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"]

Solution：Trie Tree 表示路徑

思路：
Time Complexity: O() Space Complexity: O()

Solution Code：

public class FileSystem {
    class File {
        boolean isFile = false;
        Map<String, File> children = new HashMap<>();
        String content = "";
    }
    
    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() == 0) 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() == 0) 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() == 0) 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() == 0) continue;
            if (!node.children.containsKey(dir)) {
                File file = new File();
                node.children.put(dir, file);
            }
            node = node.children.get(dir);
        }

        return node.content;
    }
}

/**
 * Your FileSystem object will be instantiated and called as such:
 * FileSystem obj = new FileSystem();
 * List<String> param_1 = obj.ls(path);
 * obj.mkdir(path);
 * obj.addContentToFile(filePath,content);
 * String param_4 = obj.readContentFromFile(filePath);
 */