https://algs4.cs.princeton.edu/mac/

Leet/Lint code reference: 
https://aaronice.gitbooks.io/lintcode/content/union_find/surrounded_regions.html

To compile:
>>javac-algs4 TestAlgs4

To run
>>java-algs4 TestAlgs4

To create directory by package name, 
为Java类添加包必须在Java源文件中通过package语句指定, mv .class 到指定文件夹是不行的
put "package lib;" on the beginning of In.java and run
>> javac -d . In.java
the class file will be under the lib directory.

But we can simply create a directory by the package name and run main()