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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Balanced Parentheses Check

What is the output of the following C code that checks if the parentheses in the string are balanced?

DSA C

#include <stdio.h>
#include <string.h>
#define MAX 100

int isBalanced(char *expr) {
    char stack[MAX];
    int top = -1;
    for (int i = 0; i < strlen(expr); i++) {
        char ch = expr[i];
        if (ch == '(') {
            stack[++top] = ch;
        } else if (ch == ')') {
            if (top == -1) return 0;
            top--;
        }
    }
    return top == -1;
}

int main() {
    char expr[] = "((())())";
    if (isBalanced(expr))
        printf("Balanced\n");
    else
        printf("Not Balanced\n");
    return 0;
}

A Not Balanced

B Runtime Error

C Compilation Error

D Balanced

💡 Hint

Check how the stack is used to track opening parentheses and matched with closing ones.

The code uses a stack to push '(' characters and pop when ')' is found. If the stack is empty at the end, parentheses are balanced.

❓ Predict Output

intermediate

Output for Unbalanced Parentheses

What will the following C program print when checking the string "(()" for balanced parentheses?

DSA C

#include <stdio.h>
#include <string.h>
#define MAX 100

int isBalanced(char *expr) {
    char stack[MAX];
    int top = -1;
    for (int i = 0; i < strlen(expr); i++) {
        char ch = expr[i];
        if (ch == '(') {
            stack[++top] = ch;
        } else if (ch == ')') {
            if (top == -1) return 0;
            top--;
        }
    }
    return top == -1;
}

int main() {
    char expr[] = "(()";
    if (isBalanced(expr))
        printf("Balanced\n");
    else
        printf("Not Balanced\n");
    return 0;
}

A Compilation Error

B Balanced

C Not Balanced

D Runtime Error

💡 Hint

Count how many '(' are left unmatched at the end.

The stack will have one '(' left unmatched, so the function returns 0 and prints "Not Balanced".

🧠 Conceptual

advanced

Why Use a Stack for Balanced Parentheses?

Why is a stack the best data structure to check for balanced parentheses in an expression?

A Because it allows checking pairs in the correct nested order using Last-In-First-Out behavior

B Because it stores all characters and sorts them alphabetically

C Because it uses First-In-First-Out behavior like a queue

D Because it can store unlimited characters without memory limits

💡 Hint

Think about how parentheses open and close in nested order.

A stack follows Last-In-First-Out (LIFO), which matches how the most recent '(' must be closed first by a ')'.

🔧 Debug

advanced

Find the Bug in Parentheses Checker

What error will the following code produce when checking the string ")(" for balanced parentheses?

DSA C

#include <stdio.h>
#include <string.h>
#define MAX 100

int isBalanced(char *expr) {
    char stack[MAX];
    int top = -1;
    for (int i = 0; i < strlen(expr); i++) {
        char ch = expr[i];
        if (ch == '(') {
            stack[++top] = ch;
        } else if (ch == ')') {
            top--;
        }
    }
    return top == -1;
}

int main() {
    char expr[] = ")(";
    if (isBalanced(expr))
        printf("Balanced\n");
    else
        printf("Not Balanced\n");
    return 0;
}

A The code will cause a compilation error

B The code will cause a stack underflow (access invalid index)

C The code will print "Balanced"

D The code will print "Not Balanced"

💡 Hint

Look at what happens when ')' is found but stack is empty.

The code decrements top without checking if the stack is empty, causing top to become -2 and invalid memory access.

🚀 Application

expert

Count Balanced Parentheses Substrings

Given the string "()()()", how many balanced parentheses substrings of length 2 are there?

A 3

B 2

C 1

D 0

💡 Hint

Look for every pair of characters that form "()".

Substrings of length 2 are: "()", "()", "()" at positions 0-1, 2-3, 4-5 respectively. All are balanced.