Palindrome check using Stack.c

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

#define MAX 100

char stack[MAX];
int top = -1;

void push(char c) {
    if (top == MAX - 1) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = c;
}

char pop() {
    if (top == -1) {
        printf("Stack Underflow\n");
        return '\0';
    }
    return stack[top--];
}

int isPalindrome(char str[]) {
    int len = strlen(str);
    for (int i = 0; i < len; i++) {
        if (isalnum(str[i])) {
            push(tolower(str[i]));
        }
    }
    for (int i = 0; i < len; i++) {
        if (isalnum(str[i])) {
            char c = pop();
            if (tolower(str[i]) != c) {
                return 0;
            }
        }
    }
    return 1;
}

int main() {
    char str[MAX];
    printf("Enter a string: ");
    fgets(str, MAX, stdin);
    str[strcspn(str, "\n")] = '\0';
    if (isPalindrome(str))
        printf("The string is a palindrome.\n");
    else
        printf("The string is NOT a palindrome.\n");
    return 0;
}