Life In USA

2. 顯示排序節點from smallest to biggest(Depth-First Search in-order) traverse:

A-B-C-D-E-F-G-I-H

• 從最左邊(最小的)開始找，如果已經找不到left child代表已經是最小的，然後print出來。
• 然後檢查有沒有right child，繼續遞迴去找。
• Big O:  O(n)

F-B-A-D-C-E-F-I-H

• 顯示current node, 然後再顯示左邊子節點, 再來右邊子結點

A-C-E-D-B-H-I-G-F

• Check first if root exists. If not then return.
• Find the node to remove.
• If node doesn't have child, direct remove it.
• If node have one child, copy child's value to it then remove child.
• If node have 2 child:
•  find a minimum value in the node's right subtree.
•  replace value of the node to be removed with found minimum.
•  remove to the right subtree to remove a duplicate(original minimum value in the node's right subtree).

GCD provides three main types of queues:

• User-interactive: This represents tasks that need to be done immediately in order to provide a nice user experience. Use it for UI updates, event handling and small workloads that require low latency. The total amount of work done in this class during the execution of your app should be small. This should run on the main thread.
• User-initiated: The represents tasks that are initiated from the UI and can be performed asynchronously. It should be used when the user is waiting for immediate results, and for tasks required to continue user interaction. This will get mapped into the high priority global queue.
• Utility: This represents long-running tasks, typically with a user-visible progress indicator. Use it for computations, I/O, networking, continous data feeds and similar tasks. This class is designed to be energy efficient. This will get mapped into the low priority global queue.
• Background: This represents tasks that the user is not directly aware of. Use it for prefetching, maintenance, and other tasks that don’t require user interaction and aren’t time-sensitive. This will get mapped into the background priority global queue.

ref: 

https://www.raywenderlich.com/148513/grand-central-dispatch-tutorial-swift-3-part-1

https://www.appcoda.com/grand-central-dispatch/

Cocoa and Cocoa Touch are the application development environments for OS X and iOS, respectively. Both Cocoa and Cocoa Touch include the Objective-C runtime and two core frameworks:

2. What's different between NULL and nil and Nil?

The difference is that while NULL represents zero for any pointer, nil is specific to objects (e.g., id) and Nil is specific to class pointers.

Swift’s nil is not the same as nil in Objective-C. In Objective-C, nil is a pointer to a non-existent object. In Swift, nil is not a pointer—it is the absence of a value of a certain type. Optionals of any type can be set to nil, not just object types.

3. What's different between 'weak' and 'strong' property?

4. What is ARC?

Automatic Reference Counting (ARC) is a compiler feature that provides automatic memory management of Objective-C objects. Rather than having to think about retain and release operations, ARC allows you to concentrate on the interesting code, the object graphs, and the relationships between objects in your application.

#automatic memory management, dealloc

5. What is atomic?

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread.

atomic vs. nonatomic refers to the thread safety of the getter and setter methods that the compiler synthesizes for the property. atomic (the default) tells the compiler to make the accessor methods thread-safe (by adding a lock before an ivar is accessed) and nonatomic does the opposite. The advantage of nonatomic is slightly higher performance.

#atomic -> thread safety, nonatomic -> faster

6. What is BLOCK?

Blocks are a way of defining a single task or unit of behavior without having to write an entire Objective-C class.

You can use blocks to compose function expressions that can be passed to API, optionally stored, and used by multiple threads. Blocks are particularly useful as a callback because the block carries both the code to be executed on callback and the data needed during that execution.

ref: https://www.appcoda.com/objective-c-blocks-tutorial/

#like closures in swift, callback API

7. What's different between CLASS and STRUCTURE in swift? 

The main difference is that structs are value types and classes are reference types. Structures cannot inherit from other types.

Structs are preferable if they are relatively small and copiable because copying is way safer than having multiple reference to the same instance as happens with classes.

Structs there is much less need to worry about memory leaks.

8. What is a protocol?

It defines a list of required and optional methods that a class must/can implement if it adopts the protocol.

#define , required or optional methods, blueprint

9. What is UIAppliction?

UIApplication is the iOS singleton application base class.  It's a major role of  app’s application object is to handle the initial routing of incoming user events.

10. What is UIView? How to create custom UIView?

UIView is an object that manages the content for a rectangular area on the screen, it responsible for recognizing touches, gestures.

To create custom uiview, just create your own class subclassing UIView, then override initWithFrame or drawRect.

11. Optional type

A type that can represent either a wrapped value or nil. It shows the wrapped type's name with a trailing question mark (?). There are couple ways to unwrapping :  if let statement , guard statement, forced unwrapping. But forced unwrapping triggers a runtime error when the optional is nil. We should try to avoid it.

A common example of implicitly unwrapped optionals is how view controller define their IBOutlets.
It would be unwieldy(笨重的) to always unwrap each view outlet inside view controllers.

#question mark, exclamation mark, unwrapping 

11.1 Optional Chain
Optional chaining is a process for querying and calling properties, methods, and subscripts on an optional that might currently be nil. If the optional contains a value, the property, method, or subscript call succeeds; if the optional is nil, the property, method, or subscript call returns nil. Multiple queries can be chained together, and the entire chain fails gracefully if any link in the chain is nil.

12. NSKeyedArchiver

Core data, NSKeyedArchiver and UserDefaults are three ways in which a programmer can persist data in between app launches. Though core data is slightly more complicated, it is useful when the stored information requires structure. NSKeyedArchiver is less complex and slower than core data, but is much simpler to use. UserDefaults is the simplest method to persist data.

13. Access Control

Open access and public access enable entities to be used within any source file from their defining module, and also in a source file from another module that imports the defining module. You typically use open or public access when specifying the public interface to a framework. The difference between open and public access is described below:

• An open class is accessible and subclassable outside of the defining module. An open class member is accessible and overridable outside of the defining module.
• A public class is accessible but not subclassable outside of the defining module. A public class member is accessible but not overridable outside of the defining module.

Internal access enables entities to be used within any source file from their defining module, but not in any source file outside of that module. You typically use internal access when defining an app’s or a framework’s internal structure.

File-private access restricts the use of an entity to its own defining source file. Use file-private access to hide the implementation details of a specific piece of functionality when those details are used within an entire file.

Private access restricts the use of an entity to the enclosing declaration, and to extensions of that declaration that are in the same file. Use private access to hide the implementation details of a specific piece of functionality when those details are used only within a single declaration.

Open access is the highest (least restrictive) access level and private access is the lowest (most restrictive) access level.

簡單來說:
keep private details of a class hidden from the rest of the app
keep internal details of a framework hidden from the client app

14. What is Core Data?

17. UIViewcontroller lifecircle

• ViewDidLoad - Called when you create the class and load from xib. Great for initial setup and one-time-only work. It might be called several times if the view is unloaded due to low memory and then loaded again.
• ViewWillAppear - Called right before your view appears, good for hiding/showing fields or any operations that you want to happen every time before the view is visible. Because you might be going back and forth between views, this will be called every time your view is about to appear on the screen.
• ViewDidAppear - Called after the view appears - great place to start an animations or the loading of external data from an API.
• ViewWillDisappear/DidDisappear - Same idea as ViewWillAppear/ViewDidAppear.

18. Singletion class

A singleton class returns the same instance no matter how many times an application requests it. A singleton object provides a global point of access to the resources of its class. Singletons are used in situations where this single point of control is desirable, such as with classes that offer some general service or resource.

use the @synchronized pattern - and @synchronized is still the best way to do simple locking in Objective-C.

19. Closures

Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.

20. Autolayout

Auto Layout dynamically calculates the size and position of all the views in your view hierarchy(階層), based on constraints placed on those views.

21. Safe area

The Safe Area is the area in between System UI elements which are Status Bar, Navigation Bar and Tool Bar or Tab Bar. After iOS11, you can use SafeAreaLayoutGuide. By add constraints between your content and safe area anchors. This prevents your content from being obscured(模糊, 遮擋) by top and bottom bars.

Assume you are an awesome parent and want to give your children some cookies. But, you should give each child at most one cookie. Each child i has a greed factor gi, which is the minimum size of a cookie that the child will be content with; and each cookie j has a size sj. If sj >= gi, we can assign the cookie j to the child i, and the child i will be content. Your goal is to maximize the number of your content children and output the maximum number.

Note:
You may assume the greed factor is always positive.
You cannot assign more than one cookie to one child.

Input: [1,2,3], [1,1]

Output: 1

Explanation: You have 3 children and 2 cookies. The greed factors of 3 children are 1, 2, 3. 
And even though you have 2 cookies, since their size is both 1, you could only make the child whose greed factor is 1 content.
You need to output 1.

Input: [1,2], [1,2,3]

Output: 2

Explanation: You have 2 children and 3 cookies. The greed factors of 2 children are 1, 2. 
You have 3 cookies and their sizes are big enough to gratify all of the children, 
You need to output 2.

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

把數字轉成文字然後拆開，遞迴相加後直到剩一個字元

--------------------

Reverse digits of an integer.

Example1: x = 123, return 321
Example2: x = -123, return -321

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)

解法: 

https://www.youtube.com/watch?v=-AMHUdZc9ss
利用前後兩個指標和最左邊開始的數相加去比對。