({[)

B ({[})

C ({[]

D ({[]})

Only '({[]})' has matching opening and closing parentheses in correct order.

What is the final check after processing all characters in the string?

A Stack should have all opening parentheses

B Stack should have one element

C Stack should be empty

D Stack should have all closing parentheses

If the stack is empty, all parentheses matched correctly.

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

Think about matching pairs and last opened parentheses.

Describe what happens if the input string has extra opening or closing parentheses when using a stack.

Consider what the stack state tells you after processing.

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