In-Memory File System

Design and implement an in-memory file system supporting `ls`, `mkdir`, `addContentToFile`, `readContentFromFile` (LC 588). Requirements ls(path) — if path is a directory, return the sorted list of files and directories in it; if path is a file, return a single-element list containing the file name. mkdir(path) — create all intermediate directories in path if they don't exist. addContentToFile(filePath, content) — append content to the file, creating it if missing. readContentFromFile(filePath) — return the entire content of the file. All paths are absolute, separated by /. The root is /. Notes The canonical implementation is a trie keyed on path components. Each node owns: A map children: name -> Node. An is_file flag. A content string (only meaningful when is_file). ls, mkdir, add, and read all reduce to walking the path components from the root, creating nodes on the way down for mkdir / add. Sorting on ls is over the immediate children's names — keep children as a hash map and sort on read, or use a sorted map for O(log N) inserts and O(N) reads. Edge cases: ls("/") on an empty FS (empty list), mkdir on an existing path (no-op), addContentToFile on an existing file (append, don't overwrite), invalid paths (the LC problem assumes well-formed input; in an interview, ask). Common stumbling points: forgetting that ls on a file path must return the file name itself (not its parent's contents), and forgetting that addContentToFile must create intermediate directories. Preparation Implement the trie-node + path-split walker in one pass; verify against the LC 588 test set. Drill the sorted-vs-hash trade-off; for an interview the hash + sort-on-read is faster to write and matches the LC time budget. Be ready to extend with delete and mv if the interviewer pushes for follow-ups. Exact API surface class FileSystem: def ls(self, path: str) -> list[str]: ... def mkdir(self, path: str) -> None: ... def addContentToFile(self, filePath: str, content: str) -> None: ... def readContentFromFile(self, filePath: str) -> str: ... ls(file_path) returns a one-element list containing the file name; ls(directory_path) returns immediate child names sorted lexicographically. mkdir creates missing parent directories. addContentToFile creates a missing file or appends to an existing file.