# Design an In-Memory File System Implement `run_file_system(operations: list[list[str]]) -> list[str]` for an initially empty in-memory hierarchy whose root is `/`. Supported operations are: - `["mkdir", path]`: create all missing directories on `path`. - `["add", file_path, content]`: append `content` to an existing file, or create the file in an existing parent directory. - `["read", file_path]`: return the file's complete content. - `["ls", path]`: if `path` is a file, return its name; if it is a directory, return its direct child names in lexicographic order, joined by commas. Return the strings produced by `read` and `ls`, in operation order. Mutating operations produce no result entry. ## Valid Input Domain - Paths are absolute and canonical, with lowercase alphanumeric names separated by `/`. - Names contain no commas. A file and directory never share a path. - Every `read` target and every `ls` target exists. The parent of an `add` target exists. ## Constraints - `0 <= operations.length <= 100,000` - Total path and content length is at most 1,000,000 characters. ## Public Examples ### Example 1 Input: `[["mkdir", "/a/b"], ["add", "/a/b/f", "hi"], ["ls", "/a"], ["read", "/a/b/f"]]` Output: `["b", "hi"]` ### Example 2 Input: `[["add", "/x", "ab"], ["add", "/x", "cd"], ["ls", "/x"], ["read", "/x"]]` Output: `["x", "abcd"]` ```hint Separate nodes by kind Choose a representation that distinguishes directories from files while sharing path traversal logic. ```