+ 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