Constructor error

gistfile1.cpp

//LIST NODE

String::ListNode * String::ListNode::stringToList(char * s){
    if (s[0] != '\0'){
        ListNode * tempHead = new ListNode(s[0],stringToList(s[1]));
        return tempHead;
    } else {
        return nullptr;
    }
}

//STRING
String::String( const char * s ){
    head = nullptr;
    head = ListNode::stringToList((char *)s);
}