93-convert-15digit-id-to-18digit-id.apex

public String convert15to18DigitId(String fifteenDigit)
{
    if(fifteenDigit == null){
    return null;
    } else if (fifteenDigit.length() <> 15){
        return '-1';
        }
    else{
    return Id.valueof(fifteenDigit);
    }
}