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.