Apex: Get all required fields of an sObject

Map<String, Schema.SObjectType> m  = Schema.getGlobalDescribe();
Schema.SObjectType s = m.get('Contact');
Schema.DescribeSObjectResult r = s.getDescribe();
Map<String,Schema.SObjectField> fields = r.fields.getMap();

for(String field : fields.keyset()) {
    Schema.DescribeFieldResult describeResult = fields.get(field).getDescribe();
    if (describeResult.isCreateable()  && !describeResult.isNillable() && !describeResult.isDefaultedOnCreate()) {
        System.debug(field);
    }
}