Design In-Memory File System

Design an in-memory file system to simulate the following functions:

ls: Given a path in string format. If it is a file path, return a list that only contains this file's name. If it is a directory path, return the list of file and directory names in this directory. Your output (file and directory names together) should in lexicographic order.

mkdir: Given a directory path that does not exist, you should make a new directory according to the path. If the middle directories in the path don't exist either, you should create them as well. This function has void return type.

addContentToFile: Given a file path and file content in string format. If the file doesn't exist, you need to create that file containing given content. If the file already exists, you need to append given content to original content. This function has void return type.

readContentFromFile: Given a file path, return its content in string format.

Example:

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:

Note:

1. You can assume all file or directory paths are absolute paths which begin with / and do not end with /except that the path is just "/".
2. 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.
3. You can assume that all directory names and file names only contain lower-case letters, and same names won't exist in the same directory.

分析：https://www.cnblogs.com/grandyang/p/6944331.html

这道题比较tricky的地方是ls这个命令，题目中的例子其实不能很好的展示出ls的要求，其对文件和文件夹的处理方式是不同的。由于这里面的文件没有后缀，所以最后一个字符串有可能是文件，也有可能是文件夹。比如a/b/c，那么最后的c有可能是文件夹，也有可能好是文件，如果c是文件夹的话，ls命令要输出文件夹c中的所有文件和文件夹，而当c是文件的话，只需要输出文件c即可。另外需要注意的是在创建文件夹的时候，路径上没有的文件夹都要创建出来，还有就是在给文件添加内容时，路径中没有的文件夹都要创建出来。

 class File {
     boolean isFile = false;
     Map<String, File> children = new HashMap<>();
     String content = "";
 }

 public class FileSystem {
     File root = null;

     public FileSystem() {
         root = new File();
     }

     public List<String> ls(String path) {
         String[] dirs = path.split("/");
         File node = root;
         List<String> result = new ArrayList<>();
         String name = "";
         for (String dir : dirs) {
             if (dir.length() == )
                 continue;
             if (!node.children.containsKey(dir)) {
                 return result;
             }
             node = node.children.get(dir);
             name = dir;
         }

         if (node.isFile) {
             result.add(name);
         } else {
             for (String key : node.children.keySet()) {
                 result.add(key);
             }
         }
         Collections.sort(result);
         return result;
     }

     public void mkdir(String path) {
         String[] dirs = path.split("/");
         File node = root;
         for (String dir : dirs) {
             if (dir.length() == ) continue;
             if (!node.children.containsKey(dir)) {
                 File file = new File();
                 node.children.put(dir, file);
             }
             node = node.children.get(dir);
         }
     }

     public void addContentToFile(String filePath, String content) {
         String[] dirs = filePath.split("/");
         File node = root;
         for (String dir : dirs) {
             if (dir.length() == ) continue;
             if (!node.children.containsKey(dir)) {
                 File file = new File();
                 node.children.put(dir, file);
             }
             node = node.children.get(dir);
         }
         node.isFile = true;
         node.content += content;
     }

     public String readContentFromFile(String filePath) {
         String[] dirs = filePath.split("/");
         File node = root;
         for (String dir : dirs) {
             if (dir.length() == )
                 continue;
             if (!node.children.containsKey(dir)) {
                 File file = new File();
                 node.children.put(dir, file);
             }
             node = node.children.get(dir);
         }

         return node.content;
     }
 }