In-Memory File System: Add cd and Relative Path Support

Implement an in-memory file system as a class, in stages. The early stages build a tree of directories and files addressed by absolute paths. The final stage adds a current working directory with a cd command and requires every operation to accept relative paths as well. That final stage is where the difficulty lies: path handling written for absolute paths has to change without breaking what already works, and relative-path bugs are easy to introduce.

The earlier stages are not specified in detail here, so Part 1 states an assumed baseline API. If your interviewer gives a different one, keep its semantics and apply Part 2 to it. Part 2 is the extension the question builds toward.

Clarifying Questions

• Are file contents plain strings, and does writing to an existing file overwrite it or append to it?
• Should ls return names in sorted order, and what should it return when the path is a file?
• How should errors be reported: exceptions, return values, or silently ignored?
• Can paths contain repeated slashes or a trailing slash, or are inputs always well formed?

Part 1 — Baseline file system over absolute paths

Implement this API. Every path is absolute: it starts with /, and / alone is the root.

class FileSystem:
 def mkdir(self, path: str) -> None: ...
 def write_file(self, path: str, content: str) -> None: ...
 def read_file(self, path: str) -> str: ...
 def ls(self, path: str) -> list[str]: ...

• mkdir creates the directory and any missing parent directories. It is an error if some component of the path is an existing file.
• write_file creates the file or overwrites its content. Its parent directory must already exist, and the path must not name a directory.
• read_file returns a file's content; the path must name an existing file.
• ls returns the names of a directory's entries in sorted order, or a one-element list with the file's own name when the path names a file.

What This Part Should Cover

• A tree in which directories map names to children and files hold content
• A single traversal helper shared by all methods
• Defined behavior for missing paths, a file used as a directory, and the root itself

Part 2 — Add cd and relative paths

Add cd(path), which changes the current working directory, and a pwd() helper that returns the current directory as an absolute path, so the behavior can be tested. The file system starts in /. From now on every method, including cd itself, must accept both absolute paths and relative paths such as docs/a.txt, ./docs or ../shared/b.txt, which are resolved against the current directory.

Clarifying Questions for this Part

• What does .. mean at the root: an error, or stay at the root as a Unix shell does?
• If cd targets a missing directory or a file, should it fail and leave the current directory unchanged?
• Must a path like missing/../docs fail because missing does not exist, or may .. be resolved purely by text?

What This Part Should Cover

• One resolution function that turns any absolute or relative input into a canonical path, used by every method
• Correct handling of ., .. (including at the root), empty components and trailing slashes
• cd validates its target before changing any state
• Tests that send relative paths through every operation, not only through cd