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# 0588. Design In-Memory File System

<https://leetcode.com/problems/design-in-memory-file-system>

## Description

Design a data structure that simulates an in-memory file system.

Implement the FileSystem class:

* `FileSystem()` Initializes the object of the system.
* `List<String> ls(String path)`
  * If `path` is a file path, returns a list that only contains this file's name.
  * If `path` is a directory path, returns the list of file and directory names **in this directory**. The answer should in **lexicographic order**.
* `void mkdir(String path)` Makes a new directory according to the given `path`. The given directory path does not exist. If the middle directories in the path do not exist, you should create them as well.
* `void addContentToFile(String filePath, String content)`
  * If `filePath` does not exist, creates that file containing given `content`.
  * If `filePath` already exists, appends the given `content` to original content.
* `String readContentFromFile(String filePath)` Returns the content in the file at `filePath`.

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/04/28/filesystem.png)

```
**Input**
["FileSystem", "ls", "mkdir", "addContentToFile", "ls", "readContentFromFile"]
[[], ["/"], ["/a/b/c"], ["/a/b/c/d", "hello"], ["/"], ["/a/b/c/d"]]
**Output**
[null, [], null, null, ["a"], "hello"]
**Explanation**
FileSystem fileSystem = new FileSystem();
fileSystem.ls("/");                         // return []
fileSystem.mkdir("/a/b/c");
fileSystem.addContentToFile("/a/b/c/d", "hello");
fileSystem.ls("/");                         // return ["a"]
fileSystem.readContentFromFile("/a/b/c/d"); // return "hello"
```

**Constraints:**

* `1 <= path.length, filePath.length <= 100`
* `path` and `filePath` are absolute paths which begin with `'/'` and do not end with `'/'` except that the path is just `"/"`.
* You can assume that all directory names and file names only contain lowercase letters, and the same names will not exist in the same directory.
* You can assume that all operations will be passed valid parameters, and users will not attempt to retrieve file content or list a directory or file that does not exist.
* `1 <= content.length <= 50`
* At most `300` calls will be made to `ls`, `mkdir`, `addContentToFile`, and `readContentFromFile`.

## ac

```java
```