Function verfication [closed]

i'm currently doing cs50 courses and i was looking for people here to check the function i wrote and tell me if there is a better way to do it.

The first function i wrote is to look for repeatead char in a given string:

bool repeatChar(char key[]){

int array[26] = {0};
int lenght = strlen(key);

for(int i = 0; i < lenght; i++)
{
    array[tolower(key[i])-'a']++;
}

for(int k = 0; k < lenght;k++)
{
    if(array[k] > 1){
    return true;
    }
}
return false;
}

Is there a better way to do the code above ? I tried to do it with while but i could not if someone could help me there please.

Now the second function :

bool isChar(char* key){

int lenght = strlen(key);

for(int i = 0; i < lenght; i++)
{
    if(!isalpha(key[i]))
    {
        return false;
    }
}

return true;
}

the second function check if the char in a given string is a character or not, but when i put special char '&' or'(' it does not work as it should.

Third Function :

bool validLenghtString(char* key){

    if(strlen(key)==26){
        return true;
    }
    return false;

}

the function above only checks if the string lenght is 26 long, is it an overkilli function ?

Last Function :

bool digitCheck(char* key){

int lenght = strlen(key);

    for(int i = 0; i < lenght; i++)
    {
        if(isdigit(key[i])){
            return true;
        }

    }
    return false;
} 

Pretty explicit function.

Is there a better way to write these functions ?