Implement an In-Memory File System

Implement an in-memory file system that supports fundamental directory and file operations. The system should not persist any data to disk.

Required Operations

1. ls(path)
   • If path refers to a file, return a list containing only its name.
   • If path refers to a directory, return the names of all files and subdirectories directly within it, sorted lexicographically.
2. mkdir(path)
   • Create a new directory at the specified absolute path.
   • Automatically create any necessary intermediate directories if they do not already exist.
3. addContentToFile(filePath, content)
   • If the file specified by filePath does not exist, create it.
   • Append the provided content string to the file's existing content.
4. readContentFromFile(filePath)
   • Return the complete content of the file specified by filePath.

Constraints and Assumptions

• All paths provided are absolute and use / as the separator.
• The root directory is represented by /.
• Directory and file names will consist solely of lowercase letters.
• The system is entirely in-memory; no persistence is required.
• Operations should aim for reasonable time complexity.