+ What is polymorphism? + What is encapsulation? + What is inversion of control? 

+ Design principles: - Separation of concerns. - Single Responsibility principle. - Principle of Least Knowledge. - DRY: Don’t repeat yourself. - Minimize upfront design. + Drawbacks of not using `separation of concerns`. + Microservices is a variant of the service-oriented architecture (SOA). + What is SOLID Principles of Object Oriented and Agile Design? - Single responsibility principle. - Open/closed principle. - Liskov substitution principle. - Interface segregation principle. - Dependency inversion principle. + Design patterns. - Creational: Builder, Object Pool, Factory Method, Signleton, Multiton, Prototype, Abstract Factory. - Structural: Adapter, Bridge, Composite, Decorator, Facade, Flyweight, Proxy. - Behavioral: Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy. + 3-tier architecture? - Presentation tier. - Application tier. - Data tier. + 3-layer architecture? - DAO (Repository). - Business (Service) layer. - Controller). + What is REST? + Idempotent operation EX: The PUT and DELETE methods are referred to as idempotent, meaning that the operation will produce the same result no matter how many times it is repeated. + nullipotent operation EX: GET method is a safe method (or nullipotent), meaning that calling it produces no side-effects. 

+ (medium) given a collection of numbers and a sum , find a matching pair that is equal to the sum - assume the numbers is sorted - assume the numbers is not sorted what is the best complexity you can come up with? can you do it in O(n^2) , O(nlog(n)) , O(n) , etc... ? where n is the length of the numbers example : [1 , 2 , 3 ,6] , Sum = 10 , no matching pair : [1 , 2 , 4 , 4] , Sum = 8 , matching pair = (4, 4) + (Easy) given a string , find the first recurret charter in the string example : 'ABCDA' , answer = 'A' : 'ABCBDA' , answer = 'B' : 'ABC' , answer = NULL find the O(n) answer