Design In-Memory File System

Build a tiny file system that lives in memory. It starts with just the root directory /, and holds directories and text files. Every path is absolute: / alone is the root, and /docs/notes/todo names todo inside notes inside docs. Implement the FileSystem class:

• FileSystem() creates a file system with an empty root directory.
• ls(path) lists names:
  • if path is a directory, it returns the names of everything directly inside it (files and directories alike, not their contents), sorted in lexicographic order;
  • if path is a file, it returns a list holding only that file's name.
• mkdir(path) creates the directory path, along with any directories on the way to it that don't exist yet.
• addContentToFile(filePath, content) appends content to the file filePath. If there is no such file yet, it creates it first, empty.
• readContentFromFile(filePath) returns everything written to the file filePath so far.

Each example lists the calls in order, then the arguments of each call, then what each call returns: null for the constructor, mkdir and addContentToFile, which return nothing.

Example 1

Input:
["FileSystem", "mkdir", "addContentToFile", "addContentToFile", "ls", "ls", "readContentFromFile"]
[[], ["/music/jazz"], ["/music/list", "blue"], ["/music/list", "train"], ["/music"], ["/music/list"], ["/music/list"]]
Output: [null, null, null, null, ["jazz", "list"], ["list"], "bluetrain"]

mkdir creates music on the way to jazz. The second write appends to the file the first one created. Listing /music shows both entries in sorted order; listing the file itself shows just its own name.

Example 2

Input:
["FileSystem", "ls", "mkdir", "mkdir", "addContentToFile", "ls", "ls"]
[[], ["/"], ["/x/yz"], ["/x/y"], ["/x/b", "ok"], ["/x"], ["/x/yz"]]
Output: [null, [], null, null, null, ["b", "y", "yz"], []]

The root starts empty. y sorts before yz because it is a prefix of it, and an empty directory lists as an empty list.

Constraints

• 1 <= path.length, filePath.length <= 100
• Every path and filePath starts with / and does not end with /, except path may be exactly "/". Names between slashes are non-empty and made of lowercase English letters.
• 1 <= content.length <= 50, and content is made of lowercase English letters.
• A directory never holds two entries (files or directories) with the same name.
• Every call is valid: ls is only given a path that exists; mkdir is only given a path that does not exist yet and does not run through a file; addContentToFile is only given a path whose parent directory already exists and that is not a directory; readContentFromFile is only given an existing file.
• At most 300 calls are made in total to ls, mkdir, addContentToFile and readContentFromFile.