Design an in-memory file system with the following operations:

• ls(path): If path is a file, return its name. If path is a directory, return all files and directories in it (sorted lexicographically).
• mkdir(path): Create a directory at path if it doesn't exist (path may have nested directories).
• addContentToFile(filePath, content): Create a file at filePath with given content if it doesn't exist, otherwise append content.
• readContentFromFile(filePath): Return the content of the file.