How To Convert 15 To 18 Digit Id Using Apex In Salesforce?

String idSalesforce = 'a0D30000001n7Pi'; // change with your 15 chars ID
if(idSalesforce.length() == 18){
system.debug('Input Id is 18 char');
}
else if(idSalesforce.length() != 15){
system.debug('Input Id error');
}
else{
String suffix = '';
String idOut= '';
String InChars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ012345';
for(Integer i = 0; i<3; i++){
Integer flags = 0;
for(Integer j = 0; j<5; j++){
String c = idSalesforce.substring(i*5+j, i*5+j+1);
if((c.compareTo('A')>=0) && (c.compareTo('Z')<=0)){
flags += 1 << j;
}
}
suffix = suffix + InChars.substring(flags, flags+1);
}
idOut = idSalesforce + suffix;
system.debug('Id Output 15: '+idSalesforce);
system.debug('Id Output 18: '+idOut);
}