In-Memory File System APIs

Design and implement an in-memory file system supporting directories and files. Implement the following APIs: ls(path) -> List[str] If path is a directory, return all immediate children names (files and subdirectories) in lexicographical order. If path is a file, return a list containing only the file name. mkdir(path): create a directory at path (create intermediate directories if missing). addContentToFile(filePath, content): append content to a file; create the file if it does not exist. readContentFromFile(filePath) -> str: return the full content of the file. Constraints: Unix-style paths, / is the root. Names contain lowercase letters only. Ensure correctness and reasonable time complexity. Provide at least 5 test sequences with expected outputs. Example Input mkdir /a/b/c ls / Output ["a"]