String longestDigitsPrefix(String inputString) {
    String temp = "";
    if (inputString.length() == 0) return "";
    for(int i = 0;i < inputString.length();i++){
        if(inputString.charAt(i) > '9' || inputString.charAt(i) < '0') break;
        else temp += inputString.charAt(i);
    }
    return temp;      
}
