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",["/a/b/c"],"addContentToFile",["/a/b/c/d","hello"],"ls",["/"],"readContentFromFile",["/a/b/c/d"]]

Output:
[null, [], null, null, ["a"], "hello"]

Explanation:
FileSystem fileSystem = new FileSystem();
fileSystem.ls("/");                         // return []
fileSystem.mkdir("/a/b/c");
fileSystem.addContentToFile("/a/b/c/d", "hello");
fileSystem.ls("/");                         // return ["a"]
fileSystem.readContentFromFile("/a/b/c/d"); // return "hello"

Constraints:

• 1 <= path.length, filePath.length <= 100
• path and filePath are absolute paths which begin with '/' and do not end with '/' except that the path is just "/".
• You can assume that all directory names and file names only contain lowercase letters, and the same names will not exist in the same directory.
• You can assume that all operations will be passed valid parameters, and users will not attempt to retrieve file content or list a directory or file that does not exist.
• You can assume that the parent directory for the file in addContentToFile will exist.
• 1 <= content.length <= 50
• At most 300 calls will be made to ls, mkdir, addContentToFile, and readContentFromFile.