Parentheses

#include <stdio.h>
#include <string.h>

void main() {
int top = -1;
int MAX = 10;
int flag = 1;
char stack[MAX];
char str[MAX];

void push(char x)
{
    if (top >= MAX)
    {
        printf("Stack Overflow");
        return;
    }
    top++;
    stack[top] = x;
}

char pop()
{
    if (top < 0) {
        printf("Stack underflow");
        return 0;
    }
    return stack[top--];
}

printf("Enter the Expression");
gets(str);

for (int i = 0; i < strlen(str); i++) {
  if (str[i] == '(' || str[i] == '{' || str[i] == '[') push(str[i]);
  if (str[i] == ')' || str[i] == '}' || str[i] == ']') {
    if (top == -1) {
      flag = 0;
    }
    else {
      char temp = pop();
      if (str[i] == ')' && (temp == '[' || temp == '{')) flag = 0;
      if (str[i] == '}' && (temp == '(' || temp == '[')) flag = 0;
      if (str[i] == ']' && (temp == '(' || temp == '{')) flag = 0;
    }
  }
  
}

if (top != -1) flag = 0;

if (flag == 1) printf("%s is a valid expression", str);
else printf("%s is an invalid expression", str);

}