Implement an In-Memory File System

Implement an In-Memory File System

Design an in-memory file system to support directory and file management. Implement the FileSystem class with the following methods:

• FileSystem(): Initializes the object with an empty root directory /.
• ls(path: str) -> list[str]:
  • If path points to a file, returns a list containing only that file's name.
  • If path points to a directory, returns the list of file and subdirectory names in that directory, sorted in lexicographical order.
• mkdir(path: str) -> None: Creates a new directory according to the given path. If the intermediate directories in the path do not exist, they should be created automatically.
• add_content_to_file(file_path: str, content: str) -> None:
  • If file_path does not exist, creates that file with the given content. Intermediate directories are created if necessary.
  • If file_path already exists, appends the given content to the existing file content.
• read_content_from_file(file_path: str) -> str: Returns the content in the file at file_path as a string.

Assumptions & Constraints

• Paths are absolute: they always start with / and do not end with / unless the path is simply /.
• Path components consist of lowercase English letters.
• You will not receive invalid operations (e.g., read_content_from_file on a directory or ls on a non-existent path).
• The total number of calls across all methods is at most 500.
• Length of any path does not exceed 100 characters.

Example

fs = FileSystem()

# Root directory is initially empty
fs.ls("/")  # Returns []

fs.mkdir("/a/b/c")
fs.add_content_to_file("/a/b/c/d", "hello")

# ls on a directory returns child names in sorted order
fs.ls("/")        # Returns ["a"]
fs.ls("/a/b")     # Returns ["c"]

# ls on a file path returns just the file name
fs.ls("/a/b/c/d") # Returns ["d"]

# Reading file content
fs.read_content_from_file("/a/b/c/d") # Returns "hello"

# Appending content
fs.add_content_to_file("/a/b/c/d", " world")
fs.read_content_from_file("/a/b/c/d") # Returns "hello world"

Edge Cases to Consider

• Operating on the root directory / directly.
• Creating nested paths where multiple parent directories do not yet exist.
• Distinguishing between an empty directory and a file with empty content.