Problema vocale/consoane sablon

#include <iostream>
#include <string.h>

using namespace std;
int isVowel(char ch);
int isConsonant(char ch);


int main() {
    char s1[51], s2[51];
    cout << "Enter first string: ";
    cin >> s1;
    cout << "Enter second string: ";
    cin >> s2;

    for (int i = 0; i < strlen(s1); i++) {
        if (isVowel(s1[i]) && isVowel(s2[i])) {
            cout << "*";
        }
        else if (isConsonant(s1[i]) && isConsonant(s2[i])) {
            cout << "#";
        }
        else {
            cout << "?";
        }
    }

    return 0;
}

int isVowel(char ch) {
    return strchr("aeiou", ch) != NULL;
}

int isConsonant(char ch) {
    if (!isVowel(ch) && (ch >= 'A' && ch <= 'Z' || ch >= 'a' && ch <= 'z')) {
        return 1;
    }
    else {
        return 0;
    }
}