Design In-Memory File System

Design a data structure that simulates an in-memory file system.

Implement the FileSystem class:

• FileSystem() Initializes the object of the system.
• List<String> ls(String path)
• If path is a file path, returns a list that only contains this file's name.
• If path is a directory path, returns the list of file and directory names in this directory.
• The answer should in lexicographic order.
• void mkdir(String path) Makes a new directory according to the given path. The given directory path does not exist. If the middle directories in the path do not exist, you should create them as well.
• void addContentToFile(String filePath, String content)
• If filePath does not exist, creates that file containing given content.
• If filePath already exists, appends the given content to original content.
• String readContentFromFile(String filePath) Returns the content in the file at filePath.

Example 1:

Input
["FileSystem", "ls", "mkdir", "addContentToFile", "ls", "readContentFromFile"]
[[], ["/"], ["/a/b/c"], ["/a/b/c/d", "hello"], ["/"], ["/a/b/c/d"]]
Output
[null, [], null, null, ["a"], "hello"]