Implement an in-memory file system

Implement an **in-memory file system** supporting basic directory and file operations. You must support the following methods (names are illustrative; any equivalent interface is fine): 1. `ls(path)` - If `path` is a file path, return a list containing **only the file name**. - If `path` is a directory path, return the names of **files and subdirectories directly under it**, sorted lexicographically. 2. `mkdir(path)` - Create a directory at the given absolute path. - Create any missing intermediate directories. 3. `addContentToFile(filePath, content)` - If the file does not exist, create it. - Append `content` to the existing file content. 4. `readContentFromFile(filePath)` - Return the full content of the file. ### Notes / Constraints - Paths are absolute and use `/` as the separator. - The root directory is `/`. - Directory and file names consist of lowercase letters (you may assume no invalid characters). - The system is in-memory only (no persistence). - Aim for reasonable time complexity per operation.